CVE-2026-44518: Out-of-Bounds Read in liboqs XMSS Signature Verification

Technical summary

CVE-2026-44518 involves an out-of-bounds read in the XMSS and XMSS^MT verification code paths within liboqs prior to 0.16.0. The vulnerability stems from insufficient input validation (CWE-20) of caller-supplied signature buffer lengths. When a signature buffer shorter than the parameter set's expected size is passed to the verification function, the implementation proceeds to read past the buffer boundary (CWE-125). The out-of-bounds bytes are consumed only as input to an internal hash computation, limiting information disclosure exposure. However, if the read crosses into an unmapped memory region, the process will segfault, resulting in denial of service. The vulnerability carries a CVSS 3.1 score of 5.3 (Medium severity, CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L), reflecting the availability impact with no confidentiality or integrity components.

Business impact

Organizations deploying liboqs in signature verification workflows face availability risk. Any service that verifies XMSS or XMSS^MT signatures using affected versions may crash if malformed or truncated signatures are processed. In post-quantum migration strategies where XMSS is being adopted for long-term signature verification, this could disrupt critical authentication or non-repudiation processes. The impact is primarily denial of service rather than cryptographic compromise; however, availability disruptions to signature verification in security-sensitive applications warrant prompt remediation.

Affected systems

The vulnerability affects liboqs versions prior to 0.16.0. The Open Quantum Safe project maintains liboqs as a broadly available C library for post-quantum cryptography. Any deployment using liboqs for XMSS or XMSS^MT signature verification—whether embedded in applications, integrated into cryptographic toolkits, or used in standalone testing environments—is potentially affected. Organizations using liboqs in non-verification contexts (e.g., key generation only) have reduced exposure.

Exploitability

Exploitation requires the ability to supply a malformed signature (specifically, one shorter than the expected size for a given XMSS parameter set) to a verification function. Attack surface depends on deployment: services accepting external signatures over a network have higher exploitability than closed environments. No authentication or user interaction is needed, and the attack is straightforward—triggering a crash by providing truncated input. However, the vulnerability does not permit code execution, cryptographic oracle attacks, or data exfiltration. CVSS reflects low complexity and network accessibility, but the absence of an information leak and high barrier to practical exploitation limit real-world risk.

Remediation

Upgrade liboqs to version 0.16.0 or later. This version includes length validation fixes that prevent out-of-bounds reads by verifying that caller-supplied buffers meet the required size before processing. Verify that your deployment environment and any dependent applications are compatible with the patched version. For organizations in early post-quantum migration phases, confirm that XMSS integration tests pass with the updated library.

Patch guidance

Apply the liboqs 0.16.0 update to all systems running the library. Test the upgrade in a staging environment to validate integration with dependent applications and any custom verification workflows. The update is recommended for all users but carries particularly high priority for services accepting external XMSS signatures. Verify against the Open Quantum Safe project's official release notes and advisories to confirm compatibility with your specific deployment.

Detection guidance

Monitor for segmentation faults or crashes in processes performing XMSS or XMSS^MT signature verification, especially following receipt of untrusted signature data. Audit logs for repeated verification failures or malformed signature handling may indicate exploitation attempts. Long-term, ensure your SBOM includes liboqs with a documented version, enabling rapid identification of deployed instances during vulnerability discovery. Static analysis or dependency scanning tools can help identify liboqs presence in codebases.

Why prioritize this

While the CVSS score is medium (5.3), the actual risk is modest for most deployments. No credential compromise, data theft, or cryptographic weakness is possible; the sole concern is availability. Prioritization should reflect your organization's reliance on XMSS signature verification: if XMSS is not yet widely deployed as part of your post-quantum roadmap, this is a routine patch. If XMSS verification is integral to authentication or compliance workflows, prioritize the update to prevent denial-of-service scenarios. The fix is straightforward and carries minimal regression risk.

Risk score, explained

The CVSS 3.1 score of 5.3 (Medium) is driven by the availability impact (A:L) and network accessibility (AV:N). Confidentiality and integrity scores are zero because the out-of-bounds read leaks no data and does not modify signatures or keys. The low attack complexity (AC:L) reflects the simplicity of triggering a crash with a truncated signature. However, the practical risk is tempered by the requirement for external signature input and the absence of code execution or privilege escalation potential. Organizations should treat this as a medium-priority availability fix rather than a critical security emergency.

Frequently asked questions