Public Vulnerability Reports

PuTTY SSH2_MSG_DEBUG Buffer Overflow Vulnerability



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


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->;
   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-> + 11, stringlen);
   buf[prefix + stringlen] = '';

-- snip --

The following debugger output shows successful control of program

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

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

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


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.


iDEFENSE has confirmed that PuTTY 0.55 is vulnerable.


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


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


The Common Vulnerabilities and Exposures (CVE) project has assigned the
names CAN-2004-1008 to these issues. This is a candidate for inclusion
in the CVE list (, which standardizes names for
security problems.


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


An anonymous contributor is credited with discovering this vulnerability.

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Copyright © 2004 Verisign, Inc.

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