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AUSCERT External Security Bulletin Redistribution
ESB-2004.0682 -- iDEFENSE Security Advisory 10.27.04
PuTTY SSH2_MSG_DEBUG Buffer Overflow Vulnerability
28 October 2004
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AusCERT Security Bulletin Summary
---------------------------------
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@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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