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Vulnerability · Published Jan 27, 2025 · Assigned by GitHub_M

CVE-2025-24356UDP traffic amplification via fastd's fast reconnect feature

Medium 6.9

In plain English

A medium-severity security flaw in fastd from neocturne. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.7% (higher than 54% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 4.0)6.9Medium
Exploit odds (30 days)0.7%top 46.3% of all CVEs
Exploited in the wildNo reportsCISA SSVC: none
Attack toolingNoneno public tooling known

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.

Affected packages & fixed versions

EcosystemPackageFixed inAdvisory
Debian:12fastdno fix listedDEBIAN-CVE-2025-24356
Debian:13fastd23-1DEBIAN-CVE-2025-24356
Debian:14fastd23-1DEBIAN-CVE-2025-24356
Ubuntu:22.04:LTSfastdno fix listedUBUNTU-CVE-2025-24356
Ubuntu:24.04:LTSfastdno fix listedUBUNTU-CVE-2025-24356

Affected products (as reported)

VendorProductVersions
neocturne fastd < 23

Weakness: CWE-405 · Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:N/SI:N/SA:L

References