UDP traffic amplification via fastd's fast reconnect feature
Key facts
- Attack vector
- Network
- Privileges required
- None
- User interaction
- None
- Attack complexity
- Low
- Attack requirements
- None
- Impact type
- —
- Confidentiality · integrity · availability
- None · None · None
- Fix status
- Fixed
Attack profile from CVSS 4.0 · CNA. Fix status from CNA.
Summary
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.
Why it matters now
SSVC decision points
CISA ADP · SSVC 2.0.3
- Exploitation
- None
- Automatable
- Yes
- Technical impact
- Partial
The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.
Affected products
| Vendor | Product | Affected versions | Fixed in | Status | Source |
|---|---|---|---|---|---|
| neocturne | fastd | before 23 | 23 | Patch available | CNA |
- fastd Vendorneocturne Affected before 23 Fixed in 23 Patch available CNA
CPE configurations NVD · 1
| CPE | Versions | Role | Vendor |
|---|---|---|---|
| cpe:2.3:a:fastd_project:fastd:*:*:*:*:*:*:*:* | before 23.0 | Vulnerable | Fastd Project |
- cpe:2.3:a:fastd_project:fastd:*:*:*:*:*:*:*:* Versions before 23.0 Vulnerable ·Fastd Project
Remediation
- Fix · CNA fastd (neocturne) Fixed in 23
Vendor assessments
Timeline
- 2026-10-02 Threatist record created from CVE-2025-24356
References
Patch 8
- 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
More Fastd Project Fastd vulnerabilities
- TI-2020-16755 receive.c in fastd before v21 allows denial of service (assertion failure) when receiving… CVSS 3.1HIGH 7.5
Sources
- CVE-2025-24356 · CVE Program
- CVE-2025-24356 · NVD
- FIRST EPSS
- CVE-2025-24356 · Red Hat security data · CC BY 4.0
This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences