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

Libreswan, via the function RSAauthenticatehashsignaturerawrsa(), 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.

— MITRE

The Libreswan Project was notified of an issue when it receives an invalidly formatted PKCS#1.5 RSA signature payload that authenticates the IKE exchange. The vulnerability is similar to CVE-2018-16151. Use of RSA signatures over certificates during X.509 certificate verifications of the remote IKE peer are not affected by this vulnerability.

When the RSA exponent is weak (eg e=3), Bleichenbacher-style signature forgeries are possible, resulting in an authentication bypass. Note that most cryptographic library versions and libreswan raw RSA key generation have not allowed weak exponents for at least a decade, so valid RSA keys with weak exponents should be very rare.

Additionally, the invalid RSA IKE authentication payload can trigger an assertion, resulting in libreswan aborting and restarting. Continued sending of such packets can result in a denial of service.

Severity : Medium Vulnerable versions : all version up to and including 5.3 Not vulnerable : 5.3.1 or later

— Red Hat