VNC Multiple Integer Overflows

1. Advisory Information

Title: VNC Multiple Integer Overflows
Advisory ID: CORE-2008-1009
Advisory URL:
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 [1] and TightVNC [2], two (open source) remote control applications derived from the popular VNC [3] 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 -
  • TightVNC - 1.3.10

6. Vendor Information, Solutions and Workarounds

VNC users connecting to untrusted servers should update their VNC viewers/clients.

The UltraVNC team has released patched binaries [4] for its viewer. Additional information can be found in the UltraVNC Forum (

The TightVNC team has released patched source code, TightVNC 1.3.10 will be released by Feb 10th 2009.

7. Credits

These vulnerabilities were discovered and researched by Ariel Futoransky, Fernando Russ and Alfredo Ortega from Core Security Technologies.

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:

  • ClientConnection::CheckBufferSize
  • ClientConnection::CheckFileZipBufferSize


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:

(ClientConnection.cpp) 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) 4189: 
{ 4190: if (m_netbufsize > bufsize) return; ... ... 


and CheckZipBufferSize looks like:

(ClientConnection.cpp) 4238: void ClientConnection::CheckFileZipBufferSize(int bufsize) 
4239: { 4240: unsigned char *newbuf; 4241: 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 > 0x2a).

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) 359: { 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; 365: 366: return TRUE; 367: } ... 

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):

(vncClient.cpp) 1848: void vncClient::UpdateClipText(LPSTR text) 1849: 
{ .. .. 1858: rfbServerCutTextMsg message; 1860: message.length = Swap32IfLE(strlen(text)); 
1861: if (!SendRFBMsg(rfbServerCutText, (BYTE *) &message, sizeof(message))) 1862: 
{ 1863: Kill(); 1864: return; 1865: } 1866: if (!m_socket->SendQueued(text, strlen(text))) 
1867: { 1868: Kill(); 1869: return; 1870: } 1871: } .. 


In line 1860 the message.length structure must be modified to some evil value like 0xFFFFFFFF.

9. Report Timeline

  • 2009-01-09: Core notifies the TightVNC team of the vulnerability.
  • 2009-01-09: Core notifies the UltraVNC team of the vulnerability.
  • 2009-01-10: The UltraVNC team asks Core for a technical description of the vulnerability.
  • 2009-01-12: Core notifies the TightVNC team of the vulnerability. The previous email sent by Core was rejected by the vendor email service.
  • 2009-01-12: Technical details sent to UltraVNC team by Core.
  • 2009-01-14: The TightVNC team asks Core for a technical description of the vulnerability.
  • 2009-01-14: Technical details sent to TightVNC team by Core.
  • 2009-01-21: 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 [5].
  • 2009-01-26: Core asks TightVNC if the fixed version will be available on 02-Feb-2009. No reply received.
  • 2009-01-26: Core asks UltraVNC team if a fixed version is available.
  • 2009-01-26: UltraVNC team notifies Core that a fixed version will probably be available on Feb 1st 2009.
  • 2009-01-30: Core notifies TightVNC and UltraVNC teams the advisory will be released on Feb 3rd 2009, given that the vulnerability was already made public [5].
  • 2009-02-02: UltraVNC team notifies Core that a fix has been produced and will be available to the users on Tuesday, Feb 3rd.
  • 2009-02-02: TightVNC team notifies Core that a patched version will be available to the users on Tuesday, Feb 10th.
  • 2009-02-03: CORE-2008-1009 advisory is published.

10. References

[4] UltraVNC binary patches: and

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

13. Disclaimer

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 is given.

14. PGP/GPG Keys

This advisory has been signed with the GPG key of Core Security Technologies advisories team.