fastd: fastd is a VPN daemon which tunnels IP packets and Ethernet frames over UDP
CVE-2025-24356 is fixed in fastd on 1 platform, first by DEBIAN-CVE-2025-24356, released January 27, 2025. It is not reported as exploited. Patch Radar priority: High.
fastd is a VPN daemon which tunnels IP packets and Ethernet frames over UDP. When receiving a data packet from an unknown IP address/port combination, fastd will assume that one of its connected peers has moved to a new address and initiate a reconnect by sending a handshake packet. This "fast reconnect" avoids having to wait for a session timeout (up to ~90s) until a new connection is established. Even a 1-byte UDP packet just containing the fastd packet type header can trigger a much larger handshake packet (~150 bytes of UDP payload). Including IPv4 and UDP headers, the resulting amplification factor is roughly 12-13. By sending data packets with a spoofed source address to fastd instances reachable on the internet, this amplification of UDP traffic might be used to facilitate a Distributed Denial of Service attack. This vulnerability is fixed in v23.
Where it's fixed
| Product | Fixed in | Fix | Released |
|---|---|---|---|
| Linux | |||
| fastd · Debian 13.5 (trixie) | 23-1 | DEBIAN-CVE-2025-24356 | Jan 27, 2025 |
| fastd · Debian 14 | 23-1 | DEBIAN-CVE-2025-24356 | Jan 27, 2025 |
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