
You Can Now Jailbreak Your AMD Zen1-Zen4 CPU Thanks to the Latest Vulnerability
Google security researchers have published comprehensive details on "EntrySign," a significant vulnerability affecting all AMD Zen processors through Zen 4. The flaw allows attackers with local administrator privileges to install custom microcode updates on affected CPUs, bypassing AMD's cryptographic verification system. The vulnerability stems from AMD's use of AES-CMAC as a hash function in its signature verification process—a critical cryptographic error. CMAC is designed as a message authentication code, not a secure hash function. The researchers discovered that AMD had been using a published example key from NIST documentation since Zen 1, allowing them to forge signatures and deploy arbitrary microcode modifications. These modifications can alter CPU behavior at the most fundamental level, enabling sophisticated attacks that persist until the next system reboot.
Google's security team has released "zentool," an open-source jailbreak toolkit allowing researchers to create, sign, and deploy custom microcode patches on vulnerable processors. The toolkit includes capabilities for microcode disassembly, patch authoring with limited assembly support, and cryptographic signing functions. As a proof-of-concept, the researchers demonstrated modifying the RDRAND instruction to consistently return predetermined values, effectively compromising the CPU's random number generation. AMD has issued microcode updates that replace the compromised validation routine with a custom secure hash function. The company's patches also leverage the AMD Secure Processor to update the validation routine before x86 cores can process potentially tampered microcode. While the attack requires local administrator access and doesn't persist through power cycles, it poses significant risks to confidential computing environments using technologies like SEV-SNP and DRTM. The researchers noted their findings could enable further CPU security research beyond exploit development, potentially allowing the implementation of new security features similar to those developed for Intel processors through similar techniques.
Google's security team has released "zentool," an open-source jailbreak toolkit allowing researchers to create, sign, and deploy custom microcode patches on vulnerable processors. The toolkit includes capabilities for microcode disassembly, patch authoring with limited assembly support, and cryptographic signing functions. As a proof-of-concept, the researchers demonstrated modifying the RDRAND instruction to consistently return predetermined values, effectively compromising the CPU's random number generation. AMD has issued microcode updates that replace the compromised validation routine with a custom secure hash function. The company's patches also leverage the AMD Secure Processor to update the validation routine before x86 cores can process potentially tampered microcode. While the attack requires local administrator access and doesn't persist through power cycles, it poses significant risks to confidential computing environments using technologies like SEV-SNP and DRTM. The researchers noted their findings could enable further CPU security research beyond exploit development, potentially allowing the implementation of new security features similar to those developed for Intel processors through similar techniques.