CVE-2026-77860

'serve-expired' can bypass Unbound 'wait-limit'

LOW

3.7

CVSS Score

Published: September 16, 2026 at 08:29 AM UTC

Updated: September 23, 2026 at 08:40 PM UTC

1 product affected across 1 vendorUnbound

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Severity

LOW3.7

Exploit prediction

EPSS 0.3%

SSVC

No known exploit

Description

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.