CVE-2025-40777 A possible assertion failure when 'stale-answer-client-timeout' is set to '0'
Attackers can cause a denial of service in BIND caching resolvers by triggering an assertion failure under specific stale-answer configurations.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2025-40777?
CVE-2025-40777 is a high-severity denial of service vulnerability in ISC BIND 9, published on 16 Jul 2025. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Weakness: CWE-617
Which versions are affected by CVE-2025-40777?
Linux distribution packages (3)
bind SUSE:Linux Enterprise Module for Basesystem 15 SP7 | All versions | 9.20.11-150700.3.6.1 |
bind SUSE:Linux Enterprise Module for Server Applications 15 SP7 | All versions | 9.20.11-150700.3.6.1 |
bind openSUSE:Tumbleweed | All versions | 9.20.11-1.1 |
How do I fix CVE-2025-40777?
Apply the vendor's patch.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2025-40777 being exploited?
There is no known exploitation of CVE-2025-40777 in the wild and no public exploit code.
Sources
Also known as ALPINE-CVE-2025-40777, AZL-65553, BELL-CVE-2025-40777, DEBIAN-CVE-2025-40777, GHSA-4x4c-8qp9-8ggh, ROOT-OS-DEBIAN-11-CVE-2025-40777 and 3 more.