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Versions: 00 01 02 03 04 05 06 07 08 09 rfc6943                         
Network Working Group                                     D. Thaler, Ed.
Internet-Draft                                                 Microsoft
Intended status: Informational                           August 24, 2012
Expires: February 25, 2013

         Issues in Identifier Comparison for Security Purposes


   Identifiers such as hostnames, URIs, and email addresses are often
   used in security contexts to identify security principals and
   resources.  In such contexts, an identifier supplied via some
   protocol is often compared against some policy to make security
   decisions such as whether the 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.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on February 25, 2013.

Copyright Notice

   Copyright (c) 2012 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
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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect

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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Canonicalization . . . . . . . . . . . . . . . . . . . . .  4
   2.  Security Uses  . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.1.  Types of Identifiers . . . . . . . . . . . . . . . . . . .  6
     2.2.  False Positives and Negatives  . . . . . . . . . . . . . .  7
     2.3.  Hypothetical Example . . . . . . . . . . . . . . . . . . .  8
   3.  Common Identifiers . . . . . . . . . . . . . . . . . . . . . .  9
     3.1.  Hostnames  . . . . . . . . . . . . . . . . . . . . . . . .  9
       3.1.1.  IPv4 Literals  . . . . . . . . . . . . . . . . . . . . 10
       3.1.2.  IPv6 Literals  . . . . . . . . . . . . . . . . . . . . 11
       3.1.3.  Internationalization . . . . . . . . . . . . . . . . . 12
       3.1.4.  Resolution for comparison  . . . . . . . . . . . . . . 12
     3.2.  Ports and Service Names  . . . . . . . . . . . . . . . . . 13
     3.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
       3.3.1.  Scheme component . . . . . . . . . . . . . . . . . . . 15
       3.3.2.  Authority component  . . . . . . . . . . . . . . . . . 15
       3.3.3.  Path component . . . . . . . . . . . . . . . . . . . . 16
       3.3.4.  Query component  . . . . . . . . . . . . . . . . . . . 16
       3.3.5.  Fragment component . . . . . . . . . . . . . . . . . . 16
       3.3.6.  Resolution for comparison  . . . . . . . . . . . . . . 17
     3.4.  Email Address-like Identifiers . . . . . . . . . . . . . . 17
   4.  General Conflation Issues  . . . . . . . . . . . . . . . . . . 18
   5.  General Internationalization Issues  . . . . . . . . . . . . . 18
   6.  General Scope Issues . . . . . . . . . . . . . . . . . . . . . 19
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 21
   10. Informative References . . . . . . . . . . . . . . . . . . . . 21
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 23

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1.  Introduction

   In computing and the Internet, various types of "identifiers" are
   used to identify humans, devices, content, etc.  Before discussing
   security issues, we first give some background on some typical
   processes involving identifiers.

   As depicted in Figure 1, there are multiple processes relevant to our
   1.  An identifier must first be generated.  If the identifier is
       intended to be unique, the generation process includes some
       mechanism, such as allocation by a central authority, to help
       ensure uniqueness.  However the notion of "unique" involves
       determining whether a putative identifier matches any other
       already-allocated identifier.  As we will see, for many types of
       identifiers, this is not simply an exact binary match.

       As a result of 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 device.
   2.  The identifier must be 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 must be used by some party.  Generally the user
       supplies the identifier which is (directly or indirectly) sent to
       the repository of identifiers.  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.

       The repository of identifiers must then attempt to match the
       user-supplied identifier with an identifier in its repository.

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

                       Typical Identifier Processes

                                 Figure 1

   One key aspect is that the identifier values passed in generation,
   distribution, and use, may all be different forms.  For example,
   generation might be exchanged in printed form, distribution done via
   voice, and use done electronically.  As such, the match process can
   be complicated.

   Furthermore, in many uses, the relationship between holder,
   repositories, and users may be more involved.  For example, when a
   hierarchy of web caches exist, 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.

