Research Archives > Security Advisories > AD20021211
PNG (Portable Network Graphics) Deflate Heap Corruption Vulnerability
12/11/2002 12:00:00 AM
12/11/2002 12:00:00 AM
We have specifically tested the following software and verified the potential for exploitation:
Microsoft Internet Explorer 5.01
Microsoft Internet Explorer 5.5
Microsoft Internet Explorer 6.0
Note: We have also successfully exploited this vulnerability via the IE web control for Microsoft Outlook.
For the purpose of completeness we have included a listing of each product that ships with the vulnerable pngfilt.dll version 6.0.2600.0 and prior. We obtained this list from Microsoft's DLL Help Database:
Access 2000 SR1
Commerce Server 2000
DirectX 6.0 SDK
Internet Explorer 4.0
Internet Explorer 4.01 SP1
Internet Explorer 4.01 SP2
Internet Explorer 5.0
Internet Explorer 5.01
Internet Explorer 5.5
Internet Explorer 5.5 Service Pack 2
Internet Explorer 6.0
Microsoft Visual Studio .NET (2002) Enterprise Architect
Microsoft Visual Studio .NET (2002) Enterprise Developer
Microsoft Visual Studio .NET (2002) Professional
Office 2000 Developer
Office 2000 SR1
Office XP Professional
Project 2002 Professional
SNA Server 4.0 SP2
SNA Server 4.0 SP3
SQL Server 7.0
SharePoint Portal Server
Small Business Server 2000
Visio 2002 Professional
Visio 2002 Standard
Visual Basic .NET Standard 2002
Visual C# .NET Standard 2002
Visual C++ .NET Standard 2002
Visual FoxPro 7.0
Visual Studio 6.0
Visual Studio 6.0 SP4
Visual Studio 6.0 SP5
Windows 2000 Datacenter Server
Windows 2000 Professional
Windows 2000 Server
Windows 95 OSR 2.5
Windows 98 Second Edition
Windows Millenium Edition
Windows NT 4.0 SP5
Windows XP Home 2002
Windows XP Professional 2002
Twas the night before Christmas, and deep in IE
A creature was stirring, a vulnerability
MS02-066 was posted on the website with care
In hopes that Team eEye would not see it there
But the engineers weren't nestled all snug in their beds,
No, PNG images danced in their heads
And Riley at his computer, with Drew's and my backing
Had just settled down for a little PNG cracking
When rendering an image, we saw IE shatter
And with just a glance we knew what was the matter
Away into SoftICE we flew in a flash
Tore open the core dumps, and threw RFC 1951 in the trash
The bug in the thick of the poorly-written code
Caused an AV exception when the image tried to load
Then what in our wondering eyes should we see
But our data overwriting all of heap memory
With heap management structures all hijacked so quick
We knew in a moment we could exploit this $#!%
More rapid than eagles our malicious pic came --
The hardest part of this exploit was choosing its name
eEye Digital Security
During a review of the PNG image format implemented in Microsoft Windows, two separate vulnerabilities were discovered related to the interpretation of PNG image data. The first vulnerability deals with the handling of the IDAT header and does not appear to be of significant threat level. The second vulnerability can be exploited to execute code when the malicious PNG image is viewed. Due to the complexity of each of these vulnerabilities we have decided only to describe the latter in detail.
A heap corruption vulnerability exists due to the way the function inflate_fast(), within pngfilt.dll, handles certain invalid data present in "deflate" data input streams in a PNG image file. The "deflate" compression specification allows for the repetition of patterns that occur in the decompressed data. This is accomplished by specifying a pair of special codes that tell the decompression routine how far back into the decompressed stream the pattern occurred (distance code), and the length of the pattern to repeat in bytes (length code). The inflate_fast() routine does not properly handle length codes marked in the specification as invalid, and as a result, a pattern can be replicated over a large portion of the heap, allowing a skilled attacker to redirect the execution of a thread into a "deflated" payload embedded in the deflate datastream within the malicious PNG image.
