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Issues in Identifier Comparison for Security Purposes
RFC 6943

Document Type RFC - Informational (May 2013)
Author Dave Thaler
Last updated 2013-05-09
RFC stream Internet Architecture Board (IAB)
RFC 6943
Internet Architecture Board (IAB)                         D. Thaler, Ed.
Request for Comments: 6943                                     Microsoft
Category: Informational                                         May 2013
ISSN: 2070-1721

         Issues in Identifier Comparison for Security Purposes


   Identifiers such as hostnames, URIs, IP addresses, and email
   addresses are often used in security contexts to identify security
   principals and resources.  In such contexts, an identifier presented
   via some protocol is often compared using some policy to make
   security decisions such as whether the security principal may access
   the resource, what level of authentication or encryption is required,
   etc.  If the parties involved in a security decision use different
   algorithms to compare identifiers, then failure scenarios ranging
   from denial of service to elevation of privilege can result.  This
   document provides a discussion of these issues that designers should
   consider when defining identifiers and protocols, and when
   constructing architectures that use multiple protocols.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Architecture Board (IAB)
   and represents information that the IAB has deemed valuable to
   provide for permanent record.  It represents the consensus of the
   Internet Architecture Board (IAB).  Documents approved for
   publication by the IAB are not a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.

Table of Contents

   1. Introduction ....................................................3
      1.1. Classes of Identifiers .....................................5
      1.2. Canonicalization ...........................................5
   2. Identifier Use in Security Policies and Decisions ...............6
      2.1. False Positives and Negatives ..............................7
      2.2. Hypothetical Example .......................................8
   3. Comparison Issues with Common Identifiers .......................9
      3.1. Hostnames ..................................................9
           3.1.1. IPv4 Literals ......................................11
           3.1.2. IPv6 Literals ......................................12
           3.1.3. Internationalization ...............................13
           3.1.4. Resolution for Comparison ..........................14
      3.2. Port Numbers and Service Names ............................14
      3.3. URIs ......................................................15
           3.3.1. Scheme Component ...................................16
           3.3.2. Authority Component ................................16
           3.3.3. Path Component .....................................17
           3.3.4. Query Component ....................................17
           3.3.5. Fragment Component .................................17
           3.3.6. Resolution for Comparison ..........................18
      3.4. Email Address-Like Identifiers ............................18
   4. General Issues .................................................19
      4.1. Conflation ................................................19
      4.2. Internationalization ......................................20
      4.3. Scope .....................................................21
      4.4. Temporality ...............................................21
   5. Security Considerations ........................................22
   6. Acknowledgements ...............................................22
   7. IAB Members at the Time of Approval ............................23
   8. Informative References .........................................23

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1.  Introduction

   In computing and the Internet, various types of "identifiers" are
   used to identify humans, devices, content, etc.  This document
   provides a discussion of some security issues that designers should
   consider when defining identifiers and protocols, and when
   constructing architectures that use multiple protocols.  Before
   discussing these security issues, we first give some background on
   some typical processes involving identifiers.  Terms such as
   "identifier", "identity", and "principal" are used as defined in

   As depicted in Figure 1, there are multiple processes relevant to our

   1.  An identifier is first generated.  If the identifier is intended
       to be unique, the generation process must include some mechanism,
       such as allocation by a central authority or verification among
       the members of a distributed authority, to help ensure
       uniqueness.  However, the notion of "unique" involves determining
       whether a putative identifier matches any other identifier that
       has already been allocated.  As we will see, for many types of
       identifiers, this is not simply an exact binary match.

       After generating the identifier, it is often stored in two
       locations: with the requester or "holder" of the identifier, and
       with some repository of identifiers (e.g., DNS).  For example, if
       the identifier was allocated by a central authority, the
       repository might be that authority.  If the identifier identifies
       a device or content on a device, the repository might be that

   2.  The identifier is distributed, either by the holder of the
       identifier or by a repository of identifiers, to others who could
       use the identifier.  This distribution might be electronic, but
       sometimes it is via other channels such as voice, business card,
       billboard, or other form of advertisement.  The identifier itself
       might be distributed directly, or it might be used to generate a
       portion of another type of identifier that is then distributed.
       For example, a URI or email address might include a server name,
       and hence distributing the URI or email address also inherently
       distributes the server name.

   3.  The identifier is used by some party.  Generally, the user
       supplies the identifier, which is (directly or indirectly) sent
       to the repository of identifiers.  The repository of identifiers
       must then attempt to match the user-supplied identifier with an
       identifier in its repository.

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       For example, using an email address to send email to the holder
       of an identifier may result in the email arriving at the holder's
       email server, which has access to the mail stores.

                          |  Holder of |     1. Generation
                          | identifier +<---------+
                          +----+-------+          |
                               |                  | Match
                               |                  v/
                               |          +-------+-------+
                               +----------+ Repository of |
                               |          |  identifiers  |
                               |          +-------+-------+
               2. Distribution |                  ^\
                               |                  | Match
                               v                  |
                     +---------+-------+          |
                     |      User of    |          |
                     |    identifier   +----------+
                     +-----------------+    3. Use

                  Figure 1: Typical Identifier Processes

   Another variation is where a user is given the identifier of a
   resource (e.g., a web site) to access securely, sometimes known as a
   "reference identifier" [RFC6125], and the server hosting the resource
   then presents its identity at the time of use.  In this case, the
   user application attempts to match the presented identity against the
   reference identifier.

   One key aspect is that the identifier values passed in generation,
   distribution, and use may all be in different forms.  For example, an
   identifier might be exchanged in printed form at generation time,
   distributed to a user via voice, and then used electronically.  As
   such, the match process can be complicated.

   Furthermore, in many cases, the relationship between holder,
   repositories, and users may be more involved.  For example, when a
   hierarchy of web caches exists, each cache is itself a repository of
   a sort, and the match process is usually intended to be the same as
   on the origin server.

   Another aspect to keep in mind is that there can be multiple
   identifiers that refer to the same object (i.e., resource, human,
   device, etc.).  For example, a human might have a passport number and
   a drivers license number, and an RFC might be available at multiple
   locations ( and  In this document, we focus

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   on comparing two identifiers to see whether they are the same
   identifier, rather than comparing two different identifiers to see
   whether they refer to the same entity (although a few issues with the
   latter are touched on in several places, such as Sections 3.1.4 and

1.1.  Classes of Identifiers

   In this document, we will refer to the following classes of

   o  Absolute: identifiers that can be compared byte-by-byte for
      equality.  Two identifiers that have different bytes are defined
      to be different.  For example, binary IP addresses are in this

   o  Definite: identifiers that have a single well-defined comparison
      algorithm.  For example, URI scheme names are required to be
      US-ASCII [USASCII] and are defined to match in a case-insensitive
      way; the comparison is thus definite, since there is a well-
      specified algorithm (Section 9.2.1 of [RFC4790]) on how to do a
      case-insensitive match among ASCII strings.

   o  Indefinite: identifiers that have no single well-defined
      comparison algorithm.  For example, human names are in this class.
      Everyone might want the comparison to be tailored for their
      locale, for some definition of "locale".  In some cases, there may
      be limited subsets of parties that might be able to agree (e.g.,
      ASCII users might all agree on a common comparison algorithm,
      whereas users of other Roman-derived scripts, such as Turkish, may
      not), but identifiers often tend to leak out of such limited

1.2.  Canonicalization

   Perhaps the most common algorithm for comparison involves first
   converting each identifier to a canonical form (a process known as
   "canonicalization" or "normalization") and then testing the resulting
   canonical representations for bitwise equality.  In so doing, it is
   thus critical that all entities involved agree on the same canonical
   form and use the same canonicalization algorithm so that the overall
   comparison process is also the same.

   Note that in some contexts, such as in internationalization, the
   terms "canonicalization" and "normalization" have a precise meaning.
   In this document, however, we use these terms synonymously in their
   more generic form, to mean conversion to some standard form.

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   While the most common method of comparison includes canonicalization,
   comparison can also be done by defining an equivalence algorithm,
   where no single form is canonical.  However, in most cases, a
   canonical form is useful for other purposes, such as output, and so
   in such cases defining a canonical form suffices to define a
   comparison method.

2.  Identifier Use in Security Policies and Decisions

   Identifiers such as hostnames, URIs, and email addresses are used in
   security contexts to identify security principals (i.e., entities
   that can be authenticated) and resources as well as other security
   parameters such as types and values of claims.  Those identifiers are
   then used to make security decisions based on an identifier presented
   via some protocol.  For example:

   o  Authentication: a protocol might match a security principal's
      identifier to look up expected keying material and then match
      keying material.

   o  Authorization: a protocol might match a resource name against some
      policy.  For example, it might look up an access control list
      (ACL) and then look up the security principal's identifier (or a
      surrogate for it) in that ACL.

   o  Accounting: a system might create an accounting record for a
      security principal's identifier or resource name, and then might
      later need to match a presented identifier to (for example) add
      new filtering rules based on the records in order to stop an

   If the parties involved in a security decision use different matching
   algorithms for the same identifiers, then failure scenarios ranging
   from denial of service to elevation of privilege can result, as we
   will see.

   This is especially complicated in cases involving multiple parties
   and multiple protocols.  For example, there are many scenarios where
   some form of "security token service" is used to grant to a requester
   permission to access a resource, where the resource is held by a
   third party that relies on the security token service (see Figure 2).
   The protocol used to request permission (e.g., Kerberos or OAuth) may
   be different from the protocol used to access the resource (e.g.,
   HTTP).  Opportunities for security problems arise when two protocols
   define different comparison algorithms for the same type of
   identifier, or when a protocol is ambiguously specified and two
   endpoints (e.g., a security token service and a resource holder)
   implement different algorithms within the same protocol.

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         | security |
         |  token   |
         | service  |
              | 1. supply credentials and
              |    get token for resource
              |                                             +--------+
         +----------+  2. supply token and access resource  |resource|
         |requester |=------------------------------------->| holder |
         +----------+                                       +--------+

                    Figure 2: Simple Security Exchange

   In many cases, the situation is more complex.  With X.509 Public Key
   Infrastructure (PKIX) certificates [RFC6125], for example, the name
   in a certificate gets compared against names in ACLs or other things.
   In the case of web site security, the name in the certificate gets
   compared to a portion of the URI that a user may have typed into a
   browser.  The fact that many different people are doing the typing,
   on many different types of systems, complicates the problem.

   Add to this the certificate enrollment step, and the certificate
   issuance step, and two more parties have an opportunity to adjust the
   encoding, or worse, the software that supports them might make
   changes that the parties are unaware are happening.

