iDEFENSE Security Advisory 10.27.04 - PuTTY SSH2_MSG_DEBUG Buffer Overflow Vulnerability

From: customer service mailbox (customerservice_at_idefense.com)
Date: 10/27/04

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    Date: Wed, 27 Oct 2004 12:44:03 -0400
To: <bugtraq@securityfocus.com>, <vulnwatch@vulnwatch.org>

    PuTTY SSH2_MSG_DEBUG Buffer Overflow Vulnerability

    iDEFENSE Security Advisory 10.27.04
    www.idefense.com/application/poi/display?id=155&type=vulnerabilities
    October 27, 2004

    I. BACKGROUND

    PuTTY is a free implementation of Telnet and SSH for Win32 and Unix
    platforms, along with an xterm terminal emulator.

    II. DESCRIPTION

    Remote exploitation of a buffer overflow vulnerability in Simon Tatham's
    PuTTY can allow attackers to execute arbitrary code.

    The vulnerability specifically exists due to insufficient bounds
    checking on SSH2_MSG_DEBUG packets. The 'stringlen' parameter is given a
    user-supplied value by reading in an integer from an offset in the
    packet data. The 'stringlen' value is incorrectly checked due to
    signedness issues as seen below.

    -- snip --
    static int ssh2_rdpkt(Ssh ssh, unsigned char **data, int *datalen)
    {
      struct rdpkt2_state_tag *st = &ssh->rdpkt2_state;
      ...
      switch (ssh->pktin.type) {
            ...
             case SSH2_MSG_DEBUG:
                 {
                          char buf[512];
                        int stringlen = GET_32BIT(ssh->pktin.data+7);
                            int prefix;
                            strcpy(buf, "Remote debug message: ");
                            prefix = strlen(buf);
                            if (stringlen > (int)(sizeof(buf)-prefix-1))
                            stringlen = sizeof(buf)-prefix-1;
    [!] memcpy(buf + prefix,
                                    ssh->pktin.data + 11, stringlen);
                            buf[prefix + stringlen] = '\0';
                            logevent(buf);
                     }
    -- snip --

    The following debugger output shows successful control of program
    execution:

    EAX CC004019
    ECX 00401909 putty.00401909
    EDX 7C9037D8 ntdll.7C9037D8
    EBX 00000000
    ESP 00129FC8
    EBP 00129FDC
    ESI 0012A0A4
    EDI 7C9037BF ntdll.7C9037BF
    EIP 0012FFBA

    SEH chain of main thread
    Address SE handler
    0012FFB0 putty.00401905

    Log data, item 0
     Address=0012FFB9
     Message=INT3 command at 0012FFB9

    III. ANALYSIS

    Exploitation allows remote attackers to execute arbitrary code under the
    privileges of the user running PuTTY. The client must be directed to
    connect to a malicious server in order to trigger the vulnerability.

    IV. DETECTION

    iDEFENSE has confirmed that PuTTY 0.55 is vulnerable.

    V. WORKAROUND

    An alternate SSH client can be used to connect to untrusted hosts.

    VI. VENDOR RESPONSE

    PuTTY 0.56 addresses this problem and is available for download at:

       http://www.chiark.greenend.org.uk/~sgtatham/putty/download.html

    VII. CVE INFORMATION

    A Mitre Corp. Common Vulnerabilities and Exposures (CVE) number has not
    been assigned yet.

    VIII. DISCLOSURE TIMELINE

    10/21/2004 Initial vendor notification
    10/21/2004 iDEFENSE clients notified
    10/22/2004 Initial vendor response
    10/27/2004 Public disclosure

    IX. CREDIT

    An anonymous contributor is credited with discovering this
    vulnerability.

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

    X. LEGAL NOTICES

    Copyright (c) 2004 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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