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

CVE-2026-77860 is fixed in unbound on 1 platform, first by DEBIAN-CVE-2026-77860, released September 19, 2026. It is not reported as exploited. Patch Radar priority: Routine.

- Web page: https://patchblox.com/security/cve/CVE-2026-77860
- Priority: Routine
- CVSS: 3.7 (low)
- EPSS: 0.0030
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Sep 16, 2026
- Weakness: Multiple Operations on Resource in Single-Operation Context
- Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-77860

## 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.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | unbound Debian 13.5 (trixie) | 1.26.1-0+deb13u1 | [DEBIAN-CVE-2026-77860](https://patchblox.com/security/data/fix/linux/debian-cve-2026-77860.md) | Sep 19, 2026 |
| Linux | unbound Debian 14 | 1.26.1-1 | [DEBIAN-CVE-2026-77860](https://patchblox.com/security/data/fix/linux/debian-cve-2026-77860.md) | Sep 19, 2026 |
| Linux | unbound opensuse-leap 16.0 | 1.26.1-160000.1.1 | [OPENSUSE-SU-2026:22082-1](https://patchblox.com/security/data/fix/linux/opensuse-su-2026-22082-1.md) | Nov 7, 2026 |
| Linux | unbound sles 15.6 | 1.26.1-150600.23.24.1 | [SUSE-SU-2026:4620-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-4620-1.md) | Nov 7, 2026 |
| Linux | unbound sles 15.7 | 1.26.1-150600.23.24.1 | [SUSE-SU-2026:4620-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-4620-1.md) | Nov 7, 2026 |
| Linux | unbound sles 16.0 | 1.26.1-160000.1.1 | [SUSE-SU-2026:24226-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-24226-1.md) | Nov 7, 2026 |

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Data as of 2026-11-08T13:06:02Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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