ExSpectre: Hiding Malware in Speculative Execution

Recently, the Spectre and Meltdown attacks revealed serious vulnerabilities in modern CPU designs, allowing an attacker to exfiltrate data from sensitive programs. These vulnerabilities take advantage of speculative execution to coerce a processor to perform computation that would otherwise not occur, leaking the resulting information via side channels to an attacker. In this paper, we extend these ideas in a different direction, and leverage speculative execution in order to hide malware from both static and dynamic analysis. Using this technique, critical portions of a malicious program's computation can be shielded from view, such that even a debugger following an instruction-level trace of the program cannot tell how its results were computed. We introduce ExSpectre, which compiles arbitrary malicious code into a seemingly-benign payload binary. When a separate trigger program runs on the same machine, it mistrains the CPU's branch predictor, causing the payload program to speculatively execute its malicious payload, which communicates speculative results back to the rest of the payload program to change its real-world behavior. We study the extent and types of execution that can be performed speculatively, and demonstrate several computations that can be performed covertly. In particular, within speculative execution we are able to decrypt memory using AES-NI instructions at over 11 kbps. Building on this, we decrypt and interpret a custom virtual machine language to perform arbitrary computation and system calls in the real world. We demonstrate this with a proof-of-concept dial back shell, which takes only a few milliseconds to execute after the trigger is issued. We also show how our corresponding trigger program can be a preexisting benign application already running on the system, and demonstrate this concept with OpenSSL driven remotely by the attacker as a trigger program. ExSpectre demonstrates a new kind of malware that evades existing reverse engineering and binary analysis techniques. Because its true functionality is contained in seemingly unreachable dead code, and its control flow driven externally by potentially any other program running at the same time, ExSpectre poses a novel threat to state-of-the-art malware analysis techniques.