CVE-2026-77860
LOWDescription
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.
How to fix
TridentStack Control can deploy fixes like this automatically across your Windows, macOS, and Linux fleet. See how it works
Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.
CVSS v3 Vector
Exploitability
Impact
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L
Exploit Intelligence
Low risk: more likely to be exploited than 20% of all known CVEs.
References
Related Vulnerabilities
Other CWE-675 vulnerabilities, ordered by exploit likelihood. View all
| CVE | Severity | CVSS | EPSS | Exploited | Fix |
|---|---|---|---|---|---|
| CVE-2019-17638 | Critical | 9.4 | 11% | - | Fix |
| CVE-2025-68973 | High | 7.8 | 0.1% | - | Fix |
Common questions
How do I fix CVE-2026-77860?
Upgrade unbound on debian unstable to 1.26.1-1 or later.
Is CVE-2026-77860 being actively exploited?
Not that we know of. CVE-2026-77860 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 0.27% is the estimated probability that it will be exploited in the next 30 days. That is higher than 20% of all scored CVEs.
How severe is CVE-2026-77860?
CVE-2026-77860 has a CVSS v3 base score of 3.7, rated low. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.
What does CVE-2026-77860 affect?
Published advisories record a fix for unbound (Debian). Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.
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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-09-16.