CVE-2025-2486: UEFI Shell exposure in Ubuntu ARM64 AAVMF undermines Secure Boot

Canonical's security team has disclosed CVE-2025-2486, a firmware-level issue in Ubuntu's edk2 packages that left the UEFI Shell accessible inside AAVMF (the ARM64 QEMU UEFI binary) even when Secure Boot was enabled — a configuration that can permit Secure Boot bypasses in affected virtualized environments and requires immediate attention from anyone using AAVMF/edk2 images on vulnerable Ubuntu releases.

Background​

The modern boot chain relies on UEFI and Secure Boot to establish a chain of trust from platform firmware to the operating system. The UEFI Shell is a small command environment that runs before the OS and can be used to execute loaders, applications, and scripts at the pre‑OS stage. That functionality is benign for debugging and development, but when left enabled in Secure Boot images it can be abused by an attacker with local privileges to load unsigned code or otherwise circumvent the Secure Boot policy.
This vulnerability is not an upstream bug in the UEFI specification itself, but a packaging/build configuration issue in the edk2-based UEFI images shipped by Ubuntu. In short, the build flag intended to remove or disable the built-in UEFI Shell for Secure Boot images was not consistently respected for AAVMF (the ARM64 firmware binary used with QEMU), leaving an exploitable surface that earlier fixes for a similar problem (tracked as CVE-2023-48733) had aimed to eliminate.
Multiple authoritative sources — including the vendor advisory and the distribution bug tracker — confirm that the problem stems from AAVMF builds not honoring the flag that disables or removes the built-in shell, and that fixes have been produced for affected Ubuntu branches.

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What exactly happened (technical overview)​

How the misconfiguration works​

• During edk2/OVMF/AAVMF builds there is a configuration option (commonly expressed as a build flag like -DBUILD_SHELL=FALSE) intended to exclude the built-in UEFI Shell from firmware images that are meant to be used with Secure Boot.
• For the x86 OVMF images the flag was applied successfully in prior mitigations. For certain AAVMF (ARM64) images, the build process did not respect that flag — the Shell binary remained present inside the firmware image even when a “secboot” image was intended.
• When the Shell remains present, and the firmware lacks runtime checks to refuse launching it under Secure Boot enforcement, an OS-resident attacker or a privileged guest user can reach the pre-OS shell and attempt to execute or load EFI applications that undermine Secure Boot protections.

Why this matters​

• Secure Boot enforces that code executed before the OS is signed and trusted. If a pre‑OS environment (the UEFI Shell) can be reached and used to load unsigned code, the chain of trust can be broken.
• The vulnerability is localized to certain packaged edk2/AAVMF images and is not a generic flaw in Secure Boot technology. But for environments that rely on AAVMF images (for example, ARM64 virtualization, emulation or CI environments), the impact is tangible: a path exists to subvert the firmware-level enforcement intended to prevent boot-time tampering.

Relation to earlier work (CVE-2023-48733)​

• This issue is an evolution of a previously disclosed problem: CVE-2023-48733. That CVE described an insecure default that left the UEFI Shell enabled in edk2 and allowed Secure Boot bypasses on some platforms.
• The 2023 mitigation patched certain builds and introduced checks that refused to start the Shell when Secure Boot was enforcing. However, downstream packaging differences (and a build configuration mismatch) meant that for some AAVMF images the earlier mitigation was incomplete — necessitating the separate identification and classification under CVE-2025-2486.

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Affected systems and scope​

Ubuntu releases and fixed package versions​

Vendor advisories and the edk2 packaging bug report indicate the following status for edk2 packages:

• 25.04 (Plucky): fixed in edk2 version 2025.02-3ubuntu1
• 24.10 (Oracular): fixed in edk2 version 2024.05-2ubuntu0.3
• 24.04 LTS (Noble): fixed in edk2 version 2024.02-2ubuntu0.3
• 22.04 LTS (Jammy): not affected (mitigation method used earlier differs)
• 20.04 LTS (Focal) and earlier LTS releases: not affected

Those package version numbers are the fixes supplied by the distribution to remove or disable the Shell from the secboot AAVMF images. Administrators must verify the edk2 package versions on their systems and apply vendor updates as required.