2.1.  False Positives and Negatives

   It is first worth discussing in more detail the effects of errors in
   the comparison algorithm.  A "false positive" results when two
   identifiers compare as if they were equal but in reality refer to two
   different objects (e.g., security principals or resources).  When
   privilege is granted on a match, a false positive thus results in an
   elevation of privilege -- for example, allowing execution of an
   operation that should not have been permitted otherwise.  When
   privilege is denied on a match (e.g., matching an entry in a
   block/deny list or a revocation list), a permissible operation is
   denied.  At best, this can cause worse performance (e.g., a cache
   miss or forcing redundant authentication) and at worst can result in
   a denial of service.

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   A "false negative" results when two identifiers that in reality refer
   to the same thing compare as if they were different, and the effects
   are the reverse of those for false positives.  That is, when
   privilege is granted on a match, the result is at best worse
   performance and at worst a denial of service; when privilege is
   denied on a match, elevation of privilege results.

   Figure 3 summarizes these effects.

                      | "Grant on match"       | "Deny on match"
       False positive | Elevation of privilege | Denial of service
       False negative | Denial of service      | Elevation of privilege

           Figure 3: Worst Effects of False Positives/Negatives

   When designing a comparison algorithm, one can typically modify it to
   increase the likelihood of false positives and decrease the
   likelihood of false negatives, or vice versa.  Which outcome is
   better depends on the context.

   Elevation of privilege is almost always seen as far worse than denial
   of service.  Hence, for URIs, for example, Section 6.1 of [RFC3986]
   states that "comparison methods are designed to minimize false
   negatives while strictly avoiding false positives".

   Thus, URIs were defined with a "grant privilege on match" paradigm in
   mind, where it is critical to prevent elevation of privilege while
   minimizing denial of service.  Using URIs in a "deny privilege on
   match" system can thus be problematic.

2.2.  Hypothetical Example

   In this example, both security principals and resources are
   identified using URIs.  Foo Corp has paid for access to
   the Stuff service.  Foo Corp allows its employees to create accounts
   on the Stuff service.  Alice gets the account
   "" and Bob gets
   "".  It turns out, however, that
   Foo Corp's URI canonicalizer includes URI fragment components in
   comparisons whereas's does not, and Foo Corp does not
   disallow the # character in the account name.  So Chuck, who is a
   malicious employee of Foo Corp, asks to create an account at with the name alice#stuff.  Foo Corp's URI logic checks
   its records for accounts it has created with stuff and sees that
   there is no account with the name alice#stuff.  Hence, in its

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   records, it associates the account alice#stuff with Chuck and will
   only issue tokens good for use with
   "" to Chuck.

   Chuck, the attacker, goes to a security token service at Foo Corp and
   asks for a security token good for
   "".  Foo Corp issues the
   token, since Chuck is the legitimate owner (in Foo Corp's view) of
   the alice#stuff account.  Chuck then submits the security token in a
   request to "".

   But uses a URI canonicalizer that, for the purposes of
   checking equality, ignores fragments.  So when looks in
   the security token to see if the requester has permission from Foo
   Corp to access the given account, it successfully matches the URI in
   the security token, "",
   with the requested resource name

   Leveraging the inconsistencies in the canonicalizers used by Foo Corp
   and, Chuck is able to successfully launch an elevation-
   of-privilege attack and access Alice's resource.

   Furthermore, consider an attacker using a similar corporation, such
   as "foocorp" (or any variation containing a non-ASCII character that
   some humans might expect to represent the same corporation).  If the
   resource holder treats them as different but the security token
   service treats them as the same, then elevation of privilege can
   occur in this scenario as well.

3.  Comparison Issues with Common Identifiers

   In this section, we walk through a number of common types of
   identifiers and discuss various issues related to comparison that may
   affect security whenever they are used to identify security
   principals or resources.  These examples illustrate common patterns
   that may arise with other types of identifiers.

3.1.  Hostnames

   Hostnames (composed of dot-separated labels) are commonly used either
   directly as identifiers, or as components in identifiers such as in
   URIs and email addresses.  Another example is in Sections 7.2 and 7.3
   of [RFC5280] (and updated in Section 3 of [RFC6818]), which specify
   use in PKIX certificates.

   In this section, we discuss a number of issues in comparing strings
   that appear to be some form of hostname.

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   It is first worth pointing out that the term "hostname" itself is
   often ambiguous, and hence it is important that any use clarify which
   definition is intended.  Some examples of definitions include:

   a.  A Fully Qualified Domain Name (FQDN),

   b.  An FQDN that is associated with address records in the DNS,

   c.  The leftmost label in an FQDN, or

   d.  The leftmost label in an FQDN that is associated with address

   The use of different definitions in different places results in
   questions such as whether "example" and "" are considered
   equal or not, and hence it is important when writing new
   specifications to be clear about which definition is meant.

   Section 3 of [RFC6055] discusses the differences between a "hostname"
   and a "DNS name", where the former is a subset of the latter by using
   a restricted set of characters (letters, digits, and hyphens).  If
   one canonicalizer uses the "DNS name" definition whereas another uses
   a "hostname" definition, a name might be valid in the former but
   invalid in the latter.  As long as invalid identifiers are denied
   privilege, this difference will not result in elevation of privilege.

   Section 3.1 of [RFC1034] discusses the difference between a
   "complete" domain name, which ends with a dot (such as
   ""), and a multi-label relative name such as
   "" that assumes the root (".") is in the suffix search
   list.  In most contexts, these are considered equal, but there may be
   issues if different entities in a security architecture have
   different interpretations of a relative domain name.

   [IAB1123] briefly discusses issues with the ambiguity around whether
   a label will be "alphabetic" -- including, among other issues, how
   "alphabetic" should be interpreted in an internationalized
   environment -- and whether a hostname can be interpreted as an IP
   address.  We explore this last issue in more detail below.

