Microsoft RPC Heap Corruption Vulnerability - Part II

Release Date:
September 10, 2003

Severity:
High (Remote Code Execution)

Systems Affected:
Microsoft Windows NT Workstation 4.0
Microsoft Windows NT Server 4.0
Microsoft Windows NT Server 4.0, Terminal Server Edition
Microsoft Windows 2000
Microsoft Windows XP
Microsoft Windows Server 2003

Description:

eEye Digital Security has discovered a critical remote vulnerability in the
way Microsoft Windows handles certain RPC requests. The RPC (Remote
Procedure Call) protocol provides an inter-process communication mechanism
allowing a program running on one computer to execute code on a remote
system.

A vulnerability exists within the DCOM (Distributed Component Object Model)
RPC interface. This interface handles DCOM object activation requests sent
by client machines to the server.

Note: this vulnerability differs from the vulnerability publicized in
Microsoft Bulletin MS03-026.
(http://www.microsoft.com/technet/security/bulletin/MS03-026.asp)
This is a new vulnerability, and a different patch that must be installed.

By sending a malformed request packet it is possible to overwrite various
heap structures and allow the execution of arbitrary code.

Technical Details:

The vulnerability can be replicated with a DCERPC "bind" packet, followed by
a malformed DCERPC DCOM object activation request packet. Issuing the API
function CoGetInstanceFromFile can generate the required request. By
manipulating the length fields within the activation packet, portions of
heap memory can be overwritten with data which may be user-defined.

Sending between 4 and 5 activation packets is generally sufficient to
trigger the overwrite.

Upon sending the sequence of packets we were able to continually cause an
exception within the usual suspect RtlAllocateHeap:

PAGE:77FC8F11                 mov     [ecx], eax
PAGE:77FC8F13                 mov     [eax+4], ecx

We control the values of the registers eax and ecx. We can write an
arbitrary dword to any address of our choosing.

Execution of code can be achieved through a number of means -- the
unhandledexceptionfilter or a PEB locking pointer for instance. For this
specific vulnerability the best route was to overwrite a pointer within the
writeable .data section of RPCSS.DLL :

.data:761BC254 off_761BC254    dd offset loc_761A1AE7  ; DATA XREF:
sub_761A19EF+1C_r
.data:761BC254                                         ; sub_761A19EF+11D_w
...
.data:761BC258 off_761BC258    dd offset loc_761A1B18  ; DATA XREF:
sub_761A19EF+108_w
.data:761BC258                                         ; sub_761A1DCF+13_r
...

At runtime these two pointers reference RtlAllocateHeap and RtlFreeHeap
respectively. By overwriting offset 0x761BC258 with our chosen EIP value, we
control the processor directly after the heap overwrite. The added benefit
in choosing this pointer is we have data from our received packet at
ebp->10h which we may modify to our liking, within reason. There is one
small obstacle that must be overcome. The first word value at that address
is the length field of our packet, this field must translate to an opcode
sequence that will allow us to reach our data that follows.

Protection:
Retina Network Security Scanner has been updated to identify this
vulnerability.
http://www.eeye.com/html/Products/Retina/index.html
Also our FREE RPC scanner tool has been updated to check for this second
vulnerability.
http://www.eeye.com/html/Research/Tools/RPCDCOM.html

Vendor Status:
Microsoft has released a patch for this vulnerability. The patch is
available at:
http://www.microsoft.com/technet/treeview/?url=/technet/security/bulletin/MS
03-039.asp

Credit:
Discovery: Barnaby Jack
Additional Research: Barnaby Jack and Riley Hassell.

Greetings:
Thanks to Riley, and utmost respect to all of the eEye massive - masters of
the black arts.
Greets to all the new people I met in Vegas this year, especially the NZ
crew, and many thanks to K2 (da bankrolla.) :)
"This is my line. This is eternal." -AFI

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