The heap overflow described above occurs in the interpretation of a compressed block that uses fixed Huffman codes (BTYPE = 1). Length codes #286 and #287, while labeled as invalid in the formal specification (RFC 1951), are not discarded by the inflation routine, and are instead treated as zero-length codes. However, due to the way the inflation routine is designed (see below), the length counter is decremented prior to being evaluated, and an integer overflow will occur. As a result, the loop will attempt to repeat the pattern we specify over all 4GB (0xFFFFFFFF) of virtual address space, filling our 32KB output buffer and proceeding to overwrite process memory until finally reaching an invalid page in memory and producing a fault.
The problem code is presented below in assembly, and C pseudo code.
Pattern-repetition loop in PNGFILT.DLL. version 5.0.2920.0:
69198FAF mov ecx,dword ptr [ebp+8]
69198FB2 mov cl,byte ptr [ecx]
69198FB4 mov byte ptr [edi],cl
69198FB6 inc edi
69198FB7 inc dword ptr [ebp+8]
69198FBA dec dword ptr [ebp+0Ch]
69198FBD jne 69198FAF
Pseudo-code representation of previous assembly:
*dest = *src;
After the process heap following the 32kb output buffer has been overwritten, numerous threads running within the Internet Explorer process attempt to free heap blocks whose memory management structures have been overwritten. By supplying a carefully crafted memory management header, we can alter any 32-bit address to which we have write access in Internet Explorer's virtual address space.
For the sake of demonstration, we will hijack the hook for the unhandled exception filter, overwriting the pointer to the handler with the address of a "CALL DWORD PTR [ESI+0x4C]" instruction present in MSHTML.DLL. We explain the purpose of this particular instruction below.
One of the threads that attempts to free a heap block with a memory management structure we now control, will cause the unhandled exception filter hook to be overwritten and will then cause an exception to be thrown by accessing an invalid address. This exception is thrown due to the following code sequence within RtlAllocateHeap() in NTDLL.DLL:
77FCB3F5 mov [ecx], eax
77FCB3F7 mov [eax+4], ecx
During this operation the eax register is set to the address within MSHTML.DLL where our "call dword ptr [esi+0x4c]" resides. The ecx register is set to the address of the unhandled exception filter hook.
After the first operation overwrites our exception filter hook, the second operation will generate an exception when it attempts to "mov" our exception handler address four bytes after the address we specified in the ".text" section of MSHTML.DLL. An exception is generated due to the fact that code sections, or ".text" sections are loaded into a process with read-only permissions.
The exception created by this operation is unhandled. The faulting thread will then attempt to call the unhandled exception filter. Since the address of this function has been changed, execution will redirect to the "call dword ptr [esi+0x4c]".
When an unhandled exception occurs, the unhandled exception filter is executed and receives, as an argument, an exception record. The "call dword ptr [esi+0x4c]" will redirect execution to an address supplied within the exception record. The data at this address is part of our decompressed stream.
The repeated pattern in our decompressed stream contains the two addresses used to overwrite the unhandled exception filter hook, along with a padding instructions and an unconditional "jmp" that will direct execution up what is essentially a jump chain formed by the pattern repetition, into the beginning of our deflated datastream and the deflated payload of choice.
During tests in our lab we noticed that under certain circumstances, race conditions occur that make exploitation very difficult. We developed intermediate solutions to these by reconstructing objects in heap so that the conflicting threads would continue long enough for our target thread to be exploited.
It should be noted that due to memory management system behavior across various Windows operating system environments, exploitation may become extremely difficult and in some cases unreliable.
Retina® Network Security Scanner has been updated to check for this vulnerability.
Microsoft was contacted in August 2002. Internet Explorer Service Pack 1 eliminates this vulnerability. Internet Explorer Service Pack 1 can be retrieved using the following URL:
Microsoft has released a security bulletin for this flaw. It is located here:
Drew Copley, Research Engineer, eEye Digital Security
Derek Soeder, Software Engineer, eEye Digital Security
Riley Hassell, Research Engineer, eEye Digital Security
Hacktivismo!, Paul L. S. at UTD, JBNZ, and Kasia
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