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3.1.1.  IPv4 Literals

   Section 2.1 of [RFC1123] states:

      Whenever a user inputs the identity of an Internet host, it SHOULD
      be possible to enter either (1) a host domain name or (2) an IP
      address in dotted-decimal ("#.#.#.#") form.  The host SHOULD check
      the string syntactically for a dotted-decimal number before
      looking it up in the Domain Name System.


      This last requirement is not intended to specify the complete
      syntactic form for entering a dotted-decimal host number; that is
      considered to be a user-interface issue.

   In specifying the inet_addr() API, the Portable Operating System
   Interface (POSIX) standard [IEEE-1003.1] defines "IPv4 dotted decimal
   notation" as allowing not only strings of the form "" but
   also allowing octal and hexadecimal, and addresses with less than
   four parts.  For example, "10.0.258", "0xA000102", and "012.0x102"
   all represent the same IPv4 address in standard "IPv4 dotted decimal"
   notation.  We will refer to this as the "loose" syntax of an IPv4
   address literal.

   In Section 6.1 of [RFC3493], getaddrinfo() is defined to support the
   same (loose) syntax as inet_addr():

      If the specified address family is AF_INET or AF_UNSPEC, address
      strings using Internet standard dot notation as specified in
      inet_addr() are valid.

   In contrast, Section 6.3 of the same RFC states, specifying

      If the af argument of inet_pton() is AF_INET, the src string shall
      be in the standard IPv4 dotted-decimal form:


      where "ddd" is a one to three digit decimal number between 0 and
      255.  The inet_pton() function does not accept other formats (such
      as the octal numbers, hexadecimal numbers, and fewer than four
      numbers that inet_addr() accepts).

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   As shown above, inet_pton() uses what we will refer to as the
   "strict" form of an IPv4 address literal.  Some platforms also use
   the strict form with getaddrinfo() when the AI_NUMERICHOST flag is
   passed to it.

   Both the strict and loose forms are standard forms, and hence a
   protocol specification is still ambiguous if it simply defines a
   string to be in the "standard IPv4 dotted decimal form".  And, as a
   result of these differences, names such as "10.11.12" are ambiguous
   as to whether they are an IP address or a hostname, and even
   "" can be ambiguous because of the "SHOULD" in the above
   text from RFC 1123, making it optional whether to treat it as an
   address or a DNS name.

   Protocols and data formats that can use addresses in string form for
   security purposes need to resolve these ambiguities.  For example,
   for the host component of URIs, Section 3.2.2 of [RFC3986] resolves
   the first ambiguity by only allowing the strict form and resolves the
   second ambiguity by specifying that it is considered an IPv4 address
   literal.  New protocols and data formats should similarly consider
   using the strict form rather than the loose form in order to better
   match user expectations.

   A string might be valid under the "loose" definition but invalid
   under the "strict" definition.  As long as invalid identifiers are
   denied privilege, this difference will not result in elevation of
   privilege.  Some protocols, however, use strings that can be either
   an IP address literal or a hostname.  Such strings are at best
   Definite identifiers, and often turn out to be Indefinite
   identifiers.  (See Section 4.1 for more discussion.)

3.1.2.  IPv6 Literals

   IPv6 addresses similarly have a wide variety of alternate but
   semantically identical string representations, as defined in
   Section 2.2 of [RFC4291] and Section 2 of [RFC6874].  As discussed in
   Section 3.2.5 of [RFC5952], this fact causes problems in security
   contexts if comparison (such as in PKIX certificates) is done between
   strings rather than between the binary representations of addresses.

   [RFC5952] specified a recommended canonical string format as an
   attempt to solve this problem, but it may not be ubiquitously
   supported at present.  And, when strings can contain non-ASCII
   characters, the same issues (and more, since hexadecimal and colons
   are allowed) arise as with IPv4 literals.

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   Whereas (binary) IPv6 addresses are Absolute identifiers, IPv6
   address literals are Definite identifiers, since string-to-address
   conversion for IPv6 address literals is unambiguous.

3.1.3.  Internationalization

   The IETF policy on character sets and languages [RFC2277] requires
   support for UTF-8 in protocols, and as a result many protocols now do
   support non-ASCII characters.  When a hostname is sent in a UTF-8
   field, there are a number of ways it may be encoded.  For example,
   hostname labels might be encoded directly in UTF-8, or they might
   first be Punycode-encoded [RFC3492] or even percent-encoded from

   For example, in URIs, Section 3.2.2 of [RFC3986] specifically allows
   for the use of percent-encoded UTF-8 characters in the hostname as
   well as the use of Internationalized Domain Names in Applications
   (IDNA) encoding [RFC3490] using the Punycode algorithm.

   Percent-encoding is unambiguous for hostnames, since the percent
   character cannot appear in the strict definition of a "hostname",
   though it can appear in a DNS name.

   Punycode-encoded labels (or "A-labels"), on the other hand, can be
   ambiguous if hosts are actually allowed to be named with a name
   starting with "xn--", and false positives can result.  While this may
   be extremely unlikely for normal scenarios, it nevertheless provides
   a possible vector for an attacker.

   A hostname comparator thus needs to decide whether a Punycode-encoded
   label should or should not be considered a valid hostname label, and
   if so, then whether it should match a label encoded in some other
   form such as a percent-encoded Unicode label (U-label).

