# CVE-2025-24356: 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.

- Web page: https://patchblox.com/security/cve/CVE-2025-24356
- Priority: High
- CVSS: 7.5 (high)
- EPSS: 0.0071
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Jan 27, 2025

## 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.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | fastd Debian 13.5 (trixie) | 23-1 | [DEBIAN-CVE-2025-24356](https://patchblox.com/security/data/fix/linux/debian-cve-2025-24356.md) | Jan 27, 2025 |
| Linux | fastd Debian 14 | 23-1 | [DEBIAN-CVE-2025-24356](https://patchblox.com/security/data/fix/linux/debian-cve-2025-24356.md) | Jan 27, 2025 |

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Data as of 2026-11-08T19:12:23Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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