1.1.  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.  Security Uses

   Identifiers such as hostnames, URIs, and email addresses are used in
   security contexts to identify principals 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 supplied via some protocol.  For example:
   o  Authentication: a protocol might match a security principal
      identifier to look up expected keying material, and then match
      keying material.
   o  Authorization: a protocol might match a resource name to look up
      an access control list (ACL), and then look up the security
      principal identifier (or a surrogate for it) in that ACL.
   o  Accounting: a system might create an accounting record for a
      security principal identifier or resource name, and then might
      later need to match a supplied identifier to allow (for example)
      law enforcement to follow up based on the records, or add new
      filtering rules based on the records in order to stop an attack.

   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 |
        +----------+                                       +--------+

                         Simple Security Exchange

                                 Figure 2

   In many cases the situation is more complex.  With certificates, 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

   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.  Types of Identifiers

   In this document we will refer to the following types of identifiers:

   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 well-defined comparison
      algorithm on which all parties agree.  For example, URI scheme
      names are required to be ASCII and are defined to match in a case-
      insensitive way; the comparison is thus definite since all parties
      agree on how to do a case-insensitive match among ASCII strings.
   o  Indefinite: identifiers that have no single comparison algorithm
      on which all parties agree.  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

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      (e.g., ASCII users might all agree on a common comparison
      algorithm whereas users of other Latin scripts, such as Turkish,
      may not), but identifiers often tend to leak out of such limited

2.2.  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.

   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

                    Effect of False Positives/Negatives

                                 Figure 3

   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: "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

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   match" system can thus be problematic.

2.3.  Hypothetical Example

   In this example, both security principals and resources are
   identified using URIs.  Foo Corp has paid example.com for access to
   the Stuff service.  Foo Corp allows its employees to create accounts
   on the Stuff service.  Alice gets the account
   "http://example.com/Stuff/FooCorp/alice" and Bob gets
   "http://example.com/Stuff/FooCorp/bob".  It turns out, however, that
   Foo Corp's URI canonicalizer includes URI fragment components in
   comparisons whereas example.com'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
   example.com 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
   records, it associates the account alice#stuff with Chuck and will
   only issue tokens good for use with
   "http://example.com/Stuff/FooCorp/alice#stuff" to Chuck.

   Chuck, the attacker, goes to a security token service at Foo Corp and
   asks for a security token good for
   "http://example.com/Stuff/FooCorp/alice#stuff".  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 "http://example.com/Stuff/FooCorp/alice".

   But example.com uses a URI canonicalizer that, for the purposes of
   checking equality, ignores fragments.  So when example.com 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, "http://example.com/Stuff/FooCorp/alice#stuff",
   with the requested resource name

   Leveraging the inconsistencies in the canonicalizers used by Foo Corp
   and example.com, 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 again elevation of privilege
   can occur.

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3.  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 [RFC5280], sections
   7.2 and 7.3 (and updated in section 3 of
   [I-D.ietf-pkix-rfc5280-clarifications]), which specify use in

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

   It is first worth pointing out that the term 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.  A FQDN that is associated with address records,
   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 "example.com" are considered
   equal or not.

   Section 3 of [RFC6055] discusses the differences between a "hostname"
   vs. a "DNS name", where the former is a subset of the latter by using
   a restricted set of characters.  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.

   [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

   [RFC1123] section 2.1 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 POSIX standard [IEEE-1003.1]
   defines "IPv4 dotted decimal notation" as allowing not only strings
   of the form "", but also allows octal and hexadecimal, and
   addresses with less than four parts.  For example, "10.0.258",
   "0xA000001", 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: ddd.ddd.ddd.ddd 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).

   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

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   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 RFC 1123
   above making it optional whether to treat it as an address or a 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 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.

   If one canonicalizer uses the "loose" definition whereas another uses
   the "strict" definition, a string 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.
   Security problems can arise, however, in protocols using strings that
   conflate IP address literals and hostnames; such strings are at best
   Definite identifiers, and often turn out to be Indefinite
   identifiers.  (See Section 4 for more discussion.)

   Furthermore, when strings can contain non-ASCII characters, they can
   contain other characters that may look like dots or digits to a human
   viewing and/or entering the identifier, especially to one who might
   expect digits to appear in his or her native script.

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 [I-D.ietf-6man-uri-zoneid].  As
   discussed in section 3.2.5 of [RFC5952], this fact causes problems in
   security contexts if comparison (such as in X.509 certificates), is
   done between strings rather than between the binary representations
   of addresses.

   [RFC5952] recently 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.

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

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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 might first be
   Punycode-encoded [RFC3492] or even percent-encoded from UTF-8.

   For example, in URIs, [RFC3986] section 3.2.2 specifically allows for
   the use of percent-encoded UTF-8 characters in the hostname, as well
   as the use of 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"
   [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 [TR36].

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.

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

   example.com.  IN A
   example.net.  CNAME example.com.
   example.org.  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.

   In addition, a comparison mechanism that relies on the ability to
   resolve identifiers such as hostnames to other identifies such as IP
   addresses leaks information about security decisions to outsiders if
   these queries are publicly observable.

   Finally, it is worth noting that determine whether two (unequal)
   identifiers refer to the same entity is a function of the use of such
   identifiers, not about actually comparing the identifiers, which is
   the focus of this document.

3.2.  Ports 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
   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

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   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, [RFC5280], section 7.4, specifies comparison
   of URIs in certificates.  Examples of URIs in security token-based
   access control systems include WS-*, SAML-P and 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.

   Also, when a URI is embedded in plain text (e.g., an email message),
   there is an additional concern because there is no termination
   criterion for a URI.  For example, consider
   Some applications that detect URIs will stop before the first '.' in
   the path, while others go to last '.', and yet others may stop at the
   ';'.  As another point of comparison, Section 2.37 of [EE] (a
   standard for history citations) specifies the use of a space after a
   URI and before the punctuation.

   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.  [RFC3986]
   section 6.2 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.

   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

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   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 ASCII and in
   section specifies that scheme names should be normalized to
   lower-case characters.

   New schemes can be defined over time.  In general two URIs with an
   unrecognized scheme cannot be safely compared, however.  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 "example.com" and "example.com:X"
   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 discussed issues with hostnames in general.  In addition,
   [RFC3986] section 3.2.2 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.  Userinfo

   [RFC3986] defines the userinfo production that allows arbitrary data
   about the user of the URI to be placed before '@' signs in URIs.  For
   example: "http://alice:bob:chuck@example.com/bar" has the value
   "alice:bob:chuck" as its userinfo.  When comparing URIs in a security

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   context, one must decide whether to treat the userinfo as being
   significant or not.  Some URI comparison services for example treat
   "http://alice:ick@example.com" and "http://example.com" as being

   When the userinfo is treated as being significant, it has additional
   considerations (e.g., whether it is case-sensitive or not) 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.  Strictly speaking, including such path
   segment values in a fully qualified URI is syntactically illegal but
   [RFC3986] section 4.1 nevertheless defines an algorithm to remove

   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 "http://example.com/foo",
   "http://example.com/foo?", and "http://example.com/foo?bar" are each
   considered equal or different.

   Similarly, it is unspecified whether the order of values matters.
   For example, should "http://example.com/blah?ick=bick&foo=bar" be
   considered equal to "http://example.com/blah?foo=bar&ick=bick"?  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 "http://example.com/blah.html#ick" means
   retrieve the resource "http://example.com/blah.html" and, once it has
   arrived locally, find the HTML anchor named ick and display that.

