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UNIRAS Brief - 601/04 - iDEFENSE Security Advisory 10.27.04 - PuTTY SSH2_MSG_DEBUG Buffer Overflow



 
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   UNIRAS (UK Govt CERT) Briefing Notice - 601/04 dated 28.10.04  Time: 12:50  
  UNIRAS is part of NISCC (National Infrastructure Security Co-ordination Centre)
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  UNIRAS material is also available from its website at www.uniras.gov.uk and
         Information about NISCC is available from www.niscc.gov.uk
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Title
=====

iDEFENSE Security Advisory 10.27.04 - PuTTY SSH2_MSG_DEBUG Buffer Overflow 
Vulnerability

Detail
====== 

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.



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===========================================================================
             
           ESB-2004.0682 -- iDEFENSE Security Advisory 10.27.04
            PuTTY SSH2_MSG_DEBUG Buffer Overflow Vulnerability
                              28 October 2004

===========================================================================

       

Product:                PuTTY 0.55
Publisher:              iDEFENSE
Operating System:       Windows
                        Linux variants
                        UNIX variants
Impact:                 Execute Arbitrary Code/Commands
Access:                 Remote/Unauthenticated
Original Bulletin URL:
  http://www.idefense.com/application/poi/display?id=155

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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@xxxxxxxxxxxx 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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UNIRAS wishes to acknowledge the contributions of iDEFENSE for the information 
contained in this Briefing. 
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This Briefing contains the information released by the original author. Some 
of the information may have changed since it was released. If the vulnerability 
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<End of UNIRAS Briefing>

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