[NT] Windows Animated Cursor Stack Overflow Vulnerability (0-Day)

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Windows Animated Cursor Stack Overflow Vulnerability (0-Day)


Determina Security Research has discovered a vulnerability in the
USER32.DLL code responsible for loading animated cursor (.ANI) files. This
vulnerability can be exploited by a malicious web page or HTML email
message and results in remote code execution with the privileges of the
logged-in user. The vulnerable code is present in all versions of Windows
up to and including Windows Vista. All applications that use the standard
Windows API for loading cursors and icons are affected. This includes
Windows Explorer, Internet Explorer, Mozilla Firefox, Outlook and others.

Microsoft fixed a closely related vulnerability with the MS05-002 security
update, but their fix was incomplete. Determina Security Research was able
to bypass the MS05-002 patch and develop a proof-of-concept exploit that
works on fully-patched Windows systems.

This vulnerability was independently disclosed and has been actively
exploited since Mar 28, prompting the release of this advisory.
Exploitation details have been omitted from the advisory until a vendor
patch is available.


Vulnerable Systems:
* Microsoft Windows 2000 Service Pack 4
* Microsoft Windows XP Service Pack 2
* Microsoft Windows XP 64-Bit Edition Version 2003 (Itanium)
* Microsoft Windows XP Professional x64 Edition
* Microsoft Windows Server 2003
* Microsoft Windows Server 2003 for Itanium-based Systems
* Microsoft Windows Server 2003 Service Pack 1
* Microsoft Windows Server 2003 Service Pack 2
* Microsoft Windows Server 2003 with SP1 for Itanium-based Systems and
Microsoft Windows Server 2003 with SP2 for Itanium-based Systems
* Microsoft Windows Server 2003 with SP2 for Itanium-based Systems
* Microsoft Windows Server 2003 x64 Edition
* Microsoft Windows Server 2003 x64 Edition Service Pack 2
* Microsoft Windows Vista

The ANI file format is used for storing animated cursors. The format is
based on the RIFF multimedia file format and consists of a series of
tagged chunks containing variable sized data. Each chunk starts with a 4
byte ASCII tag, followed by a dword specifying the size of the data
contained in the chunk.

struct ANIChunk
char tag[4]; // ASCII tag
DWORD size; // length of data in bytes
char data[size]; // variable sized data

One of the chunks in an ANI file is the anih chunk, which contains a
36-byte animation header structure. The buffer overflow fixed in
MS05-002 was in the LoadCursorIconFromFileMap function. The vulnerable
code did not validate the length of the anih chunk before reading the
chunk data into fixed size buffer on the stack.

The pseudo code of the vulnerable function is given below:
int LoadCursorIconFromFileMap(struct MappedFile* file, ...)
struct ANIChunk chunk;
struct ANIHeader header; // 36 byte structure


// read the first 8 bytes of the chunk
ReadTag(file, &chunk);

if (chunk.tag == 'anih') {

+ if (chunk.size != 36) // added in MS05-002
+ return 0;

// read chunk.size bytes of data into the header struct
ReadChunk(file, &chunk, &header);

For more information about the
MS05-002 vulnerability, please refer to the
<http://research.eeye.com/html/advisories/published/AD20050111.html> eEye
advisory that describes the issue in great detail.

If the animation header is valid, LoadCursorIconFromFileMap will call the
LoadAniIcon function to process the rest of the chunks in the ANI file.
LoadAniIcon uses the same ReadTag and ReadChunk functions as
LoadCursorIconFromFileMap and contains the same vulnerability in the code
that reads the anih header. This vulnerability was left unpatched in the
MS05-002 security update.

int LoadAniIcon(struct MappedFile* file, ...)
struct ANIChunk chunk;
struct ANIHeader header; // 36 byte structure


while (1) {
// read the first 8 bytes of the chunk
ReadTag(file, &chunk);

switch (chunk.tag) {
case 'seq ':

case 'LIST':

case 'rate':

case 'anih':
// read chunk.size bytes of data into the header struct
ReadChunk(file, &chunk, &header);

This code relies on the check in LoadCursorIconFromFileMap to catch the
malformed anih chunk before it reaches the LoadAniIcon function.
Unfortunately, LoadCursorIconFromFileMap validates only the first anih
chunk, but LoadAniIcon processes all chunks in the file. By creating a
file with two anih chunks, one valid and one malformed, it is possible to
reach the vulnerable code in LoadAniIcon.

Reading the second anih chunk with the ReadChunk function will result in a
classic buffer overflow, overwriting the return address of LoadAniChunk
and allowing the attacker to take control of the code execution.

The exploitation of this vulnerability is interesting in light of the
protection features built in the latest versions of Windows XP, 2003 and
Vista. It is a stack overflow and should be detected by the /GS security
check. Unfortunately, the Visual Studio compiler adds the /GS check only
to functions that contain certain types of arrays, assuming that most
buffer overflows are a result of out-of-bounds array access. The
LoadAniIcon function uses a structure as a destination buffer for the data
it reads and as a result its return address is not protected by the /GS
stack check. This allows an attacker to overwrite the return address and
take control of the program execution on Windows XP SP2, 2003 and Vista in
the same way as on Windows 2000.

In addition to the missing /GS check, the vulnerable code in USER32.DLL is
wrapped in an exception handler that can recover from access violations.
If the exploit is unsuccessful, for example due to the Vista ASLR, the
process will not terminate and the attacker can simply try again. This
gives the attacker an easy way to bypass the ASLR protection and increase
the reliability of the exploit.

The call to ReadChunk in the LoadAniIcon function should be preceeded by a
check similar to the one introduced in MS05-002:
if (chunk.size != 36)
return 0;

ReadChunk(file, &chunk, &header);


The information has been provided by Determina Security Research.
The original article can be found at:

<http://www.determina.com/security.research/vulnerabilities/ani-header.html> http://www.determina.com/security.research/vulnerabilities/ani-header.html


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