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Attackers Exploiting Windows XP Zero-Day Use Multiple Techniques to Thwart Analysis

Attackers exploiting a recently-publicized vulnerability in older versions of Windows are using a number of techniques to thwart analysis, according to Trend Micro.

Attackers exploiting a recently-publicized vulnerability in older versions of Windows are using a number of techniques to thwart analysis, according to Trend Micro.

On Nov. 27, Microsoft warned it was investigating reports of a vulnerability in a kernel component in Windows XP and Windows Server 2003. If exploited, the vulnerability can be used to enable an attacker to escalate privileges and run arbitrary code in kernel mode – permitting them to install programs, view data or take other actions with full administrative rights.

The issue is being exploited using a malicious PDF file to deliver a backdoor that has a few tricks up its sleeve.

“As a first step to thwart debugging, the malware creates a new thread which continues its malicious routines,” blogged Jayronn Christian Bucu, senior research engineer at Trend Micro. “The original thread is then deleted, blocking debugging. It also makes an API call to NtSetInformationThread for the newly-created thread it created. This is commonly used to set the execution priority of a thread. However, the malware sets the THREADINFOCLASS to 0×11 (ThreadHideFromDebugger). This prevents debug information from the new thread from being sent to the debugger.”

“This is normally done by RtlQueryProcessDebugInformation to prevent deadlock in certain debugging scenarios, such as remote debugging,” the researcher added. “However, here it prevents the debugger from gathering information, making analysis more difficult.”

The malware also reuses many portions of its existing code, which poses both positives and negatives for analysts. The benefit, Bucu explained, is that since the code to be injected is a copy of the currently running code, it eliminates the need for re-analysis. The downside however is that if the attacker has placed several breakpoints on the current executing code and wasn’t able to remove them, the malware code would be injected with the patched breakpoint. Meanwhile, debuggers implement breakpoints by putting INT3 in the code being debugged.

“The end result would be that the patched shellcode would be patched with multiple occurrences of INT3. This would mean that the target process would itself be frozen,” Bucu blogged. “Consider a case where the injected process is explorer.exe. When INT3 is executed in its context, explorer.exe would freeze, affecting the debugging environment (unless it, too was attached to the debugger.) This complicates the debugging process for an unwary analyst.”

The malware’s author or authors also took the step of designing the creation so that it refuses to run its malicious routines if it finds any of the following network tool processes running:

  • dsniff.exe
  • ethereal.exe
  • ettercap.exe
  • snoop.exe
  • tcpdump.exe
  • windump.exe
  • wireshark.exe

“Our findings indicate that whoever was responsible for this attack was capable of some sophistication,” Bucu noted. “Not only was BKDR_TAVDIG.GUD delivered by a malware exploiting a zero-day vulnerability, the backdoor also uses multiple techniques in order to try and make analysis more difficult.”

The researcher advised Windows XP and Windows Server 2003 users to be mindful of Microsoft’s plans to end support in 2014 and 2015, respectively.

While Microsoft works on a patch for the vulnerability, the company advises Windows XP and Windows Server 2003 users to reroute the NDProxy service to Null.sys.

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