Executive brief
strongSwan is a widely used open-source software for creating secure VPN connections. A flaw in how it handles certain authentication messages allows an unauthenticated remote attacker to crash the VPN service. This results in a denial of service, preventing users from connecting to the network and disrupting secure communications.
Technical details
An integer underflow vulnerability exists in the eap-ttls plugin of strongSwan when processing Attribute-Value Pairs (AVPs). The parser fails to validate that the 3-byte length field in the AVP header is at least 8 bytes (the size of the header itself) before subtracting 8 to determine the payload length. By sending a crafted AVP with a length between 0 and 7, an unauthenticated remote attacker triggers a 32-bit integer underflow. This leads to an extremely large memory allocation attempt (~4 GiB) via chunk_alloc(). Because the software does not verify if the allocation succeeded before attempting to write to the resulting buffer, a NULL pointer dereference occurs, crashing the charon IKE daemon. The vulnerability is reachable during the IKEv2 authentication phase before the client has authenticated. Fixed in version 6.0.5.
Affected products
- strongSwan strongSwan 4.5.0 prior to 6.0.5
Timeline
- 2026-03-23: disclosed: Vulnerability reported by Kazuma Matsumoto
- 2026-03-23: patched: strongSwan 6.0.5 released with fix
- 2026-03-23: advisory
References
- https://www.strongswan.org/blog/2026/03/23/strongswan-6.0.5-released.html
- https://www.strongswan.org/blog/2026/03/23/strongswan-vulnerability-(cve-2026-25075).html
- https://www.vulncheck.com/advisories/strongswan-eap-ttls-avp-parsing-integer-underflow
- https://y637f9qq2x.com/posts/cve-2026-25075/
- https://lists.debian.org/debian-lts-announce/2026/03/msg00016.html