CVE-2025-24356 UDP traffic amplification via fastd's fast reconnect feature
Attackers can exploit fastd's fast reconnect feature to amplify UDP traffic, facilitating Distributed Denial of Service attacks against reachable instances.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2025-24356?
CVE-2025-24356 is a medium-severity denial of service vulnerability in neocturne fastd, published on 27 Jan 2025. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Weakness: CWE-405
Which versions are affected by CVE-2025-24356?
It affects 1 product.
Products include Fastd Project Fastd.
How do I fix CVE-2025-24356?
Upgrade to a fixed version.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2025-24356 being exploited?
There is no known exploitation of CVE-2025-24356 in the wild and no public exploit code.
Sources
Also known as DEBIAN-CVE-2025-24356, GHSA-pggg-vpfv-4rcv, UBUNTU-CVE-2025-24356.