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.
Weak RSA exponent (e=3) in use, enabling Bleichenbacher signature forgery
Problem types
CWE-347: Improper Verification of Cryptographic Signature
Product status
Any version
5.3.1 (semver)
Timeline
| 2026-03-24: | Libreswan notified of the issue via security@libreswan.org |
| 2026-06-16: | Advanced notice given to supported customers and distributions |
| 2026-06-24: | Public announcement and release of libreswan 5.3.1 |
Credits
Yeonghyeon Choi
Duyeong Kim
Andrew Cagney (The Libreswan Team)
References
libreswan.org/security/CVE-2026-50721/CVE-2026-50721.txt (Libreswan Security Advisory CVE-2026-50721)
libreswan.org/security/CVE-2026-50721/ (Libreswan CVE-2026-50721 Patches)
libreswan.org/security/CVE-2026-50722/CVE-2026-50722.txt (Related: CVE-2026-50722 (IKEv2 variant))
www.rfc-editor.org/rfc/rfc2313 (RFC 2313 - PKCS #1: RSA Encryption Version 1.5)