Network Working Group P. Hoffman
Internet-Draft VPN Consortium
Intended status: Informational A. Melnikov
Expires: August 25, 2008 Isode Ltd.
February 22, 2008
Security Requirements for HTTP
draft-ietf-httpbis-security-properties-01.txt
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Abstract
Recent IESG practice dictates that IETF protocols must specify
mandatory-to-implement security mechanisms, so that all conformant
implementations share a common baseline. This document examines all
widely deployed HTTP security technologies, and analyzes the trade-
offs of each.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Existing HTTP Security Mechanisms . . . . . . . . . . . . . . 3
2.1. Forms And Cookies . . . . . . . . . . . . . . . . . . . . 3
2.2. HTTP Access Authentication . . . . . . . . . . . . . . . . 4
2.2.1. Basic Authentication . . . . . . . . . . . . . . . . . 5
2.2.2. Digest Authentication . . . . . . . . . . . . . . . . 5
2.2.3. Other Access Authentication Schemes . . . . . . . . . 6
2.3. Centrally-Issued Tickets . . . . . . . . . . . . . . . . . 6
2.4. Web Services . . . . . . . . . . . . . . . . . . . . . . . 6
2.5. Transport Layer Security . . . . . . . . . . . . . . . . . 7
3. Revisions To HTTP . . . . . . . . . . . . . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 7
5. Normative References . . . . . . . . . . . . . . . . . . . . . 7
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 8
Appendix B. Document History . . . . . . . . . . . . . . . . . . 9
B.1. Changes between draft-sayre-http-security-variance-00
and draft-ietf-httpbis-security-properties-00 . . . . . . 9
B.2. Changes between -00 and -01 . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . . . . 11
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1. Introduction
Recent IESG practice dictates that IETF protocols are required to
specify mandatory to implement security mechanisms. "The IETF
Standards Process" [RFC2026] does not require that protocols specify
mandatory security mechanisms. "Strong Security Requirements for
IETF Standard Protocols" [RFC3365] requires that all IETF protocols
provide a mechanism for implementers to provide strong security. RFC
3365 does not define the term "strong security".
"Security Mechanisms for the Internet" [RFC3631] is not an IETF
procedural RFC, but it is perhaps most relevant. Section 2.2 states:
We have evolved in the IETF the notion of "mandatory to implement"
mechanisms. This philosophy evolves from our primary desire to
ensure interoperability between different implementations of a
protocol. If a protocol offers many options for how to perform a
particular task, but fails to provide for at least one that all
must implement, it may be possible that multiple, non-interoperable
implementations may result. This is the consequence of the
selection of non-overlapping mechanisms being deployed in the
different implementations.
This document examines the effects of applying security constraints
to Web applications, documents the properties that result from each
method, and will make Best Current Practice recommendations for HTTP
security in a later document version. At the moment, it is mostly a
laundry list of security technologies and tradeoffs.
2. Existing HTTP Security Mechanisms
For HTTP, the IETF generally defines "security mechanisms" as some
combination of access authentication and/or a secure transport.
[[ There is a suggestion that this section be split into "browser-
like" and "automation-like" subsections. ]]
[[ NTLM (shudder) was brought up in the WG a few times in the
discussion of the -00 draft. Should we add a section on it? ]]
2.1. Forms And Cookies
Almost all HTTP authentication that involves a human using a web
browser is accomplished through HTML forms, with session identifiers
stored in cookies. For cookies, most implementations rely on the
"Netscape specification", which is described loosely in section 10 of
"HTTP State Management Mechanism" [RFC2109]. The protocol in RFC
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2109 is relatively widely implemented, but most clients don't
advertise support for it. RFC 2109 was later updated [RFC2965], but
the newer version is not widely implemented.
Forms and cookies have many properties that make them an excellent
solution for some implementers. However, many of those properties
introduce serious security trade-offs.
HTML forms provide a large degree of control over presentation, which
is an imperative for many websites. However, this increases user
reliance on the appearance of the interface. Many users do not
understand the construction of URIs [RFC3986], or their presentation
in common clients [PhishingHOWTO]. As a result, forms are extremely
vulnerable to spoofing.
HTML forms provide acceptable internationalization if used carefully,
at the cost of being transmitted as normal HTTP content in all cases
(credentials are not differentiated in the protocol).
HTML forms provide a facility for sites to indicate that a password
should never be pre-populated. [[ More needed here on autocomplete ]]
The cookies that result from a successful form submission make it
unnecessary to validate credentials with each HTTP request; this
makes cookies an excellent property for scalability. Cookies are
susceptible to a large variety of XSS (cross-site scripting) attacks,
and measures to prevent such attacks will never be as stringent as
necessary for authentication credentials because cookies are used for
many purposes. Cookies are also susceptible to a wide variety of
attacks from malicious intermediaries and observers. The possible
attacks depend on the contents of the cookie data. There is no
standard format for most of the data.
