[VulnWatch] Corsaire Security Advisory - Multiple vendor MIME Content-Transfer-Encoding mechanism issue

From: advisories (advisories_at_corsaire.com)
Date: 09/13/04

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    To: <advisories@corsaire.com>
    Date: Mon, 13 Sep 2004 12:46:14 +0100

    -- Corsaire Security Advisory --

    Title: Multiple vendor MIME Content-Transfer-Encoding mechanism issue
    Date: 04.08.03
    Application: various
    Environment: various
    Author: Martin O'Neal [martin.oneal@corsaire.com]
    Audience: General distribution
    Reference: c030804-005

    -- 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

    -- History --

    Discovered: 04.08.03 (Martin O'Neal)
    NISCC notified: 28.01.04
    Document released: 13.09.04

    -- 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 MIME encapsulation techniques
    centred on both standard & non-standard Content-Transfer-Encoding
    mechanisms, 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    (superseded by RFC2822 [3]).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. RFC2045 provides the Content-Transfer-Encoding field, which
    allows the specification of an encoding type to allow 8bit data to
    travel successfully through 7bit transport mechanisms.

    Although the standard itself only provides for a limited set of encoding
    mechanisms (7bit, 8bit, binary, quoted-printable and base64), there are
    also a number of defacto mechanisms that are in use (uuencode, mac-
    binhex40 and yenc).

    The implementation of these encoding standards has not been universal by
    all of the vendors, and additionally there has also been a degree of
    variation in the actual mechanism value used within the Content-
    Transfer-Encoding header field. For example the uuencode mechanism may
    be specified interchangeably as "uue", "x-uue" or "x-uuencode" (plus

    For many products, such as email clients and browsers, this scope for
    variation 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 implemented 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 an
    unknown Content-Transfer-Encoding mechanism, it tends to respond in one
    of two broad ways:

    - It identifies the MIME message as malformed and blocks it.
    - It fails to interpret the MIME field (or message).

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

    The Content-Transfer-Encoding mechanism issue has been observed to
    affect many of the security products. To use this issue as an attack
    vector, all that is required is to identify a target that has a client
    agent that successfully interprets the chosen Content-Transfer-Encoding
    mechanism, where any security products that protect it do not.

    -- 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-2004-0051 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

    Copyright 2003 Corsaire Limited. All rights reserved.


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