CVE-2025-24356

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.

CVSS v3.1 Score

7.5

HIGH

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Weakness Type (CWE)

CWE-405 CWE-405

Affected Products

Vendor	Product	Versions
fastd_project	fastd	< 23.0

References

Advisories & Patches

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 https://github.com/neocturne/fastd/security/advisories/GHSA-pggg-vpfv-4rcv

Frequently Asked Questions

What is CVE-2025-24356? +

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. It has a CVSS v3.1 base score of 7.5 (HIGH).

How severe is CVE-2025-24356? +

CVE-2025-24356 has a CVSS v3.1 score of 7.5 out of 10, rated HIGH. This is a high-severity vulnerability that should be prioritized for patching.

What products are affected by CVE-2025-24356? +

CVE-2025-24356 affects products from fastd_project, specifically: fastd. Check the affected products table above for specific version ranges.

How do I check if I'm vulnerable to CVE-2025-24356? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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