HTML forms and cookies provide flexible ways of ending a session from
the client.
HTML forms require an HTML rendering engine for which many protocols
have no use.
2.2. HTTP Access Authentication
HTTP 1.1 provides a simple authentication framework, "HTTP
Authentication: Basic and Digest Access Authentication" [RFC2617],
which defines two optional mechanisms. Both of these mechanisms are
extremely rarely used in comparison to forms and cookies, but some
degree of support for one or both is available in many
implementations. Neither scheme provides presentation control,
logout capabilities, or interoperable internationalization.
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2.2.1. Basic Authentication
Basic Authentication (normally called just "Basic") transmits
usernames and passwords in the clear. It is very easy to implement,
but not at all secure unless used over a secure transport.
Basic has very poor scalability properties because credentials must
be revalidated with every request, and because secure transports
negate many of HTTP's caching mechanisms. Some implementations use
cookies in combination with Basic credentials, but there is no
standard method of doing so.
Since Basic credentials are clear text, they are reusable by any
party. This makes them compatible with any authentication database,
at the cost of making the user vulnerable to mismanaged or malicious
servers, even over a secure channel.
Basic is not interoperable when used with credentials that contain
characters outside of the ISO 8859-1 repertoire.
2.2.2. Digest Authentication
In Digest Authentication, the client transmits the results of hashing
user credentials with properties of the request and values from the
server challenge. Digest is susceptible to man-in-the-middle attacks
when not used over a secure transport.
Digest has some properties that are preferable to Basic and Cookies.
Credentials are not immediately reusable by parties that observe or
receive them, and session data can be transmitted along side
credentials with each request, allowing servers to validate
credentials only when absolutely necessary. Authentication data
session keys are distinct from other protocol traffic.
Digest includes many modes of operation, but only the simplest modes
enjoy any degree of interoperability. For example, most
implementations do not implement the mode that provides full message
integrity. Perhaps one reason is that implementation experience has
shown that in some cases, especially those involving large requests
or responses such as streams, the message integrity mode is
impractical because it requires servers to analyze the full request
before determining whether the client knows the shared secret or
whether message-body integrity has been violated and hence whether
the request can be processed.
Digest is extremely susceptible to offline dictionary attacks, making
it practical for attackers to perform a namespace walk consisting of
a few million passwords [[ CITATION NEEDED ]].
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Many of the most widely-deployed HTTP/1.1 clients are not compliant
when GET requests include a query string [Apache_Digest].
Digest either requires that authentication databases be expressly
designed to accommodate it, or requires access to cleartext
passwords. As a result, many authentication databases that chose to
do the former are incompatible, including the most common method of
storing passwords for use with Forms and Cookies.
Many Digest capabilities included to prevent replay attacks expose
the server to Denial of Service attacks.
Digest is not interoperable when used with credentials that contain
characters outside of the ISO 8859-1 repertoire.
2.2.3. Other Access Authentication Schemes
There are many niche schemes that make use of the HTTP Authentication
framework, but very few are well documented. Some are bound to
transport layer connections.
2.2.3.1. Negotiate (GSS-API) Authentication
[[ A discussion about "SPNEGO-based Kerberos and NTLM HTTP
Authentication in Microsoft Windows" [RFC4559] goes here. ]]
2.3. Centrally-Issued Tickets
Many large Internet services rely on authentication schemes that
center on clients consulting a single service for a time-limited
ticket that is validated with undocumented heuristics. Centralized
ticket issuing has the advantage that users may employ one set of
credentials for many services, and clients don't send credentials to
many servers. This approach is often no more than a sophisticated
application of forms and cookies.
All of the schemes in wide use are proprietary and non-standard, and
usually are undocumented. There are many standardization efforts in
progress, as usual.
2.4. Web Services
Many security properties mentioned in this document have been recast
in XML-based protocols, using HTTP as a substitute for TCP. Like the
amalgam of HTTP technologies mentioned above, the XML-based protocols
are defined by an ever-changing combination of standard and vendor-
produced specifications, some of which may be obsoleted at any time
[WS-Pagecount] without any documented change control procedures.
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These protocols usually don't have much in common with the
Architecture of the World Wide Web. It's not clear why the term "Web"
is used to group them, but they are obviously out of scope for HTTP-
based application protocols.
