fastd: Asymmetric Resource Consumption (Amplification)
CVE-2025-24356 is a high-severity asymmetric resource consumption (amplification) vulnerability in fastd from Neocturne, with a CVSS 3.1 base score of 7.5 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, without any action from a user. Affected fastd versions: < 23. The record does not name a fixed version, so check the vendor advisory. There is no confirmed exploitation so far. EPSS estimates a 0.71% chance of exploitation within 30 days, higher than 54% of scored CVEs.
High severity with no evidence of exploitation so far.
What to do
- Check which version of fastd you run against the affected versions below, and install the vendor's latest release.
- Read the advisory on github.com for workarounds and exact fixed builds.
- Until you can update, restrict network access to the affected service so it is not reachable from untrusted networks.
Affected products and versions
Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.
How the severity score breaks down
- Attack vector
- NetworkCan be exploited remotely over a network, including the internet.
- Attack complexity
- LowNo special conditions are needed; the attack works reliably.
- Privileges required
- NoneNo account or login is needed.
- User interaction
- NoneNo action by a victim is required.
- Scope
- UnchangedImpact stays within the vulnerable component.
- Confidentiality
- NoneNo data is disclosed.
- Integrity
- NoneNothing can be modified.
- Availability
- HighThe component can be taken down completely.
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H. Scored by NVD.
| Scoring system | Score | Severity | Scored by |
|---|---|---|---|
| CVSS 4.0 | 6.9 | Medium | CNA (github.com) |
| CVSS 3.1 | 7.5 | High | NVD |
Official 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.
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Asymmetric Resource Consumption (Amplification)
The product does not properly control situations in which an adversary can cause the product to consume or produce excessive resources without requiring the adversary to invest equivalent work or otherwise prove authorization, i.e., the adversary's influence is asymmetric.
How developers prevent this class of flaw
- An application must make resources available to a client commensurate with the client's access level.
- An application must, at all times, keep track of allocated resources and meter their usage appropriately.
- Consider disabling resource-intensive algorithms on the server side, such as Diffie-Hellman key exchange.
More Asymmetric Resource Consumption (Amplification) vulnerabilities (CWE-405)
Timeline
- CVE record published.
- CISA published its SSVC assessment (exploitation: none, automatable: yes, technical impact: partial).
- Record last changed materially (score, affected products, status or references).
References
Advisories
Patches and release notes
- github.com/neocturne/fastd/commit/1f233bee76b722c0b3f9024f2c39c72e9f7e5843
- github.com/neocturne/fastd/commit/3940150e801d0c91460491bec32cbcc5bbc89d5f
- github.com/neocturne/fastd/commit/5f63fcfc18ae9cad023fa463b152d5e14192b5a8
- github.com/neocturne/fastd/commit/9df7e516378441d2d17b89f9db5c27c8312d8f12
- github.com/neocturne/fastd/commit/c1a07b3f2b9066c3713c68547da700b85d60f4f7
- github.com/neocturne/fastd/commit/ce1b79b12dbfa796743b5f3a50789ade965b7023
- github.com/neocturne/fastd/commit/d03a0a17347efb5293e42fde7d982781e90f14ef
Recent Asymmetric Resource Consumption (Amplification) vulnerabilities
- 7.5 High severity
- 7.4 High severity
- 7.5 High severity
- 7.5 High severity
- 7.5 High severity
- 7.5 High severity
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