Executive brief
Libreswan is a widely used tool for creating secure VPN connections. A flaw in how it handles specific types of encrypted network traffic (IKEv2 fragments) allows a remote attacker to crash the service. If an attacker repeatedly sends these malformed messages, they can cause a sustained denial of service, preventing users from connecting to the VPN.
Technical details
A vulnerability exists in the Libreswan pluto daemon within the reassemble_v2_incoming_fragments() function. The function fails to properly validate the number of unknown outer payloads before storing them in the fixed-size msg_digest.digest[PAYLIMIT] array. An off-by-one error in a PASSERT check (using < instead of <=) causes the daemon to abort and restart when processing specifically malformed IKEv2 fragments. This is a remote, unauthenticated denial-of-service vulnerability; however, remote code execution is not possible. The issue is resolved in version 5.3.1.
Affected products
- The Libreswan Project libreswan 4.6 to 5.3
Timeline
- 2026-06-16: disclosed: Libreswan notified of the issue
- 2026-06-24: patched: Public announcement and release of version 5.3.1
- 2026-07-02: advisory: NVD publication date