   For example, Section 3 of "Transport Layer Security (TLS) Extensions:
   Extension Definitions" [RFC6066] states:

      "HostName" contains the fully qualified DNS hostname of the
      server, as understood by the client.  The hostname is represented
      as a byte string using ASCII encoding without a trailing dot.
      This allows the support of internationalized domain names through
      the use of A-labels defined in [RFC5890].  DNS hostnames are case-
      insensitive.  The algorithm to compare hostnames is described in
      [RFC5890], Section

   For some additional discussion of security issues that arise with
   internationalization, see Section 4.2 and [TR36].

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3.1.4.  Resolution for Comparison

   Some systems (specifically Java URLs [JAVAURL]) use the rule that if
   two hostnames resolve to the same IP address(es) then the hostnames
   are considered equal.  That is, the canonicalization algorithm
   involves name resolution with an IP address being the canonical form.

   For example, if resolution was done via DNS, and DNS contained:

               IN A
               IN A

   then the algorithm might treat all three names as equal, even though
   the third name might refer to a different entity.

   With the introduction of dynamic IP addresses; private IP addresses;
   multiple IP addresses per name; multiple address families (e.g., IPv4
   vs. IPv6); devices that roam to new locations; commonly deployed DNS
   tricks that result in the answer depending on factors such as the
   requester's location and the load on the server whose address is
   returned; etc., this method of comparison cannot be relied upon.
   There is no guarantee that two names for the same host will resolve
   the name to the same IP addresses; nor that the addresses resolved
   refer to the same entity, such as when the names resolve to private
   IP addresses; nor even that the system has connectivity (and the
   willingness to wait for the delay) to resolve names at the time the
   answer is needed.  The lifetime of the identifier, and of any cached
   state from a previous resolution, also affects security (see
   Section 4.4).

   In addition, a comparison mechanism that relies on the ability to
   resolve identifiers such as hostnames to other identifiers such as IP
   addresses leaks information about security decisions to outsiders if
   these queries are publicly observable.  (See [PRIVACY-CONS] for a
   deeper discussion of information disclosure.)

   Finally, it is worth noting that resolving two identifiers to
   determine if they refer to the same entity can be thought of as a use
   of such identifiers, as opposed to actually comparing the identifiers
   themselves, which is the focus of this document.

3.2.  Port Numbers and Service Names

   Port numbers and service names are discussed in depth in [RFC6335].
   Historically, there were port numbers, service names used in SRV
   records, and mnemonic identifiers for assigned port numbers (known as
   port "keywords" at [IANA-PORT]).  The latter two are now unified, and

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   various protocols use one or more of these types in strings.  For
   example, the common syntax used by many URI schemes allows port
   numbers but not service names.  Some implementations of the
   getaddrinfo() API support strings that can be either port numbers or
   port keywords (but not service names).

   For protocols that use service names that must be resolved, the
   issues are the same as those for resolution of addresses in
   Section 3.1.4.  In addition, Section 5.1 of [RFC6335] clarifies that
   service names/port keywords must contain at least one letter.  This
   prevents confusion with port numbers in strings where both are

3.3.  URIs

   This section looks at issues related to using URIs for security
   purposes.  For example, Section 7.4 of [RFC5280] specifies comparison
   of URIs in certificates.  Examples of URIs in security-token-based
   access control systems include WS-*, SAML 2.0 [OASIS-SAMLv2-CORE],
   and OAuth Web Resource Authorization Profiles (WRAP) [OAuth-WRAP].
   In such systems, a variety of participants in the security
   infrastructure are identified by URIs.  For example, requesters of
   security tokens are sometimes identified with URIs.  The issuers of
   security tokens and the relying parties who are intended to consume
   security tokens are frequently identified by URIs.  Claims in
   security tokens often have their types defined using URIs, and the
   values of the claims can also be URIs.

   URIs are defined with multiple components, each of which has its own
   rules.  We cover each in turn below.  However, it is also important
   to note that there exist multiple comparison algorithms.  Section 6.2
   of [RFC3986] states:

      A variety of methods are used in practice to test URI equivalence.
      These methods fall into a range, distinguished by the amount of
      processing required and the degree to which the probability of
      false negatives is reduced.  As noted above, false negatives
      cannot be eliminated.  In practice, their probability can be
      reduced, but this reduction requires more processing and is not
      cost-effective for all applications.

      If this range of comparison practices is considered as a ladder,
      the following discussion will climb the ladder, starting with
      practices that are cheap but have a relatively higher chance of
      producing false negatives, and proceeding to those that have
      higher computational cost and lower risk of false negatives.

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   The ladder approach has both pros and cons.  On the pro side, it
   allows some uses to optimize for security, and other uses to optimize
   for cost, thus allowing URIs to be applicable to a wide range of
   uses.  A disadvantage is that when different approaches are taken by
   different components in the same system using the same identifiers,
   the inconsistencies can result in security issues.

3.3.1.  Scheme Component

   [RFC3986] defines URI schemes as being case-insensitive US-ASCII and
   in Section specifies that scheme names should be normalized
   to lowercase characters.

   New schemes can be defined over time.  In general, however, two URIs
   with an unrecognized scheme cannot be safely compared.  This is
   because the canonicalization and comparison rules for the other
   components may vary by scheme.  For example, a new URI scheme might
   have a default port of X, and without that knowledge, a comparison
   algorithm cannot know whether "" and ""
   should be considered to match in the authority component.  Hence, for
   security purposes, it is safest for unrecognized schemes to be
   treated as invalid identifiers.  However, if the URIs are only used
   with a "grant access on match" paradigm, then unrecognized schemes
   can be supported by doing a generic case-sensitive comparison, at the
   expense of some false negatives.

