CVE-2025-24356

Name	CVE-2025-24356
Description	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.
Source	CVE (at NVD; CERT, LWN, oss-sec, fulldisc, Red Hat, Ubuntu, Gentoo, SUSE bugzilla/CVE, GitHub advisories/code/issues, web search, more)

Vulnerable and fixed packages

The table below lists information on source packages.

Source Package	Release	Version	Status
fastd (PTS)	bullseye	21-1	vulnerable
	bookworm, trixie	22-4	vulnerable
	sid	23-1	fixed

The information below is based on the following data on fixed versions.

Package	Type	Release	Fixed Version	Urgency	Origin	Debian Bugs
fastd	source	(unstable)	23-1

Notes

[bookworm] - fastd <no-dsa> (Minor issue)
[bullseye] - fastd <no-dsa> (Minor issue)
https://github.com/neocturne/fastd/security/advisories/GHSA-pggg-vpfv-4rcv
Fixed by: https://github.com/neocturne/fastd/commit/ce1b79b12dbfa796743b5f3a50789ade965b7023 (v23)
Fixed by: https://github.com/neocturne/fastd/commit/1f233bee76b722c0b3f9024f2c39c72e9f7e5843 (v23)
Fixed by: https://github.com/neocturne/fastd/commit/d03a0a17347efb5293e42fde7d982781e90f14ef (v23)
Fixed by: https://github.com/neocturne/fastd/commit/c1a07b3f2b9066c3713c68547da700b85d60f4f7 (v23)
Fixed by: https://github.com/neocturne/fastd/commit/9df7e516378441d2d17b89f9db5c27c8312d8f12 (v23)
Fixed by: https://github.com/neocturne/fastd/commit/5f63fcfc18ae9cad023fa463b152d5e14192b5a8 (v23)
Fixed by: https://github.com/neocturne/fastd/commit/3940150e801d0c91460491bec32cbcc5bbc89d5f (v23)