Critical 9.5 severity CVE-2026-97631 found in TianoCore EDK II UEFI firmware PXE boot stack

Security researchers Alex Matrosov of Anthropic and Dan Holmes of Google discovered CVE-2026-97631, a memory-corruption vulnerability rated 9.5 out of 10 in severity, residing in the TianoCore EDK II open-source UEFI reference implementation. The flaw exists in the PXE-Base Code driver (NetworkPkg/UefiPxeBcDxe) and enables attackers with local network access to send crafted ICMP error messages during PXE boot, triggering a buffer overflow that executes malicious code at the CPU's most privileged ring-0 level before the operating system loads. Because exploitation occurs prior to Secure Boot and TPM initialization, attackers can subvert the entire chain of trust, potentially extracting disk encryption keys or installing persistent bootkits.

The vulnerability's impact is exceptionally broad: EDK II is the dominant open-source UEFI codebase, used by independent BIOS vendors AMI, Insyde, and Phoenix, as well as Microsoft's hardened Project Mu fork and countless hardware OEMs and cloud providers. A premature public disclosure occurred when AMI published a pull request containing the vulnerability details and patch code before the coordinated January disclosure date, giving attackers immediate access to the information needed to develop working exploits. Patching is complicated by the fragmented UEFI supply chain, with millions of devices requiring firmware updates from thousands of vendors.

Vulnerability mechanics and attack surface

• Buffer overflow location: The flaw exists in the PXE Base Code driver's ICMP error handling, specifically in functions PxeBcIcmpErrorDpcHandle (IPv4) and PxeBcIcmp6ErrorDpcHandle (IPv6), which copy received ICMP messages into a fixed 504-byte Mode.IcmpError field without proper bounds checking.
• Root cause: The copy loop advances the destination pointer by cumulative rather than per-fragment length, and no earlier receive path restricts message length, allowing oversized ICMP messages to write attacker-controlled bytes past the buffer boundary.
• Exploitation vector: Attackers can deliver malicious ICMP errors via simple unicast to a target's IP address during the PXE boot sequence, requiring only low-level network access; successful exploitation yields code execution in ring 0 within the UEFI firmware environment.
• Chain-of-trust impact: Because exploitation occurs during pre-boot, attackers bypass Secure Boot attestation and gain access to sensitive components including the TPM, enabling potential extraction of disk encryption keys or installation of persistent early-boot payloads.
• Disclosure failure: Coordinated disclosure scheduled for January was disrupted when AMI published a pull request with vulnerability details and patch code, followed by a public TianoCore advisory, giving advanced attackers immediate access to the technical information needed to build exploits.

Why it matters

For Linux system administrators and cloud operators, this vulnerability represents a fundamental threat to infrastructure security assumptions, as it compromises the hardware root of trust that underpins Secure Boot, measured boot, and remote attestation workflows. The open-source nature of EDK II means the fix is publicly auditable and available, but the fragmented firmware supply chain creates a severe patching lag—most organizations depend on OEMs and IBVs to integrate and distribute updates, leaving systems exposed for months. The incident also highlights ongoing gaps in security review of critical open-source firmware components, despite previous fuzzing initiatives following the similar PixieFail vulnerability that failed to detect this 16-year-old flaw.