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Core Security Technologies - CoreLabs Advisory
VNC Multiple Integer Overflows
1. *Advisory Information*
Title: VNC Multiple Integer Overflows
Advisory ID: CORE-2008-1009
Advisory URL: http://www.coresecurity.com/content/vnc-integer-overflows
Date published: 2009-02-03
Date of last update: 2009-02-03
Vendors contacted: UltraVNC, TightVNC
Release mode: Coordinated release
2. *Vulnerability Information*
Class: Integer overflow
Remotely Exploitable: Yes
Locally Exploitable: No
Bugtraq ID: 33568
CVE Name: CVE-2009-0388
3. *Vulnerability Description*
Multiple integer overflow vulnerabilities have been discovered in
UltraVNC  and TightVNC , two (open source) remote control
applications derived from the popular VNC  software.
The vulnerabilities cause a miscalculation of a buffer size on the heap,
allowing an attacker to corrupt a VNC client heap and can probably allow
code execution (exploitation is very likely).
4. *Vulnerable packages*
. UltraVNC - 1.0.2
. UltraVNC - 1.0.5
. TightVnc - 1.3.9
. Older versions are probably affected too, but they were not tested
5. *Non-vulnerable packages*
. UltraVNC - 188.8.131.52
. TightVNC - 1.3.10
6. *Vendor Information, Solutions and Workarounds*
VNC users connecting to untrusted servers should update their VNC
The UltraVNC team has released patched binaries  for its viewer.
Additional information can be found in the UltraVNC Forum
The TightVNC team has released patched source code in . TightVNC
1.3.10 will be released by Feb 10th 2009.
These vulnerabilities were discovered and researched by Ariel
Futoransky, Fernando Russ and Alfredo Ortega from Core Security
8. *Technical Description / Proof of Concept Code*
Multiple integer overflow vulnerabilities have been discovered in
UltraVNC and TightVNC. The vulnerable functions are located in
'ClientConnection.cpp', and they are:
These functions are used in UltraVNC - 1.0.2 (and previous versions):
. 'ClientConnection::ReadServerCutText() : 3859'
. 'ClientConnection::Authenticate() : 1701'
And in TightVNC - 1.3.9 (and previous versions):
. 'ClientConnection::ReadServerCutText() : 2951'
. 'ClientConnection::ReadFailureReason() : 3066'
Other versions may be vulnerable too.
Multiple VNC clients are affected, as they share the vulnerable code.
The integer overflow follows this pattern:
unsigned int len; /* note the *unsigned int* */
// read len from the net
len = network.read_placeholder();
// check the size to ensure the network related read buffer is of the
bigger as need
CheckBufferSize( len ); // or CheckZipBufferSize(len);
// use network related red buffer
where 'CheckBufferSize' looks like:
4185: // Makes sure netbuf is at least as big as the specified size.
4186: // Note that netbuf itself may change as a result of this call.
4187: // Throws an exception on failure.
4188: void ClientConnection::CheckBufferSize(int bufsize)
4190: if (m_netbufsize > bufsize) return;
and 'CheckZipBufferSize' looks like:
4238: void ClientConnection::CheckFileZipBufferSize(int bufsize)
4240: unsigned char *newbuf;
4242: if (m_filezipbufsize > bufsize) return;
Also, other functions like 'CheckFileZipBufferSize()' and
'CheckFileChunkBufferSize()' follow the same vulnerable pattern. The
integer overflow will ensue a heap corruption in the function
'ReadString()', often called after the bug in 'CheckBufferSize()'. This
is not a comprehensive list of possible memory corruptions caused by
this bug, as the vulnerable function is used in many places.
The integer overflow is caused because the data types of the argument
'bufsize' (signed int) and the buffers size member (unsigned long),
'm_netbufsize' and 'm_filezipbufsize'. Both are 'unsigned long', so:
'(unsigned long)-1 > (int)42 == TRUE'
because all the comparison was "casted" to unsigned long... (0xFFFFFFFF
Steps to reproduce:
The quickest way to reproduce this bug is by modifying the VNC server to
send crafted evil packets as:
(from the TightVNC vncClient.cpp sourcecode...)
