CVE-2025-24356
HIGHDescription
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.
How to fix
TridentStack Control can deploy fixes like this automatically across your Windows, macOS, and Linux fleet. See how it works
Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.
CVSS v3 Vector
Exploitability
Impact
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Exploit Intelligence
Moderate risk: more likely to be exploited than 49% of all known CVEs.
References
Related Vulnerabilities
Other CWE-405 vulnerabilities, ordered by exploit likelihood. View all
| CVE | Severity | CVSS | EPSS | Exploited | Fix |
|---|---|---|---|---|---|
| CVE-2019-11479 | High | 7.5 | 92% | - | Fix |
| CVE-2024-11187 | High | 7.5 | 15% | - | Fix |
| CVE-2025-8677 | High | 7.5 | 11% | - | Fix |
| CVE-2021-38447 | High | 8.6 | 2.1% | - | Fix |
| CVE-2021-21359 | Medium | 5.9 | 1.7% | - | Fix |
| CVE-2018-15492 | High | 7.5 | 1.1% | - | - |
Common questions
How do I fix CVE-2025-24356?
Upgrade fastd on debian 13 to 23-1 or later.
Is CVE-2025-24356 being actively exploited?
Not that we know of. CVE-2025-24356 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 0.67% is the estimated probability that it will be exploited in the next 30 days. That is higher than 49% of all scored CVEs.
How severe is CVE-2025-24356?
CVE-2025-24356 has a CVSS v3 base score of 7.5, rated high. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.
What does CVE-2025-24356 affect?
Published advisories record a fix for fastd (Debian). Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.
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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2025-08-27.