   So, for example, when a user clicks on the link
   "http://example.com/blah.html#baz" a browser will check its cache by
   doing a URI comparison for "http://example.com/blah.html" and, if the
   resource is present in the cache, a match is declared.

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   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.  For example, there is at
   least one well known site today (Twitter) that requires the fragment
   component in order to uniquely identify a user profile.

3.3.6.  Resolution for comparison

   As with Section 3.1.4 for hostnames, it may be tempting to define a
   URI comparison algorithm based on whether they resolve to the same
   content.  Similar problems exist, however, including content that
   dynamically changes over time or based on factors such as the
   requester's location, potential lack of external connectivity at the
   time/place comparison is done, potentially undesirable delay
   introduced, 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.  [RFC5280], section 7.5, further discusses the
   use of internationalized email addresses in certificates.

   [RFC6532] use in certificates points to [RFC6530], where Section 13
   of that document contains a discussion of many issues resulting from

   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, covered earlier.

   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
   foo@example.com is the same person as FOO@example.com.  Thus email-
   like identifiers are typically Indefinite identifiers.

   To avoid false positives, some security mechanisms (such as
   [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.

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4.  General Conflation Issues

   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., for IPv4 vs IPv6 address literals).  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 are unambiguous.  For example, Section
   of [RFC4291] defines a range of the IPv6 address space for
   representing IPv4 addresses.

5.  General Internationalization Issues

   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 ASCII (Basic Latin) strings
   including regional spelling variations such as "color" and "colour"
   and 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 affect 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

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   inspection for security is unreliable.  In addition to the security
   issues, visual confusability also adversely affects the usability of
   identifiers distributed via visual mediums.  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

   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, NFKC, or non-shortest-form UTF-8) delimiters
   such as '@' in an email address-like identifier, or a '.' in a

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

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

   Finally, the set of permitted characters and the canonical form of
   the characters (and hence the canonicalization algorithm) sometimes
   varies by protocol today, even when the intent is to use the same
   identifier, such as when one protocol passes identifiers to the
   other.  See [I-D.ietf-precis-problem-statement] for further

6.  General Scope Issues

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

      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 a non-globally-unique identifier 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

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   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 "example.com" (as in
   "alice@example.com") to uniquely identify the context outside of that

   It is also worth noting that non-globally-scoped IPv6 addresses 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

7.  Security Considerations

   This entire document is about security considerations.

   To minimize elevation of privilege issues, 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 the figure in Section 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.  An area for future work is how to deal with such
   security risks in current systems.

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8.  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, Leslie
   Daigle, Mark Davis, Russ Housley, Magnus Nystrom, and Chris Weber.

9.  IANA Considerations

   This document requires no actions by the IANA.

10.  Informative References

   [EE]       Mills, E., "Evidence Explained: Citing History Sources
              from Artifacts to Cyberspace", 2007.

              Carpenter, B. and R. Hinden, "Representing IPv6 Zone
              Identifiers in Address Literals and Uniform Resource
              Identifiers", draft-ietf-6man-uri-zoneid-02 (work in
              progress), July 2012.

              Yee, P., "Updates to the Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", draft-ietf-pkix-rfc5280-clarifications-08
              (work in progress), August 2012.

              Blanchet, M. and A. Sullivan, "Stringprep Revision and
              PRECIS Problem Statement",
              draft-ietf-precis-problem-statement-07 (work in progress),
              August 2012.

   [IAB1123]  IAB, "The interpretation of rules in the ICANN gTLD
              Applicant Guidebook", February 2012, <http://www.iab.org/

              IANA, "PORT NUMBERS", June 2011,


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              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",
              2011, <http://docs.oracle.com/javase/7/docs/api/java/net/

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

   [RFC2277]  Alvestrand, H., "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.

   [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.

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

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   [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.

   [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.

   [TR36]     Unicode Consortium, "Unicode Security Considerations",
              Unicode Technical Report 36, August 2004.

   [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
   Email: dthaler@microsoft.com

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