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Windows MRXSMB.SYS MrxSmbCscIoctlCloseForCopyChunk DoS



Microsoft Windows Operating System is system software for Intel based
PCs. More information can be found at the vendor website:



Local exploitation of an access validation error in Microsoft Corp.'s
Windows Operating System could allow attackers to cause a denial of
service (DoS) condition.

The vulnerability specifically exists due to a logic error in the
Microsoft Client Side Caching (CSCDLL.DLL) and Microsoft Server Message
Block Redirector Driver (MRXSMB.SYS) code. The Microsoft Client Side
Caching infrastructure provides the user-mode portion of the "offline
files" subsystem that allows interaction with network files while
offline and preserves file system permissions. The Microsoft Server
Message Block Redirector Driver is the kernel-mode file system driver
that provides the network redirector functionality utilized by CSC.

MRXSMB.SYS functions are exposed via IOCTL commands. An access
validation error exists in the MrxSmbCscIoctlCloseForCopyChunk()
function. In order to establish communication with the MRXSMB subsystem,
a file handle to a shadow device is created. If the
MrxSmbCscIoctlCloseForCopyChunk() function is passed the file handle to
the shadow device, a deadlock occurs, resulting in an unkillable


Exploitation could result in the creation of unkillable processes. This
attack can be used as protection against anti-virus or other host-based
intrusion prevention systems.


iDefense has confirmed the existence of this vulnerability in Microsoft
Windows XP SP2. It is suspected that all versions of Microsoft Windows
are vulnerable.


iDefense is unaware of any effective workaround for this issue.


The vendor security advisory and appropriate patches are available at:



The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CAN- 2006-2374 to this issue. This is a candidate for inclusion in
the CVE list (http://cve.mitre.org), which standardizes names for
security problems.


02/07/2006   Initial vendor notification
02/07/2006   Initial vendor response
06/13/2006   Coordinated public disclosure


iDefense credits Rubén Santamarta with the discovery of this

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Copyright © 2006 Verisign, Inc.

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