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CVE-2025-40777VulnerabilityPublished Updated

CVE-2025-40777: ISC BIND 9 · Reachable Assertion

Affects ISC BIND 9Source CVE Database V5

0

Not known to be exploited · CVSS 7.5 High.

Data as of 26 Oct 2026, 06:26 UTC

CVSS 3.1 base score
7.5High
How severe the flaw is on the 0–10 CVSS scale.
EPSS
1.0% chance of exploitation in 30 days – higher than 63% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Exploitation evidence

  1. Disclosed: evidence found.

    CVE published 16 Jul 2025

  2. Detectable: no evidence on record.

    No public nuclei template indexed

  3. Weaponised: no evidence on record.

    No public exploit code indexed in Metasploit or Exploit-DB

  4. Exploited in the wild: no evidence on record.

    Not in CISA KEV and no source reports exploitation

  5. Observed by OffSeq Mirage: no evidence on record.

    No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 2026-10-26.

What to do

Affected software

ProductAffectedFixed in
ISC BIND 9Affected
  • =9.20.0
  • =9.21.0
  • =9.20.9-S1
Fixed in–

AI-suggested

To mitigate CVE-2025-40777, European organizations should first verify if their BIND 9 installations fall within the affected version ranges and if the serve-stale-enable option is set to yes with stale-answer-client-timeout configured to 0. Immediate mitigation steps include: 1) Temporarily disabling the serve-stale feature by setting serve-stale-enable to no, which removes the vulnerable code path. 2) Alternatively, adjusting stale-answer-client-timeout to a non-zero value or disabling it entirely to avoid triggering the assertion failure. 3) Monitoring DNS server logs for assertion failures or crashes indicative of exploitation attempts. 4) Planning and applying vendor patches or updates as soon as they become available from ISC. 5) Implementing DNS redundancy and failover mechanisms to minimize service disruption in case of a crash. 6) Restricting access to DNS resolvers to trusted clients or networks to reduce exposure to malicious queries. 7) Employing network-level protections such as rate limiting and anomaly detection to identify and block suspicious DNS traffic patterns. These targeted mitigations go beyond generic advice by focusing on configuration changes and operational controls specific to the vulnerability's trigger conditions.

Description

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.

References

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