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

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. A remote, unauthenticated attacker could exploit this vulnerability by sending a specially crafted IKEv1 packet containing a shorter-than-expected hash payload.

This triggers an assertion failure within the Libreswan daemon, causing it to crash and restart, leading to a persistent Denial of Service (DoS) condition if malicious packets are continuously transmitted.

Additionally, if the target system relies on RSA keys with weak public exponents (e.g., e=3), a Bleichenbacher-style signature forgery attack may be feasible, potentially allowing the attacker to bypass authentication entirely.