358: BOOL vncClientThread::SendTextStringMessage(const char *str)
360: CARD32 len = Swap32IfLE(strlen(str));
361: if (!m_socket->SendExact((char *)&len, sizeof(len)))
362: return FALSE;
363: if (!m_socket->SendExact(str, strlen(str)))
364: return FALSE;
366: return TRUE;
modifying the line 360, a crafted length like 0xFFFFFFFF triggers an
exception in the following functions:
. In the case of UltraVNC, in 'ClientConnection::Authenticate()'
. In the case of TightVNC, in 'ClientConnection::ReadFailureReason()'
To trigger the bug in the function 'ClientConnection::CheckBufferSize'
located in the file 'ClientConnection.cpp' (both vendors):
1848: void vncClient::UpdateClipText(LPSTR text)
1858: rfbServerCutTextMsg message;
1860: message.length = Swap32IfLE(strlen(text));
1861: if (!SendRFBMsg(rfbServerCutText, (BYTE *) &message, sizeof(message)))
1866: if (!m_socket->SendQueued(text, strlen(text)))
In line 1860 the 'message.length' structure must be modified to some
evil value like 0xFFFFFFFF.
9. *Report Timeline*
Core notifies the TightVNC team of the vulnerability.
Core notifies the UltraVNC team of the vulnerability.
The UltraVNC team asks Core for a technical description of the
Core notifies the TightVNC team of the vulnerability. The previous email
sent by Core was rejected by the vendor email service.
Technical details sent to UltraVNC team by Core.
The TightVNC team asks Core for a technical description of the
Technical details sent to TightVNC team by Core.
TightVNC team notifies Core that a fix has been produced, but the
release of the fixed version (TightVNC 1.3.10) will be available early
February. TightVNC team releases the fix for its SVN users .
Core asks TightVNC if the fixed version will be available on
02-Feb-2009. No reply received.
Core asks UltraVNC team if a fixed version is available.
UltraVNC team notifies Core that a fixed version will probably be
available on Feb 1st 2009.
Core notifies TightVNC and UltraVNC teams the advisory will be released
on Feb 3rd 2009, given that the vulnerability was already made public .
UltraVNC team notifies Core that a fix has been produced and will be
available to the users on Tuesday, Feb 3rd.
TightVNC team notifies Core that a patched version will be available to
the users on Tuesday, Feb 10th.
CORE-2008-1009 advisory is published.
 UltraVNC binary patches:
11. *About CoreLabs*
CoreLabs, the research center of Core Security Technologies, is charged
with anticipating the future needs and requirements for information
security technologies. We conduct our research in several important
areas of computer security including system vulnerabilities, cyber
attack planning and simulation, source code auditing, and cryptography.
Our results include problem formalization, identification of
vulnerabilities, novel solutions and prototypes for new technologies.
CoreLabs regularly publishes security advisories, technical papers,
project information and shared software tools for public use at:
12. *About Core Security Technologies*
Core Security Technologies develops strategic solutions that help
security-conscious organizations worldwide develop and maintain a
proactive process for securing their networks. The company's flagship
product, CORE IMPACT, is the most comprehensive product for performing
enterprise security assurance testing. CORE IMPACT evaluates network,
endpoint and end-user vulnerabilities and identifies what resources are
exposed. It enables organizations to determine if current security
investments are detecting and preventing attacks. Core Security
Technologies augments its leading technology solution with world-class
security consulting services, including penetration testing and software
security auditing. Based in Boston, MA and Buenos Aires, Argentina, Core
Security Technologies can be reached at 617-399-6980 or on the Web at
The contents of this advisory are copyright (c) 2009 Core Security
Technologies and (c) 2009 CoreLabs, and may be distributed freely
provided that no fee is charged for this distribution and proper credit
14. *PGP/GPG Keys*
This advisory has been signed with the GPG key of Core Security
Technologies advisories team, which is available for download at
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