CVE-2026-50721

A flaw was found in Libreswan's implementation of IKEv1 authentication via raw RSA signatures. When processing an IKEv1 packet using PKCS #1 v1.5 RSA encryption, the RSA_authenticate_hash_signature_raw_rsa() function fails to properly validate the length of the authentication hash. 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.

Red Hat Product Security rates this as having an Moderate security impact, because this vulnerability directly affects the availability of Libreswan VPN gateways utilizing legacy IKEv1 tunnels. IKEv1 is not default in modern RHEL; modern profiles enforce IKEv2. Because the crash occurs during the unauthenticated phase of the IKE negotiation, any exposed Libreswan service (server or client) accepting standard IKEv1 connections via the default (authby=rsasig) option is vulnerable to the Denial of Service aspect of this flaw.

Additionally, the risk of the authentication bypass vector is mitigated to low on Red Hat Enterprise Linux due to system-wide Crypto-Policies that strictly disallow the generation and use of keys with weak public exponents. This completely neutralizes the worst-case Authentication Bypass and Impersonation vectors for CVE-2026-50721, confining the threat strictly to a DoS process crash.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

To mitigate this vulnerability, consider the following strategic workarounds if upgrading Libreswan immediately is not viable:

• Migrate to IKEv2: Disable IKEv1 entirely and migrate all connections to IKEv2. The vulnerable code path is tied specifically to IKEv1’s rigid handling of PKCS#1 v1.5 RSA-SHA1.

• Switch to Pre-Shared Keys (PSK): Additionally, if the configuration is for static tunnels, and not for a group of Remote Access VPN Clients, the authentication can be changed to use PSK via "authby=secret" after coordination with the remote peer.