Windows 2000 and NT4 IIS .HTR Remote Buffer Overflow
Release Date: June 12, 2002
Severity: High (Remote code execution)
Systems Affected: Microsoft Windows NT 4.0 Internet Information Services 4.0 Microsoft Windows 2000 Internet Information Services 5.0
A vulnerability in transfer chunking, in combination with the processing of HTR request sessions can be exploited to remotely execute code of an attackers choice on the vulnerable machine. By sending a carefully crafted session, an attacker can overwrite a section of the heap. Data structures in the overwritten heap can be manipulated to move attacker-supplied data to attacker supplied memory addresses, thereby altering the flow of execution into an attacker supplied payload.
This is a very serious vulnerability and eEye suggests that administrators install the Microsoft supplied patch as soon as possible.
The following example will show the vulnerable condition. The dllhost.exe child process will silently die because the developers have replaced the default exception filter. So if you want to examine this closer, load a debugger up on the dllhost child process before you send this example session over the wire.
**Begin Session**** POST /EEYE.htr HTTP/1.1 Host: 0day.big5.com Transfer-Encoding: chunked
20 XXXXXXXXXXXXXXXXXXXXXXXXEEYE2002 0 [enter] [enter] **End Session******
The example session above overwrites a section of the heap that contains data structures related to the memory management system. By manipulating the content of these structures we can overwrite an arbitrary 4 bytes of memory with an attacker supplied address.
While many may believe that the risk for these types of vulnerabilities is fairly low due to the fact that addressing is dynamic and brute force techniques would need to be use in an attack, eEye strongly disagrees. This premise is false as successful exploitation can be made with one attempt, across dll versions. An attacker can overwrite static global variables, stored function pointers, process management structures, memory management structures, or any number of data types that will allow him to gain control of the target application in one session.
SecureIIS(tm) Application Firewall for Microsoft IIS
It should be noted that clients using any version of SecureIIS from eEye Digital Security are secure from this vulnerability. This vulnerability was discovered by the eEye team while testing a new version of SecureIIS to help further its protection abilities from similar classes of attack. To learn more visit http://www.eeye.com/SecureIIS
Vendor Status: Microsoft has released a security bulletin and patch: http://www.microsoft.com/technet/security/
Beyond installing the Microsoft security patch it is also recommend to disable the .htr ISAPI filter if you have not already done so. Microsoft’s security advisory references more information on the steps of how to disable the .htr ISAPI filter.
Credit: Riley Hassell
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