CVE-2025-40777 | 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.

Published: 2025-07-16 Last update: 2026-04-15 Assigner: [email protected] Source: [email protected]
NVD Status：Deferred，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2025-40777 is rated Low Risk (39.7/100): CVSS High severity, with low exploitation likelihood (EPSS 0.10%). Monitor for updates and reassess as exploit intelligence or EPSS changes.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Exploit prediction scoring system (EPSS) score for CVE-2025-40777

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-05-12 0.02% 0.10% +0.09%
2 2025-07-17 — 0.02% —

Full EPSS history (2 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2025-40777

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
7.5 3.1 HIGH
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N)
Doesn’t really leak secrets in a meaningful way.
Integrity (I:N)
Data isn’t meaningfully altered or forged.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 3.9 3.6 [email protected]

Weakness enumeration for CVE-2025-40777

OS Trackers for CVE-2025-40777

vendor priority summary link
alpine high CVE-2025-40777: 1 source package rows (bind); 2 state rows across 2 repos (3.22-main, edge-main); fixed 2, open 0. https://security.alpinelinux.org/vuln/CVE-2025-40777
debian not yet assigned CVE-2025-40777 not yet assigned priority: Debian including 1 source packages (bind9), 5 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): resolved 3, open 2. https://security-tracker.debian.org/tracker/CVE-2025-40777
redhat medium — https://access.redhat.com/security/cve/CVE-2025-40777
suse high — https://www.suse.com/security/cve/CVE-2025-40777/
ubuntu medium CVE-2025-40777 medium priority: Ubuntu including 3 source packages (bind9, bind9-libs, isc-dhcp), 21 status rows across 8 suites (bionic, focal, jammy, noble, plucky, trusty, upstream, xenial): not-affected 15, DNE 2, needs-triage 2, released 2. https://ubuntu.com/security/CVE-2025-40777

Affected software / configurations for CVE-2025-40777

Vendor Product Version Raw CPE
No affected products in dataset.

References for CVE-2025-40777

cvelogic Threat Intelligence