[Full-disclosure] iDEFENSE Security Advisory 09.13.05: Linksys WRT54G Router Remote Administration apply.cgi Buffer Overflow Vulnerability

From: iDEFENSE Labs (labs-no-reply_at_idefense.com)
Date: 09/13/05

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    Date: Tue, 13 Sep 2005 17:29:06 -0400
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    Linksys WRT54G Router Remote Administration apply.cgi Buffer Overflow
    Vulnerability

    iDEFENSE Security Advisory 09.13.05
    www.idefense.com/application/poi/display?id=305&type=vulnerabilities
    September 13, 2005

    I. BACKGROUND

    The Linksys WRT54G is a combination wireless access point, switch and
    router. More information is available at the following URL:

     http://www.linksys.com/products/product.asp?prid=508

    II. DESCRIPTION

    Remote exploitation of a buffer overflow vulnerability in multiple
    versions of the firmware for Cisco Systems Inc.'s Linksys WRT54G
    wireless router may allow unauthenticated execution of arbitrary
    commands as the root user.

    The vulnerability specifically exists in the 'apply.cgi' handler of the
    httpd running on the internal interfaces, including the by default the
    wireless interface. This handler is used by the many of the
    configuration pages to perform the configuration management of the
    router.

    If an unauthenticated remote attacker sends a POST request to the
    apply.cgi page on the router with a content length longer than 10000
    bytes, an exploitable buffer overflow may occur.

    III. ANALYSIS

    Successful exploitation of this vulnerability would allow an
    unauthenticated user to execute arbitrary commands on the affected
    router with root privileges. This could allow any operation to be
    performed on the router, including changing passwords and firewall
    configuration, installation of new firmware with other features, or
    denial of service. Exploitation of this vulnerability requires that an
    attacker can connect to the web management port of the router. The
    httpd is running by default but is only accessible via the LAN ports or
    the WLAN (wireless LAN). An attacker who can associate via the wireless
    interface to the network running a vulnerable httpd could send an
    exploit from a wireless device, and so not require direct physical
    access to an affected network. Additionally, if the httpd is configured
    to listen on the WAN (internet) interface, this vulnerability would be
    exploitable remotely over the internet.

    On some versions of the WRT54G firmware the buffer used to store the
    POST input, 'post_buf', is before a structure in memory containing
    pointers to the 'mime_handlers' structure, which contains function
    pointers for handling the various types of input. By overwriting this
    structure so some function pointers point into post_buf, it is possible
    to execute arbitrary commands. Overwriting these values with nulls will
    prevent access to the httpd on the system until the router is
    restarted. Overwriting these values with 'garbage' values will cause
    the httpd to crash but it will be restarted by a system monitoring
    process within 2 minutes, allowing multiple exploitation attempts.

    Although authentication checks are performed on access to this page, the

    code which reads in the buffer is executed even if authentication fails,

    so as to clear the input buffer from the client before returning an
    error message. This may allow an unauthenticated user to exploit the
    vulnerability.

    IV. DETECTION

    iDEFENSE has confirmed the existence of this vulnerability in version
    3.01.03 of the firmware of the Linksys WRT54G, and has identified the
    same code is present in version 3.03.6. All versions prior to 4.20.7 may

    be affected.

    As this firmware is Open Source, and based on a reference implementation

    supplied by the original hardware maker, there may be other affected 3rd

    party firmware which use the same or similar code, and are thus also
    affected.

    V. WORKAROUND

    In order to mitigate exposure of the internal network to outside
    attackers, ensure encryption is enabled on the wireless interface. The
    exact settings to use are dependent on your wireless deployment
    policies.

    VI. VENDOR RESPONSE

    This vulnerability is addressed in firmware version 4.20.7 available for
    download at:

    http://www.linksys.com/servlet/Satellite?childpagename=US%2FLayout
     
    &packedargs=c%3DL_Download_C2%26cid%3D1115417109974%26sku%3D112491680264
    5
     &pagename=Linksys%2FCommon%2FVisitorWrapper

    VII. CVE INFORMATION

    The Common Vulnerabilities and Exposures (CVE) project has assigned the
    name CAN-2005-2799 to this issue. This is a candidate for inclusion in
    the CVE list (http://cve.mitre.org), which standardizes names for
    security problems.

    VIII. DISCLOSURE TIMELINE

    06/07/2005 Initial vendor notification
    06/07/2005 Initial vendor response
    09/13/2005 Coordinated public disclosure

    IX. CREDIT

    This vulnerability was discovered by Greg MacManus of iDEFENSE Labs.

    Get paid for vulnerability research
    http://www.idefense.com/poi/teams/vcp.jsp

    Free tools, research and upcoming events
    http://labs.idefense.com

    X. LEGAL NOTICES

    Copyright (c) 2005 iDEFENSE, Inc.

    Permission is granted for the redistribution of this alert
    electronically. It may not be edited in any way without the express
    written consent of iDEFENSE. If you wish to reprint the whole or any
    part of this alert in any other medium other than electronically, please
    email customerservice@idefense.com for permission.

    Disclaimer: The information in the advisory is believed to be accurate
    at the time of publishing based on currently available information. Use
    of the information constitutes acceptance for use in an AS IS condition.
    There are no warranties with regard to this information. Neither the
    author nor the publisher accepts any liability for any direct, indirect,
    or consequential loss or damage arising from use of, or reliance on,
    this information.
    _______________________________________________
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