CVE-2026-50721 The Libreswan Project CVE debrief

Who should care

Users of Libreswan, particularly those using IKEv1 with small public exponents, should verify their configurations and ensure proper mitigations are in place. This includes updating Libreswan to the latest version, verifying IKEv1 configurations for small public exponents, and implementing compensating controls to detect and prevent Bleichenbacher attacks. Security teams and vulnerability management teams should also review the official advisory and CVE record to validate affected scope, severity, and vendor guidance.

Technical summary

The Libreswan implementation did not correctly verify the length of the authentication hash when the SIG payload of an IKEv1 packet was encoded using PKCS #1 RSA Encryption as per RFC 2313. This allows for a variation of the Bleichenbacher attack, potentially leading to impersonation when small public exponents are used. Additionally, encoding a shorter than expected hash in the SIG payload can trigger an assertion, leading to a denial-of-service. The vulnerability is limited to IKEv1 packets and does not affect X.509 certificate verifications of remote IKE peers.

Defensive priority

High priority should be given to updating Libreswan to mitigate the Bleichenbacher attack and denial-of-service vulnerabilities. Security teams should review the official advisory and CVE record to validate affected scope, severity, and vendor guidance.

Recommended defensive actions

• Update Libreswan to the latest version
• Verify IKEv1 configurations for small public exponents
• Implement compensating controls to detect and prevent Bleichenbacher attacks
• Monitor for unusual SIG payload lengths
• Review compensating controls for exposed systems while remediation is scheduled and verified
• Check relevant monitoring, detection, and logs for exposed assets that need extra review
• Track exceptions, retest remediated assets, and close the item only after evidence is documented

Evidence notes

The CVE record was published on 2026-07-02T22:16:43.367Z and was last modified on 2026-07-07T18:16:39.147Z. The NVD entry is currently Undergoing Analysis. Evidence is limited, and defenders should verify the official CVE record and NVD entry for the latest information. The Libreswan implementation did not correctly verify the length of the authentication hash when the SIG payload of an IKEv1 packet was encoded using PKCS #1 RSA Encryption as per RFC 2313. This allows for a variation of the Bleichenbacher attack, potentially leading to impersonation when small public exponents are used. Additionally, encoding a shorter than expected hash in the SIG payload can trigger an assertion, leading to a denial-of-service.

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.