3.3.2.  Authority Component

   The authority component is scheme-specific, but many schemes follow a
   common syntax that allows for userinfo, host, and port.  Host

   Section 3.1 discusses issues with hostnames in general.  In addition,
   Section 3.2.2 of [RFC3986] allows future changes using the IPvFuture
   production.  As with IPv4 and IPv6 literals, IPvFuture formats may
   have issues with multiple semantically identical string
   representations and may also be semantically identical to an IPv4 or
   IPv6 address.  As such, false negatives may be common if IPvFuture is
   used.  Port

   See discussion in Section 3.2.

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   [RFC3986] defines the userinfo production that allows arbitrary data
   about the user of the URI to be placed before '@' signs in URIs.  For
   example, "" has the value "alice:bob"
   as its userinfo.  When comparing URIs in a security context, one must
   decide whether to treat the userinfo as being significant or not.
   Some URI comparison services, for example, treat
   "" and "" as being equal.

   When the userinfo is treated as being significant, it has additional
   considerations (e.g., whether or not it is case sensitive), which we
   cover in Section 3.4.

3.3.3.  Path Component

   [RFC3986] supports the use of path segment values such as "./" or
   "../" for relative URIs.  As discussed in Section of
   [RFC3986], they are intended only for use within a reference relative
   to some other base URI, but Section 5.2.4 of [RFC3986] nevertheless
   defines an algorithm to remove them as part of URI normalization.

   Unless a scheme states otherwise, the path component is defined to be
   case sensitive.  However, if the resource is stored and accessed
   using a filesystem using case-insensitive paths, there will be many
   paths that refer to the same resource.  As such, false negatives can
   be common in this case.

3.3.4.  Query Component

   There is the question as to whether "",
   "", and "" are each
   considered equal or different.

   Similarly, it is unspecified whether the order of values matters.
   For example, should "" be
   considered equal to ""?  And
   if a domain name is permitted to appear in a query component (e.g.,
   in a reference to another URI), the same issues in Section 3.1 apply.

3.3.5.  Fragment Component

   Some URI formats include fragment identifiers.  These are typically
   handles to locations within a resource and are used for local
   reference.  A classic example is the use of fragments in HTTP URIs
   where a URI of the form "" means
   retrieve the resource "" and, once it has
   arrived locally, find the HTML anchor named "ick" and display that.

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   So, for example, when a user clicks on the link
   "", a browser will check its cache by
   doing a URI comparison for "" and, if the
   resource is present in the cache, a match is declared.

   Hence, comparisons for security purposes typically ignore the
   fragment component and treat all fragments as equal to the full
   resource.  However, if one were actually trying to compare the piece
   of a resource that was identified by the fragment identifier,
   ignoring it would result in potential false positives.

3.3.6.  Resolution for Comparison

   It may be tempting to define a URI comparison algorithm based on
   whether URIs resolve to the same content, along the lines of
   resolving hostnames as described in Section 3.1.4.  However, such an
   algorithm would result in similar problems, including content that
   dynamically changes over time or that is based on factors such as the
   requester's location, potential lack of external connectivity at the
   time or place that comparison is done, introduction of potentially
   undesirable delay, etc.

   In addition, as noted in Section 3.1.4, resolution leaks information
   about security decisions to outsiders if the queries are publicly

3.4.  Email Address-Like Identifiers

   Section 3.4.1 of [RFC5322] defines the syntax of an email address-
   like identifier, and Section 3.2 of [RFC6532] updates it to support
   internationalization.  Section 7.5 of [RFC5280] further discusses the
   use of internationalized email addresses in certificates.

   Regarding the security impact of internationalized email headers,
   [RFC6532] points to Section 14 of [RFC6530], which contains a
   discussion of many issues resulting from internationalization.

   Email address-like identifiers have a local part and a domain part.
   The issues with the domain part are essentially the same as with
   hostnames, as covered earlier in Section 3.1.

   The local part is left for each domain to define.  People quite
   commonly use email addresses as usernames with web sites such as
   banks or shopping sites, but the site doesn't know whether is the same person as  Thus, email
   address-like identifiers are typically Indefinite identifiers.

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   To avoid false positives, some security mechanisms (such as those
   described in [RFC5280]) compare the local part using an exact match.
   Hence, like URIs, email address-like identifiers are designed for use
   in grant-on-match security schemes, not in deny-on-match schemes.

   Furthermore, when such identifiers are actually used as email
   addresses, Section 2.4 of [RFC5321] states that the local part of a
   mailbox must be treated as case sensitive, but if a mailbox is stored
   and accessed using a filesystem using case-insensitive paths, there
   may be many paths that refer to the same mailbox.  As such, false
   negatives can be common in this case.

4.  General Issues

4.1.  Conflation

   There are a number of examples (some in the preceding sections) of
   strings that conflate two types of identifiers, using some heuristic
   to try to determine which type of identifier is given.  Similarly,
   two ways of encoding the same type of identifier might be conflated
   within the same string.

