CVE-2026-77860
Received
Received - Intake
Integer Overflow in Unbound DNS Resolver
Vulnerability report for CVE-2026-77860, including description, CVSS score, EPSS score, affected products, exploitability, helpful resources, and attack-flow context.
Publication date: 2026-09-16
Last updated on: 2026-09-16
Assigner: NLnet Labs
Description
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.
CVSS Scores
EPSS Scores
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Meta Information
Affected Vendors & Products
| Vendor | Product | Version / Range |
|---|---|---|
| nlnetlabs | unbound | to 1.26.0 (inc) |
| nlnetlabs | unbound | 1.26.1 |
Helpful Resources
Exploitability
CWE
KEV
| CWE ID | Description |
|---|---|
| CWE-675 | The product performs the same operation on a resource two or more times, when the operation should only be applied once. |
