CVE-2026-50722: IKEv2 Denial of Service via RSA-SHA1 (PKCS#1 RSASSA-PKCS1-v1_5) authentication payload

Libreswan, via the function RSAauthenticatehashsignaturepkcs115rsa(), did not correctly verify the DER encoding of the ASN.1 digest when the IKEv2 AUTH payload was encoded using RSASSA-PKCS1-v15 (RFC 8017). A remote attacker can use a variation on the Bleichenbacher attack to forge the AUTH payload when small public exponents are used (e.g., e=3), leading to impersonation. Additionally, a remote attacker, by encoding a shorter than expected hash in the AUTH 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 the remote IKE peer 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