CVE-2026-12413
Description
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.
Statement
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.
Mitigation
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.
Common Vulnerability Scoring System (CVSS) Score Details
Info alert:Important note
CVSS scores for open source components depend on vendor-specific factors (e.g. version or build chain). Therefore, Red Hat's score and impact rating can be different from NVD and other vendors. Red Hat remains the authoritative CVE Naming Authority (CNA) source for its products and services (see Red Hat classifications).
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 7.5 | N/A | 7.5 |
| Attack Vector | Network | N/A | Network |
| Attack Complexity | Low | N/A | Low |
| Privileges Required | None | N/A | None |
| User Interaction | None | N/A | None |
| Scope | Unchanged | N/A | Unchanged |
| Confidentiality | None | N/A | None |
| Integrity Impact | None | N/A | None |
| Availability Impact | High | N/A | High |
Vector
Red Hat: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
cve.org: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Understanding the Weakness (CWE)
Availability
Technical Impact: DoS: Crash, Exit, or Restart; DoS: Resource Consumption (CPU); DoS: Resource Consumption (Memory); DoS: Instability
This weakness will generally lead to undefined behavior and therefore crashes. In the case of overflows involving loop index variables, the likelihood of infinite loops is also high.
Integrity
Technical Impact: Modify Memory
If the value in question is important to data (as opposed to flow), simple data corruption has occurred. Also, if the wrap around results in other conditions such as buffer overflows, further memory corruption may occur.
Confidentiality,Availability,Access Control
Technical Impact: Execute Unauthorized Code or Commands; Bypass Protection Mechanism
This weakness can sometimes trigger buffer overflows which can be used to execute arbitrary code. This is usually outside the scope of a program's implicit security policy.
Frequently Asked Questions
Not sure what something means? Check out our Security Glossary.
Want to get errata notifications? Sign up here.
