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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). |
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Credit:
The information has been provided by CORE Security Technologies Advisories.
The original article can be found at: http://www.coresecurity.com/content/vnc-integer-overflows
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Vulnerable Systems:
* UltraVNC version 1.0.2
* UltraVNC version 1.0.5
* TightVnc version 1.3.9
Immune Systems:
* UltraVNC version 1.0.5.4
* TightVNC version 1.3.10
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 (http://forum.ultravnc.info/).
The TightVNC team has released patched source code in [5]. TightVNC 1.3.10 will be released by Feb 10th 2009.
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'
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).
Exploit:
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.
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.
References:
[1] http://www.uvnc.com.
[2] http://www.tightvnc.com.
[3] http://www.realvnc.com.
[4] UltraVNC binary patches: http://support1.uvnc.com/download/vncviewer_1054_w32.zip and http://support1.uvnc.com/download/vncviewer_1054_X64.zip.
[5] http://vnc-tight.svn.sourceforge.net/viewvc/vnc-tight?view=rev&revision=3564.
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