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Vulnerability · Published Sep 16, 2026 · Assigned by NLnet Labs

CVE-2026-77860'serve-expired' can bypass Unbound 'wait-limit'

Low 3.7

In plain English

A low-severity security flaw in Unbound from NLnet Labs. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 23% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)3.7Low
Exploit odds (30 days)0.3%top 77.0% of all CVEs
Exploited in the wildNo reportsCISA SSVC: none
Attack toolingNoneno public tooling known

Official 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.

Affected packages & fixed versions

EcosystemPackageFixed inAdvisory
Debian:12unboundno fix listedDEBIAN-CVE-2026-77860
Debian:13unbound1.26.1-0+deb13u1DEBIAN-CVE-2026-77860
Debian:14unbound1.26.1-1DEBIAN-CVE-2026-77860
Ubuntu:22.04:LTSunboundno fix listedUBUNTU-CVE-2026-77860
Ubuntu:24.04:LTSunboundno fix listedUBUNTU-CVE-2026-77860
Ubuntu:26.04:LTSunboundno fix listedUBUNTU-CVE-2026-77860
Ubuntu:26.10unboundno fix listedUBUNTU-CVE-2026-77860

Affected products (as reported)

VendorProductVersions
NLnet Labs Unbound 1.20.0 – before 1.26.1

Weakness: CWE-675 · Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L

References