Attack vector and complexity​

• Attack Vector: Local (the attacker must have code execution within the guest or OS, or otherwise local access to the VM)
• Attack Complexity: High — successfully exploiting the issue requires precise actions at the pre-OS stage and often prior local access.
• Privileges required: In many scenarios, an attacker with normal OS-level privileges might be able to abuse the Shell if the guest is not protected; in others, elevated privileges may be required depending on guest isolation.
• Remote exploitation: There is no indication this is remotely exploitable without first achieving local access inside the virtual machine.

Practical risk profile​

• The immediate integrity risk is significant for environments where untrusted users can launch or manage VMs using AAVMF images — a malicious tenant or dev user could use an available shell to alter the boot process.
• For single-tenant desktop or tightly controlled servers (where only trusted admins can create or boot VMs), the risk is lower but still present until patched.
• There are no verifiable reports of active exploitation in the wild as of the available advisories, but that absence of evidence should not be interpreted as proof of safety.

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Verification and cross-checks​

The key technical claims and package fixes were verified against the vendor’s security advisory and the upstream packaging bug discussion in the distribution's bug tracker. Third‑party vulnerability trackers and CVE aggregators were consulted to confirm the CVE assignment and to reconcile scoring differences.

• Vendor advisory confirmed the affected Ubuntu releases and fixed package versions.
• The distribution bug report provides the root-cause explanation (AAVMF not respecting the build flag to remove the Shell) and the mitigation decisions.
• Independent CVE trackers reflect the same description and show differing severity assessments, which highlights variation in scoring methodology among vendors.

Where scoring or impact details diverge between trackers, the article notes the differences and explains why they arise — because scoring systems (CVSS v3, v4, vendor priorities) can incorporate different assumptions about attack feasibility and privilege requirements. Any claim not independently verifiable in public advisories (for example, in-the-wild exploitation) is explicitly flagged as unverified.

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Recommended remediation (actionable steps)​

The only reliable remediation is to apply the vendor-supplied package updates that remove or disable the UEFI Shell from AAVMF secboot images. For administrators and users running Ubuntu images that include edk2/AAVMF, follow these steps:

1. Confirm whether your system/package is vulnerable:
   • Check installed edk2 package versions:
     dpkg -l | grep edk2
   • Or query apt for the candidate version:
     apt policy edk2
2. Update package lists and upgrade edk2 (general approach):
   

   sudo apt update
   sudo apt install --only-upgrade edk2

   • For full system upgrades:
     

     sudo apt update
     sudo apt upgrade
     sudo reboot

3. If you need to require a specific safe package version (example):
   • Upgrade to the distribution-fix version (replace with the exact version number relevant to your release):
     sudo apt install edk2=2025.02-3ubuntu1
   • After installing a package-level fix, ensure VMs that boot using the updated AAVMF images are restarted so they pick up fixed firmware.
4. If you maintain custom or CI-built firmware images:
   • Rebuild AAVMF/edk2 images ensuring -DBUILD_SHELL=FALSE (or equivalent configuration) is honored.
   • Run automated tests that confirm the Shell is absent in secboot images and that the firmware refuses to launch the Shell under Secure Boot.
5. For QEMU-based deployments that use images from distributions or packaging repositories:
   • Pull updated packages from the distribution's official repositories (avoid ephemeral or manually altered images).
   • Rebase or replace any golden CI images that contained the vulnerable AAVMF firmware.
6. If you cannot update immediately:
   • Restrict access to build or management systems that can create or boot AAVMF-based VMs.
   • Enforce stricter RBAC controls in virtualization management to prevent unprivileged users from creating VMs from untrusted images.

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Detection, forensics, and verification after patching​

• Confirm the absence of the built-in Shell in the secboot AAVMF image:
  • Inspect the AAVMF binary (strings, file list) to ensure the Shell binary or commands are not present.
  • Boot a test VM with Secure Boot enabled and verify the firmware does not offer a Shell prompt.
• Audit VM images and CI pipelines:
  • Identify any VM images or CI artifacts that include edk2/AAVMF images and update them.
  • Run the new edk2 test cases (packaging tests referenced in the bug report) to ensure compliance.
• Look for signs of tampering:
  • Forensic indicators include unexpected unsigned EFI apps used during boot, modified firmware images, or boot-time artifacts that persist through reboots.
  • Check virtualization logs and CI logs for unapproved boot argument usage or firmware-flashing actions.

