PNG (Portable Network Graphics) Deflate Heap Corruption Vulnerability Dec 12 2002 12:16AM
Marc Maiffret (marc eeye com)
PNG (Portable Network Graphics) Deflate Heap Corruption Vulnerability

Release Date:
December 11, 2002

High (Code Execution)

Systems Affected:
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
BackOffice 4.5
Commerce Server 2000
DirectX 6.0 SDK
DirectX 6.0 SDK
Internet Explorer 4.0
Internet Explorer 4.01 SP1
Internet Explorer 4.01 SP1
Internet Explorer 4.01 SP2
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 Architect
Microsoft Visual Studio .NET (2002) Enterprise Developer
Microsoft Visual Studio .NET (2002) Professional
Office 2000 Developer
Office 2000 SR1
Office 2000 SR1
Office XP Professional
Project 2002 Professional
Publisher 98
Publisher 98
SNA Server 4.0 SP2
SNA Server 4.0 SP2
SNA Server 4.0 SP3
SNA Server 4.0 SP3
SQL Server 7.0
SQL Server 7.0
SharePoint Portal Server
Small Business Server 2000
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
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 95 OSR 2.5
Windows 98
Windows 98 Second Edition
Windows Millenium Edition
Windows NT 4.0 SP5
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

Derek Soeder
Software Engineer
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.

General Description:
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.

Technical Description:
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;
while (--len);

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

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 execut
ed 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

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.

Mitigating Factors:
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

Vendor Status:
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

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

Copyright (c) 1998-2002 eEye Digital Security
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