CNAME Chain Assertion Failure in BIND 9 9.20.0–9.20.10 Caching Resolver
CVE-2025-40777 Published on July 16, 2025
A possible assertion failure when 'stale-answer-client-timeout' is set to '0'
If a `named` caching resolver is configured with `serve-stale-enable` `yes`, and with `stale-answer-client-timeout` set to `0` (the only allowable value other than `disabled`), and if the resolver, in the process of resolving a query, encounters a CNAME chain involving a specific combination of cached or authoritative records, the daemon will abort with an assertion failure.
This issue affects BIND 9 versions 9.20.0 through 9.20.10, 9.21.0 through 9.21.9, and 9.20.9-S1 through 9.20.10-S1.
Vulnerability Analysis
CVE-2025-40777 can be exploited with network access, and does not require authorization privileges or user interaction. This vulnerability is considered to have a low attack complexity. The potential impact of an exploit of this vulnerability is considered to have no impact on confidentiality and integrity, and a high impact on availability.
Weakness Type
What is an assertion failure Vulnerability?
The product contains an assert() or similar statement that can be triggered by an attacker, which leads to an application exit or other behavior that is more severe than necessary.
CVE-2025-40777 has been classified to as an assertion failure vulnerability or weakness.
Products Associated with CVE-2025-40777
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Affected Versions
ISC BIND 9:- Version 9.20.0, <= 9.20.10 is affected.
- Version 9.21.0, <= 9.21.9 is affected.
- Version 9.20.9-S1, <= 9.20.10-S1 is affected.
- Version 9.18.0, <= 9.18.37 is unaffected.
- Version 9.18.11-S1, <= 9.18.37-S1 is unaffected.
Exploit Probability
EPSS (Exploit Prediction Scoring System) scores estimate the probability that a vulnerability will be exploited in the wild within the next 30 days. The percentile shows you how this score compares to all other vulnerabilities.