CVE-2026-77860
Low CVSS 3.7Published 2026-09-16 ยท last changed 2026-09-23
The fix
We hold no Microsoft remediation data for CVE-2026-77860, which normally means it is not a Windows CVE. For non-Microsoft software the fix is the vendor's own updated release, and the affected versions are listed below.
What it is
In NLnetLabs Unbound 1.20.0 up to and including 1.26.0, a vulnerability on the 'serve-expired' code path can cause a double decrement on the 'wait-limit' counter per client IP essentially bypassing one of the counter measures that was introduced for DNSBomb (CVE-2024-33655). A malicious actor can exploit this by controlling an authoritative zone with short TTL, so cached entries expire quickly. Each 'slow' query, one the attacker's authoritative never answers, is followed by one query for an expired cached name, which is answered immediately via the 'serve-expired' path and decrements the counter twice. This second query was named 'pump'. By alternating slow queries and pumps, the attacker keeps the per-client counter at or below the configured 'wait-limit' indefinitely, and can hold an arbitrary number of pending queries from a single source IP, up to the global mesh quota (num-queries-per-thread); eventually bypassing one of the counter measures introduced for DNSBomb (CVE-2024-33655). This vulnerability is present on the 'serve-expired' code path.
Sources
CVSS vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L