# CVE-2025-24356

## Summary

- **CVE ID:** CVE-2025-24356
- **Severity:** MEDIUM
- **CVSS Score:** 6.9 (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)
- **CWE:** CWE-405
- **Published:** Jan 27, 2025
- **Last Modified:** Mar 12, 2026

## 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 Products

- neocturne — fastd (< 23)

## References

- [CNA](https://github.com/neocturne/fastd/security/advisories/GHSA-pggg-vpfv-4rcv)
- [CNA](https://github.com/neocturne/fastd/commit/1f233bee76b722c0b3f9024f2c39c72e9f7e5843)
- [CNA](https://github.com/neocturne/fastd/commit/3940150e801d0c91460491bec32cbcc5bbc89d5f)
- [CNA](https://github.com/neocturne/fastd/commit/5f63fcfc18ae9cad023fa463b152d5e14192b5a8)
- [CNA](https://github.com/neocturne/fastd/commit/9df7e516378441d2d17b89f9db5c27c8312d8f12)
- [CNA](https://github.com/neocturne/fastd/commit/c1a07b3f2b9066c3713c68547da700b85d60f4f7)
- [CNA](https://github.com/neocturne/fastd/commit/ce1b79b12dbfa796743b5f3a50789ade965b7023)
- [CNA](https://github.com/neocturne/fastd/commit/d03a0a17347efb5293e42fde7d982781e90f14ef)

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.68%
- **EPSS Percentile:** 50.3

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_Exported from OnDuty AI Vulnerability Intelligence on 2026-09-10._