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Broader implications and threat modelling​

Firmware-level trust is brittle​

This CVE underscores an important reality: firmware trust is only as strong as the build and packaging process. An upstream fix can be rendered ineffective by a downstream packaging or build-system difference. For security teams, it is a reminder to:

• Treat firmware/artifact build pipelines as security-critical code paths.
• Use reproducible builds and guardrails that prevent misconfiguration from producing “insecure-by-default” firmware images.
• Include firmware integrity checks in the CI/CD pipeline and infrastructure as code.

Virtualization and multi-tenant risk​

In cloud or multi-tenant virtualization environments where different users can instantiate VMs using pre-built images, a vulnerable AAVMF image opens a pathway for a relatively privileged tenant to attempt firmware-level manipulations that could affect VM integrity. Cloud providers and platform teams should:

• Ensure base images and virtualization firmware are up to date and centrally managed.
• Limit who can create or run VMs with custom firmware images.
• Include firmware checks as part of image signing and registries.

Persistent threats and bootkits​

While this specific issue requires local access to the VM or guest OS, its exploitation can facilitate persistence via boot-stage code execution. Once Secure Boot is bypassed, sophisticated attackers can attempt to install bootkits or implants that survive OS reinstallation — a high-integrity impact scenario. That makes the vulnerability materially important for high-value targets and for systems that rely on firmware-based attestation.

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Why scoring and severity can vary (interpretation guidance)​

Different trackers and vendors have assigned different severity levels and CVSS scores for CVE-2025-2486. The variation stems from:

• Different assumptions about required privileges (some scorers assume an attacker already has local access; others assume weaker privilege levels).
• Differences in whether downstream mitigations (e.g., other distro-specific checks) are considered in the scoring.
• Disparate views on the real-world exploitability of AAVMF-configured systems (some environments simply never use AAVMF).

Practical takeaway: treat the vulnerability as an integrity risk that warrants prompt patching if you run affected firmware images, even if your environment seems low‑risk on paper.

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Recommended short-term and long-term mitigations​

• Short-term:
  • Apply the distribution vendor updates for edk2 as soon as possible on affected Ubuntu releases.
  • Replace or rebuild any custom AAVMF images created from vulnerable packages.
  • Restrict access to systems that allow VM instantiation with custom firmware.
• Long-term:
  • Harden CI pipelines and enforce build-time flags via automated checks.
  • Maintain an inventory of firmware binaries used by virtualization stacks and scan for unexpected components (e.g., the UEFI Shell).
  • Adopt reproducible firmware builds and cryptographic signing for firmware artifacts.
  • Include AAVMF/OVMF images in regular vulnerability scanning and asset management.

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Final assessment and practical advice​

CVE-2025-2486 is a classic example of a low-level integrity risk created by a configuration/build omission rather than a code-level vulnerability. The root cause — the AAVMF build not respecting the flag to remove the UEFI Shell — is straightforward to remediate, and vendor-supplied package updates already exist for affected Ubuntu releases. The primary operational risk is the window of exposure: any environment that still runs vulnerable AAVMF firmware images while Secure Boot is expected to protect the boot chain should prioritize applying the fixed edk2 packages and replacing any affected images.
Action checklist for administrators and platform owners:

1. Identify whether your environment uses AAVMF (ARM64) edk2 firmware images for virtualization or CI.
2. Check edk2 package versions and apply the vendor updates immediately if your release is listed as affected.
3. Rebuild or replace any VM/CI golden images that contain the vulnerable AAVMF firmware.
4. Harden access controls to image build and VM lifecycle operations.
5. Add automated verification to ensure future edk2/AAVMF builds do not include the Shell in secboot images.

This issue highlights how distribution packaging decisions and build system behavior can materially affect security guarantees long after an upstream patch exists. For administrators and platform engineers, the pragmatic response is immediate patching plus tightening build and image governance to prevent repeat regressions that undermine Secure Boot assurances.

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Conclusion
The presence of a UEFI Shell inside AAVMF secboot images undermines Secure Boot guarantees and presents a non‑trivial integrity risk in virtualized ARM64 environments. The vulnerability has been acknowledged and fixed in vendor packages for affected Ubuntu releases; the correct mitigation is to apply those updates, rebuild any dependent images, and tighten build pipelines so Secure Boot artifacts remain enforceable.

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Source: MSRC Security Update Guide - Microsoft Security Response Center