   Some examples include:

   1.  A string that might be an IPv4 address literal or an IPv6 address

   2.  A string that might be an IP address literal or a hostname

   3.  A string that might be a port number or a service name

   4.  A DNS label that might be literal or be Punycode-encoded

   Strings that allow such conflation can only be considered Definite if
   there exists a well-defined rule to determine which identifier type
   is meant.  One way to do so is to ensure that the valid syntax for
   the two is disjoint (e.g., distinguishing IPv4 vs. IPv6 address
   literals by the use of colons in the latter).  A second way to do so
   is to define a precedence rule that results in some identifiers being
   inaccessible via a conflated string (e.g., a host literally named
   "xn--de-jg4avhby1noc0d" may be inaccessible due to the "xn--" prefix
   denoting the use of Punycode encoding).  In some cases, such
   inaccessible space may be reserved so that the actual set of
   identifiers in use is unambiguous.  For example, Section of
   [RFC4291] defines a range of the IPv6 address space for representing
   IPv4 addresses.

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4.2.  Internationalization

   In addition to the issues with hostnames discussed in Section 3.1.3,
   there are a number of internationalization issues that apply to many
   types of Definite and Indefinite identifiers.

   First, there is no DNS mechanism for identifying whether
   non-identical strings would be seen by a human as being equivalent.
   There are problematic examples even with US-ASCII (Basic Latin)
   strings, including regional spelling variations such as "color" and
   "colour", and with many non-English cases, including partially
   numeric strings in Arabic script contexts, Chinese strings in
   Simplified and Traditional forms, and so on.  Attempts to produce
   such alternate forms algorithmically could produce false positives
   and hence have an adverse effect on security.

   Second, some strings are visually confusable with others, and hence
   if a security decision is made by a user based on visual inspection,
   many opportunities for false positives exist.  As such, using visual
   inspection for security is unreliable.  In addition to the security
   issues, visual confusability also adversely affects the usability of
   identifiers distributed via visual media.  Similar issues can arise
   with audible confusability when using audio (e.g., for radio
   distribution, accessibility to the blind, etc.) in place of a visual
   medium.  Furthermore, when strings conflate two types of identifiers
   as discussed in Section 4.1, allowing non-ASCII characters can cause
   one type of identifier to appear to a human as another type of
   identifier.  For example, characters that may look like digits and
   dots may appear to be an IPv4 literal to a human (especially to one
   who might expect digits to appear in his or her native script).
   Hence, conflation often increases the chance of confusability.

   Determining whether a string is a valid identifier should typically
   be done after, or as part of, canonicalization.  Otherwise, an
   attacker might use the canonicalization algorithm to inject (e.g.,
   via percent encoding, Normalization Form KC (NFKC), or non-shortest-
   form UTF-8) delimiters such as '@' in an email address-like
   identifier, or a '.' in a hostname.

   Any case-insensitive comparisons need to define how comparison is
   done, since such comparisons may vary by the locale of the endpoint.
   As such, using case-insensitive comparisons in general often results
   in identifiers being either Indefinite or, if the legal character set
   is restricted (e.g., to US-ASCII), Definite.

   See also [WEBER] for a more visual discussion of many of these

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   Finally, the set of permitted characters and the canonical form of
   the characters (and hence the canonicalization algorithm) sometimes
   vary by protocol today, even when the intent is to use the same
   identifier, such as when one protocol passes identifiers to the
   other.  See [RFC6885] for further discussion.

4.3.  Scope

   Another issue arises when an identifier (e.g., "localhost",
   "", etc.) is not globally unique.  Section 1.1 of
   [RFC3986] states:

      URIs have a global scope and are interpreted consistently
      regardless of context, though the result of that interpretation
      may be in relation to the end-user's context.  For example,
      "http://localhost/" has the same interpretation for every user of
      that reference, even though the network interface corresponding to
      "localhost" may be different for each end-user: interpretation is
      independent of access.

   Whenever an identifier that is not globally unique is passed to
   another entity outside of the scope of uniqueness, it will refer to a
   different resource and can result in a false positive.  This problem
   is often addressed by using the identifier together with some other
   unique identifier of the context.  For example, "alice" may uniquely
   identify a user within a system but must be used with ""
   (as in "") to uniquely identify the context outside
   of that system.

   It is also worth noting that IPv6 addresses that are not globally
   scoped can be written with, or otherwise associated with, a "zone ID"
   to identify the context (see [RFC4007] for more information).
   However, zone IDs are only unique within a host, so they typically
   narrow, rather than expand, the scope of uniqueness of the resulting

4.4.  Temporality

   Often, identifiers are not unique across all time but have some
   lifetime associated with them after which they may be reassigned to
   another entity.  For example, might be assigned to an
   employee of the Example company, but if he leaves and another Bob is
   later hired, the same identifier might be reused.  As another
   example, IP address might be assigned to one subscriber
   and then later reassigned to another subscriber.  Security issues can
   arise if updates are not made in all entities that store the
   identifier (e.g., in an access control list as discussed in
   Section 2, or in a resolution cache as discussed in Section 3.1.4).

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   This issue is similar to the issue of scope discussed in Section 4.3,
   except that the scope of uniqueness is temporal rather than

5.  Security Considerations

   This entire document is about security considerations.

   To minimize issues related to elevation of privilege, any system that
   requires the ability to use both deny and allow operations within the
   same identifier space should avoid the use of Indefinite identifiers
   in security comparisons.

   To minimize future security risks, any new identifiers being designed
   should specify an Absolute or Definite comparison algorithm, and if
   extensibility is allowed (e.g., as new schemes in URIs allow), then
   the comparison algorithm should remain invariant so that unrecognized
   extensions can be compared.  That is, security risks can be reduced
   by specifying the comparison algorithm, making sure to resolve any
   ambiguities pointed out in this document (e.g., "standard dotted

   Some issues (such as unrecognized extensions) can be mitigated by
   treating such identifiers as invalid.  Validity checking of
   identifiers is further discussed in [RFC3696].

