CVE-2026-12413

A flaw was found in Libreswan's IKEv2 fragment reassembly mechanism. When a VPN gateway processes incoming split network packets (fragments) containing unexpected data, an off-by-one boundary validation error triggers an internal program safety check (assertion failure). A remote, unauthenticated attacker can exploit this by sending a specific sequence of malformed IKEv2 fragments to an exposed gateway, causing the Libreswan daemon to immediately crash and restart. While this flaw does not allow data theft or unauthorized system access, a continuous stream of these packets will lead to a persistent Denial of Service (DoS) for legitimate VPN users.

Red Hat Product Security rates this vulnerability as having an Important impact, primarily because it can be exploited remotely without authentication. However, the actual exposure depends entirely on your specific VPN configuration:

Affected Configurations: This vulnerability only impacts IKEv2 connections. By default, Libreswan enables packet fragmentation (fragmentation=yes) to handle large encryption keys over restrictive network paths. Any default IKEv2 tunnel that do not set fragmentation=no are vulnerable.

Unaffected Configurations: IKEv1 connections are completely unaffected by this flaw.

Crucially, this vulnerability does not impact environments running Libreswan versions 4.5 and older. The vulnerable fragment reassembly engine was introduced during a major codebase refactor in version 4.6. As a result, older product branches—such as those shipped in rhel-6, rhel-7, rhel-8.6.z and prior do not contain the flawed code path and are inherently immune to this attack.

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.

If upgrading to Libreswan is not an option, you can mitigate the vulnerability by disabling IKEv2 fragment processing:

Add the following directive to your global or connection-specific configuration files in /etc/ipsec.conf: fragmentation=no

Warning: Disabling fragmentation may cause larger IKEv2 payloads (such as those carrying large X.509 certificate chains) to be dropped by intermediate network routers if they exceed the path MTU.