CVE-2025-2296: 
Rocky Linux vulnerability analysis and mitigation

CVE-2025-2296 is a Secure Boot bypass vulnerability in EDK2 (TianoCore) caused by improper input validation (CWE-20) in the BIOS firmware's direct boot mode. When Secure Boot is enabled and a Linux kernel's signature is not found in the DB, DxeImageVerification returns EFI_ACCESS_DENIED, but the code falls back to a legacy loader — effectively bypassing Secure Boot entirely. The vulnerability affects EDK2 versions up to and including edk2-stable202502, with the fix introduced in edk2-stable202505. It was published on December 9, 2025, with a CVSS v3.1 score of 8.2 (High) and a CVSS v4.0 score of 8.4 (High) (GitHub Advisory, Microsoft MSRC).

The root cause is improper input validation (CWE-20) in the QemuLoadKernelImage() function within OvmfPkg/Library/X86QemuLoadImageLib/X86QemuLoadImageLib.c. When Secure Boot is active and DxeImageVerification denies loading an unsigned or improperly signed kernel image with EFI_ACCESS_DENIED, the code introduced in commit dafce29 (2020-06-06) falls back to a legacy kernel loader that does not enforce signature verification. This fallback path allows an attacker with high privileges and local access to load an unverified kernel, altering control flow in unexpected ways and enabling arbitrary command execution. The vulnerability specifically affects QEMU-based virtual machine environments using OvmfPkg, including Microsoft's CBL-Mariner (Azure Linux) distributions (GitHub Advisory).

Successful exploitation allows an attacker with high privileges to bypass Secure Boot protections and load an unsigned or malicious kernel image, leading to arbitrary command execution at the firmware/OS boundary. This results in a full compromise of confidentiality, integrity, and availability of the affected virtual machine. Because the attack subverts a foundational security control (Secure Boot), it can enable persistent implants, kernel-level rootkits, or complete takeover of the guest OS, with potential for lateral movement within virtualized infrastructure (GitHub Advisory, Microsoft MSRC).

1. Prerequisite: Obtain high-privilege (e.g., root or hypervisor-level) local access to a system running an affected EDK2-based QEMU virtual machine (e.g., Azure Linux 2 or 3 with OvmfPkg).
2. Prepare unsigned kernel: Obtain or craft a Linux kernel image that is not enrolled in the Secure Boot DB (i.e., lacks a trusted signature).
3. Trigger direct boot mode: Configure the QEMU VM to use direct kernel boot mode (e.g., via QEMU -kernel parameter), which invokes QemuLoadKernelImage() in OvmfPkg/Library/X86QemuLoadImageLib/X86QemuLoadImageLib.c.
4. Exploit fallback path: When DxeImageVerification returns EFI_ACCESS_DENIED for the unsigned kernel, the vulnerable code falls back to the legacy loader, which loads the kernel without signature verification — bypassing Secure Boot.
5. Achieve arbitrary execution: The unsigned/malicious kernel boots successfully, granting the attacker full control of the guest OS environment, enabling persistent access, data exfiltration, or further lateral movement within the virtualized infrastructure (GitHub Advisory).

• Logs: UEFI/firmware logs showing EFI_ACCESS_DENIED from DxeImageVerification followed immediately by a successful kernel load event — indicating the fallback path was triggered.
• Logs: Hypervisor or QEMU logs showing -kernel direct boot invocations with kernel images not matching enrolled Secure Boot DB certificates.
• File System: Presence of unsigned or unexpectedly modified kernel images in VM boot directories or QEMU configuration files referencing non-standard kernel paths.
• Process/Configuration: QEMU process command lines containing -kernel flags pointing to kernel binaries not signed by a trusted authority, combined with Secure Boot enabled in firmware settings.
• Network: Unusual outbound connections from a guest VM shortly after boot, potentially indicating post-exploitation activity following a Secure Boot bypass.

The upstream fix has been merged into EDK2 via pull request #10628, and the patched release is edk2-stable202505. Microsoft has released patches for affected Azure Linux (CBL-Mariner) packages: cbl2_edk2, azl3_edk2, cbl2_qemu, azl3_qemu, and cbl2_hvloader. Organizations should update to the patched EDK2 version and apply vendor-specific firmware/package updates promptly. As interim mitigations: restrict local and privileged access to systems running affected QEMU/EDK2 configurations, avoid using QEMU direct kernel boot mode (-kernel) in Secure Boot-enabled environments, and monitor for unauthorized kernel load events in firmware logs (GitHub Advisory, Microsoft MSRC).

The vulnerability received coverage from The Hacker Wire, which highlighted the Secure Boot bypass risk in EDK2-based BIOS environments (The Hacker Wire). A technical analysis was published by Gerd Hoffmann at kraxel.org, providing deeper insight into the fallback mechanism (Kraxel Blog). Community discussion appeared on Mastodon/infosec.exchange and Bluesky, with security researchers noting the significance of a Secure Boot bypass in virtualized environments. The openSUSE security mailing list and Yocto Project security list also circulated advisories, indicating broad awareness across Linux distribution communities.