What’s Vulnerable › Vulnerabilities

CVE-2026-50721: IKEv1 Denial of Service via RSA-SHA1 (PKCS#1 Version 1.5 Encrypted) authentication payload

• Severity: medium (CVSS 5.9 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.4% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-07-02 · updated 2026-10-25
• Weakness: CWE-347

Description

Libreswan, via the function RSA_authenticate_hash_signature_raw_rsa(), 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. A remote attacker can use a variation on the Bleichenbacher attack to forge the SIG payload when small public exponents are being used (e.g., e=3), which could lead to impersonation. Additionally, a remote attacker, by encoding a shorter than expected hash in the SIG payload, could trigger an assertion leading to denial-of-service. The daemon aborts and restarts; continued exploitation causes sustained denial of service. Remote code execution is not possible. X.509 certificate verifications of remote IKE peers are not affected.

Affected products

• The Libreswan Project libreswan: before 5.3.1 — fixed in 5.3.1

How to fix

Upgrade to a fixed version: The Libreswan Project libreswan 5.3.1. Check the vendor advisory for mitigations.

References

• https://libreswan.org/security/CVE-2026-50721/
• https://libreswan.org/security/CVE-2026-50721/CVE-2026-50721.txt
• https://libreswan.org/security/CVE-2026-50722/CVE-2026-50722.txt
• https://www.rfc-editor.org/rfc/rfc2313

Get alerted when CVE-2026-50721 or the next vulnerability affects your stack