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Trigger-based malware is designed to remain dormant and undetected unless a specific trigger occurs. Such behavior occurs in prevalent threats such as backdoors and environment-dependent (targeted) malware. Currently, trigger-based malicious code is often hidden in rarely exercised code paths in benign host binaries, and relies upon a lack of code inspection to remain undetected. However, recent advances in automatic backdoor detection make this approach unsustainable. We introduce a new code hiding approach for trigger-based malware, which conceals malicious code inside spurious code fragments in such a way that it is invisible to disassemblers and static backdoor detectors. Furthermore, we implement stealthy control transfers to the hidden code by crafting trigger-dependent bugs, which jump to the hidden code only if provided with the correct trigger. Thus, the hidden code also remains invisible under dynamic analysis if the correct trigger is unknown. We demonstrate the feasibility of our approach by crafting a hidden backdoor for the Nginx HTTP server module. |