[[ This section could really use a good definition of "Web Services"
to differentiate it from REST. ]]
2.5. Transport Layer Security
[[ A discussion of HTTP over TLS needs to be added here. ]]
[[ Discussion of connection confidentiality should be separate from
the discussion of access authentication based on mutual
authentication with certificates in TLS. ]]
3. Revisions To HTTP
Is is possible that HTTP will be revised in the future. "HTTP/1.1"
[RFC2616] and "Use and Interpretation of HTTP Version Numbers"
[RFC2145] define conformance requirements in relation to version
numbers. In HTTP 1.1, all authentication mechanisms are optional,
and no single transport substrate is specified. Any HTTP revision
that adds a mandatory security mechanism or transport substrate will
have to increment the HTTP version number appropriately. All widely
used schemes are non-standard and/or proprietary.
4. Security Considerations
This entire document is about security considerations.
5. Normative References
[Apache_Digest]
Apache Software Foundation, "Apache HTTP Server -
mod_auth_digest", <http://httpd.apache.org/docs/1.3/mod/
mod_auth_digest.html>.
[PhishingHOWTO]
Gutmann, P., "Phishing Tips and Techniques",
February 2008,
<http://www.cs.auckland.ac.nz/~pgut001/pubs/phishing.pdf>.
[RFC2026] Bradner, S., "The Internet Standards Process -- Revision
3", BCP 9, RFC 2026, October 1996.
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[RFC2109] Kristol, D. and L. Montulli, "HTTP State Management
Mechanism", RFC 2109, February 1997.
[RFC2145] Mogul, J., Fielding, R., Gettys, J., and H. Nielsen, "Use
and Interpretation of HTTP Version Numbers", RFC 2145,
May 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
Leach, P., Luotonen, A., and L. Stewart, "HTTP
Authentication: Basic and Digest Access Authentication",
RFC 2617, June 1999.
[RFC2965] Kristol, D. and L. Montulli, "HTTP State Management
Mechanism", RFC 2965, October 2000.
[RFC3365] Schiller, J., "Strong Security Requirements for Internet
Engineering Task Force Standard Protocols", BCP 61,
RFC 3365, August 2002.
[RFC3631] Bellovin, S., Schiller, J., and C. Kaufman, "Security
Mechanisms for the Internet", RFC 3631, December 2003.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC4559] Jaganathan, K., Zhu, L., and J. Brezak, "SPNEGO-based
Kerberos and NTLM HTTP Authentication in Microsoft
Windows", RFC 4559, June 2006.
[WS-Pagecount]
Bray, T., "WS-Pagecount", September 2004, <http://
www.tbray.org/ongoing/When/200x/2004/09/21/WS-Research>.
Appendix A. Acknowledgements
Much of the material in this document was written by Rob Sayre, who
first promoted the topic. Many others on the HTTPbis Working Group
have contributed to this document in the discussion.
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Appendix B. Document History
[This entire section is to be removed when published as an RFC.]
B.1. Changes between draft-sayre-http-security-variance-00 and
draft-ietf-httpbis-security-properties-00
Changed the authors to Paul Hoffman and Alexey Melnikov, with
permission of Rob Sayre.
Made lots of minor editorial changes.
Removed what was section 2 (Requirements Notation), the reference to
RFC 2119, and any use of 2119ish all-caps words.
In 3.2.1 and 3.2.2, changed "Latin-1 range" to "ISO 8859-1
repertoire" to match the definition of "TEXT" in RFC 2616.
Added minor text to the Security Considerations section.
Added URLs to the two non-RFC references.
B.2. Changes between -00 and -01
Fixed some editorial nits reported by Iain Calder.
Added the suggestions about splitting for browsers and automation,
and about adding NTLM, to be beginning of 2.
In 2.1, added "that involves a human using a web browser" in the
first sentence.
In 2.1, changed "session key" to "session identifier".
In 2.2.2, changed
Digest includes many modes of operation, but only the simplest modes
enjoy any degree of interoperability. For example, most
implementations do not implement the mode that provides full message
integrity. Additionally, implementation experience has shown that
the message integrity mode is impractical because it requires servers
to analyze the full request before determining whether the client
knows the shared secret.
to
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Digest includes many modes of operation, but only the simplest
modes enjoy any degree of interoperability. For example, most
implementations do not implement the mode that provides full message
integrity. Perhaps one reason is that implementation experience has
shown that in some cases, especially those involving large requests
or responses such as streams, the message integrity mode is
impractical because it requires servers to analyze the full request
before determining whether the client knows the shared secret or
whether message-body integrity has been violated and hence whether
the request can be processed.
In 2.4, asked for a definition of "Web Services".
In A, added the WG.
Authors' Addresses
Paul Hoffman
VPN Consortium
Email: paul.hoffman@vpnc.org
Alexey Melnikov
Isode Ltd.
Email: alexey.melnikov@isode.com
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