Corsaire Security Advisory - Multiple vendor MIME field multiple occurrence issue

From: advisories (advisories_at_corsaire.com)
Date: 09/14/04

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Date: Tue, 14 Sep 2004 15:57:26 +0100

    -- Corsaire Security Advisory --

    Title: Multiple vendor MIME field multiple occurrence issue
    Date: 04.08.03
    Application: various
    Environment: various
    Author: Martin O'Neal [martin.oneal@corsaire.com]
    Audience: General distribution
    Reference: c030804-002

    -- Scope --

    The aim of this document is to clearly define a MIME content evasion
    issue that affects a variety of products including; browsers, proxy
    servers, email clients, content security gateways and antivirus
    products.

    -- History --

    Discovered: 04.08.03 (Martin O'Neal)
    CERT/CC notified: 18.08.03
    NISCC notified: 24.10.03
    Document released: 13.09.04

    This advisory has been created from merging Corsaire advisories with
    references c030804-001, c030806-001 and c030806-002.

    -- Overview --

    There are a number of content security gateway and antivirus products
    available that provide policy based security functionality. Part of this
    functionality allows the products to block embedded file attachments
    based on their specific content type, such as executables or those
    containing viruses. However, by using malformed MIME encapsulation
    techniques centred on the presence of multiple occurrences of fields,
    this functionality can be evaded.

    -- Analysis --

    The MIME standards are intended to provide a common mechanism to
    exchange data between systems and are used extensively by protocols such
    as HTTP and SMTP. The structure of a MIME message is defined in RFC2045
    [1], which in turn makes use of concepts introduced in RFC822 [2]
    (superseded by RFC2822 [3]).

    The standards define a range of fields that control how data is encoded
    within the transport, and how it should be interpreted by the receiving
    agent. RFC822 states "This specification permits multiple occurrences of
    most fields. Except as noted, their interpretation is not specified
    here, and their use is discouraged."

    As usual, this lack of clarity within an RFC has been interpreted in
    various ways by the assorted vendors. For many products, such as email
    clients and browsers, this scope for interpretation might only result in
    some unreliable behaviour. However, for a collection of security
    products, being unaware of the various ways that the standard has been
    interpreted can lead to more serious results, as the products may fail
    to detect a threat within the data stream.

    When a receiving agent is presented with a MIME message that contains
    multiple occurrences of a field, it tends to respond in one of four
    broad ways:

    - It identifies the MIME message as malformed and blocks it.
    - It fails to interpret the MIME field (or message).
    - It interprets the first occurrence of a field and ignores all those
      that follow it.
    - It interprets the last occurrence of a field and ignores all those
      that precede it.

    The first of the four would be the correct behaviour for a security
    conscious product, but based on empirical research this is not the
    common result.

    The MIME field multiple occurrence issue has been observed to affect
    most of the headers, parameters and values defined within the standard.
    To use this issue as an attack mechanism, all that is required is to
    identify a target that has a client agent that interprets the chosen
    MIME field in a different way to any security products that protect it.

    -- Recommendations --

    To be effective tools, the security products must not only be able to
    process encoding techniques implemented as per the relevant standard,
    but also common misinterpretations and deliberate corruptions.

    As an ongoing process, a study project should be undertaken by the
    vendors to identify applications that routinely decode MIME objects and
    have a liberal interpretation of the MIME standard.

    NISCC have produced a document consolidating a number of vendor
    statements on these issues [4]. Contact your vendor directly to
    establish whether you are affected by these issues.

    -- Background --

    This issue was discovered using a custom SMTP/HTTP vulnerability
    analysis tool developed by Corsaire's security assessment team. This
    tool is not available publicly, but is an example of the specialist
    approach used by Corsaire's consultants as part of a commercial security
    assessment. To find out more about the cutting edge services provided by
    Corsaire simply visit our web site at http://www.corsaire.com

    -- CVE --

    The Common Vulnerabilities and Exposures (CVE) project has assigned
    the name CAN-2003-1014 to this issue. This is a candidate for
    inclusion in the CVE list (http://cve.mitre.org), which standardises
    names for security problems.

    -- References --

    [1] http://www.faqs.org/rfcs/rfc2045.html
    [2] http://www.faqs.org/rfcs/rfc822.html
    [3] http://www.faqs.org/rfcs/rfc2822.html
    [4] http://www.uniras.gov.uk/vuls/2004/380375/mime.htm

    -- Revision --

    a. Initial release.
    b. Released.

    -- Distribution --

    This security advisory may be freely distributed, provided that it
    remains unaltered and in its original form.

    -- Disclaimer --

    The information contained within this advisory is supplied "as-is" with
    no warranties or guarantees of fitness of use or otherwise. Corsaire
    accepts no responsibility for any damage caused by the use or misuse of
    this information.

    -- About Corsaire --

    Corsaire are a leading information security consultancy, founded in 1997
    in Guildford, Surrey, UK. Corsaire bring innovation, integrity and
    analytical rigour to every job, which means fast and dramatic security
    performance improvements. Our services centre on the delivery of
    information security planning, assessment, implementation, management
    and vulnerability research.

    A free guide to selecting a security assessment supplier is available at
    http://www.penetration-testing.com

    Copyright 2003 Corsaire Limited. All rights reserved.

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