   Perhaps the hardest issues arise when multiple protocols are used
   together, such as in Figure 2, where the two protocols are defined or
   implemented using different comparison algorithms.  When constructing
   an architecture that uses multiple such protocols, designers should
   pay attention to any differences in comparison algorithms among the
   protocols in order to fully understand the security risks.  How to
   deal with such security risks in current systems is an area for
   future work.

6.  Acknowledgements

   Yaron Goland contributed to the discussion on URIs.  Patrik Faltstrom
   contributed to the background on identifiers.  John Klensin
   contributed text in a number of different sections.  Additional
   helpful feedback and suggestions came from Bernard Aboba, Fred Baker,
   Leslie Daigle, Mark Davis, Jeff Hodges, Bjoern Hoehrmann, Russ
   Housley, Christian Huitema, Magnus Nystrom, Tom Petch, and Chris

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7.  IAB Members at the Time of Approval

   Bernard Aboba
   Jari Arkko
   Marc Blanchet
   Ross Callon
   Alissa Cooper
   Spencer Dawkins
   Joel Halpern
   Russ Housley
   David Kessens
   Danny McPherson
   Jon Peterson
   Dave Thaler
   Hannes Tschofenig

8.  Informative References

   [IAB1123]  Internet Architecture Board, "IAB Statement: 'The
              interpretation of rules in the ICANN gTLD Applicant
              Guidebook'", February 2012, <

              IANA, "Service Name and Transport Protocol Port Number
              Registry", March 2013,

              IEEE and The Open Group, "The Open Group Base
              Specifications, Issue 6, IEEE Std 1003.1, 2004 Edition",
              IEEE Std 1003.1, 2004.

   [JAVAURL]  Oracle, "Class URL", Java(TM) Platform Standard Ed. 7,
              2013, <

              Cantor, S., Ed., Kemp, J., Ed., Philpott, R., Ed., and E.
              Maler, Ed., "Assertions and Protocols for the OASIS
              Security Assertion Markup Language (SAML) V2.0", OASIS
              Standard saml-core-2.0-os, March 2005,

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              Hardt, D., Ed., Tom, A., Eaton, B., and Y. Goland, "OAuth
              Web Resource Authorization Profiles", Work in Progress,
              January 2010.

              Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", Work in Progress,
              April 2013.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [RFC1123]  Braden, R., "Requirements for Internet Hosts - Application
              and Support", STD 3, RFC 1123, October 1989.

   [RFC2277]  Alvestrand, H.T., "IETF Policy on Character Sets and
              Languages", BCP 18, RFC 2277, January 1998.

   [RFC3490]  Faltstrom, P., Hoffman, P., and A. Costello,
              "Internationalizing Domain Names in Applications (IDNA)",
              RFC 3490, March 2003.

   [RFC3492]  Costello, A., "Punycode: A Bootstring encoding of Unicode
              for Internationalized Domain Names in Applications
              (IDNA)", RFC 3492, March 2003.

   [RFC3493]  Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
              Stevens, "Basic Socket Interface Extensions for IPv6",
              RFC 3493, February 2003.

   [RFC3696]  Klensin, J., "Application Techniques for Checking and
              Transformation of Names", RFC 3696, February 2004.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC4007]  Deering, S., Haberman, B., Jinmei, T., Nordmark, E., and
              B. Zill, "IPv6 Scoped Address Architecture", RFC 4007,
              March 2005.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

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   [RFC4790]  Newman, C., Duerst, M., and A. Gulbrandsen, "Internet
              Application Protocol Collation Registry", RFC 4790,
              March 2007.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              RFC 4949, August 2007.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, May 2008.

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              October 2008.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              October 2008.

   [RFC5952]  Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
              Address Text Representation", RFC 5952, August 2010.

   [RFC6055]  Thaler, D., Klensin, J., and S. Cheshire, "IAB Thoughts on
              Encodings for Internationalized Domain Names", RFC 6055,
              February 2011.

   [RFC6066]  Eastlake, D., "Transport Layer Security (TLS) Extensions:
              Extension Definitions", RFC 6066, January 2011.

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, March 2011.

   [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
              Cheshire, "Internet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", BCP 165,
              RFC 6335, August 2011.

   [RFC6530]  Klensin, J. and Y. Ko, "Overview and Framework for
              Internationalized Email", RFC 6530, February 2012.

   [RFC6532]  Yang, A., Steele, S., and N. Freed, "Internationalized
              Email Headers", RFC 6532, February 2012.

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   [RFC6818]  Yee, P., "Updates to the Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 6818, January 2013.

   [RFC6874]  Carpenter, B., Cheshire, S., and R. Hinden, "Representing
              IPv6 Zone Identifiers in Address Literals and Uniform
              Resource Identifiers", RFC 6874, February 2013.

   [RFC6885]  Blanchet, M. and A. Sullivan, "Stringprep Revision and
              Problem Statement for the Preparation and Comparison of
              Internationalized Strings (PRECIS)", RFC 6885, March 2013.

   [TR36]     Unicode Consortium, "Unicode Security Considerations",
              Unicode Technical Report #36, Revision 11, July 2012,

   [USASCII]  American National Standards Institute, "Coded Character
              Sets -- 7-bit American Standard Code for Information
              Interchange (7-bit ASCII)", ANSI X3.4, 1986.

   [WEBER]    Weber, C., "Attacking Software Globalization", March 2010,

Author's Address

   Dave Thaler (editor)
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052

   Phone: +1 425 703 8835

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