Mandiant researchers, part of Google’s cybersecurity division, have uncovered several critical security bugs in Microsoft’s Time Travel Debugging (TTD) framework.
The findings reveal significant instruction emulation inaccuracies that could potentially compromise security analyses and incident response investigations, leading analysts to overlook threats or draw incorrect conclusions during malware analysis.
Time Travel Debugging is a powerful usermode record-and-replay framework developed by Microsoft that allows security researchers and developers to capture a comprehensive recording of a process execution.
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TTD works by injecting a dynamic-link library (DLL) into the target process and capturing each state of execution, storing it in a trace file for later analysis.
The framework relies on a CPU emulation layer called Nirvana runtime engine to accurately simulate and replay program executions by translating guest instructions into host-level micro-operations.
While the researchers at Google’s Mandiant detected that the discovered bugs could cause significant security and reliability issues.
More concerning, attackers could potentially exploit these inaccuracies to intentionally evade detection or disrupt forensic analyses, severely compromising investigative outcomes.
The researchers identified four distinct bugs in TTD’s instruction emulation and functioning, including emulation issues with pop r16, push segment, and lodsb/lodsw instructions, as well as an output capture truncation problem in the WinDbg TTDAnalyze debugging extension.
The investigation began when researchers noticed a heavily obfuscated 32-bit Windows executable crashed under TTD instrumentation but ran normally in real and virtual machine environments.
This led them to suspect an issue with TTD’s instruction emulation capabilities rather than the executable itself.
Initial Pop Instruction Bug
Through meticulous analysis, the researchers identified that the pop r16 instruction was being incorrectly emulated in TTD.
While on a real CPU the instruction preserves the upper 16 bits of the register, TTD incorrectly cleared these bits, causing critical discrepancies between native execution and TTD instrumentation.
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A simple proof-of-concept code confirmed this discrepancy:-
int main()
{
int val = 0x0;
_asm
{
mov eax, 0xffffffff
push eax
pop ax
sub esp, 2
mov val, eax
}
printf("Value: %08x\n", val);
return 0;
}When executed natively, this code outputs “Value: ffffffff” but under TTD instrumentation it produces “Value: 0000ffff” – confirming the emulation bug.
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The results of running the code both natively and with TTD instrumentation, clearly demonstrating the difference in behavior.
After discovering this initial bug, Mandiant researchers created a fuzzing harness to execute random sequences of instructions and compare the results between real CPU execution and TTD instrumentation.
This led to the discovery of additional instruction emulation problems, including issues with push segment instructions that revealed differences between Intel and AMD CPU implementations.
All bugs discussed in the report have been resolved as of TTD version 1.11.410, with Microsoft addressing the issues after Mandiant’s responsible disclosure.
The researchers acknowledged Microsoft’s Time Travel Debugging team for their readiness and support in addressing the reported issues, highlighting their prompt communication and commitment to maintaining TTD as a robust and reliable security tool for Windows security research.
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