CVE-2025-5994
HIGHDescription
A multi-vendor cache poisoning vulnerability named 'Rebirthday Attack' has been discovered in caching resolvers that support EDNS Client Subnet (ECS). Unbound is also vulnerable when compiled with ECS support, i.e., '--enable-subnet', AND configured to send ECS information along with queries to upstream name servers, i.e., at least one of the 'send-client-subnet', 'client-subnet-zone' or 'client-subnet-always-forward' options is used. Resolvers supporting ECS need to segregate outgoing queries to accommodate for different outgoing ECS information. This re-opens up resolvers to a birthday paradox attack (Rebirthday Attack) that tries to match the DNS transaction ID in order to cache non-ECS poisonous replies.
How to fix
This vulnerability affects an unusually large number of packages. The highest-confidence fixes are shown above; the full list is longer. Check the referenced advisories for complete coverage.
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Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.
CVSS v4 Vector
Exploitability
Impact
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:U/V:C/RE:X/U:X
Exploit Intelligence
Low risk: more likely to be exploited than 10% of all known CVEs.
References
Related Vulnerabilities
Other CWE-349 vulnerabilities, ordered by exploit likelihood. View all
| CVE | Severity | CVSS | EPSS | Exploited | Fix |
|---|---|---|---|---|---|
| CVE-2019-9535 | Critical | 9.8 | 2.5% | - | - |
| CVE-2026-32162 | High | 8.4 | 1.9% | - | Fix |
| CVE-2018-1131 | High | 8.8 | 1.3% | - | - |
| CVE-2021-21374 | High | 8.1 | 1.0% | - | Fix |
| CVE-2024-21094 | Low | 3.7 | 0.8% | - | Fix |
| CVE-2025-40778 | High | 8.6 | 0.7% | - | Fix |
Common questions
How do I fix CVE-2025-5994?
Published advisories record a fix for 48 affected products. The "How to fix" section on this page lists the fixed version and source advisory for each one, so apply the entry matching what you actually run.
Is CVE-2025-5994 being actively exploited?
Not that we know of. CVE-2025-5994 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 0.20% is the estimated probability that it will be exploited in the next 30 days. That is higher than 10% of all scored CVEs.
How severe is CVE-2025-5994?
CVE-2025-5994 has a CVSS v4 base score of 8.7, 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-2025-5994 affect?
Published advisories record a fix for python2-unbound (Amazon), python3-unbound (Amazon), python3-unbound-debuginfo (Amazon), unbound (Amazon), and 44 more. 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 2026-06-17.