---
id: CVE-2026-77860
title: >-
  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 in…
summary: >-
  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 in…
severity: low
cvss: 3.7
cvssVector: 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L'
cwe:
  - CWE-675
  - CWE-911
vendor: nlnetlabs
product: unbound
affected:
  - 'unbound >= 1.20.0, < 1.26.1'
patched:
  - unbound 1.26.1
published: '2026-09-16'
updated: '2026-09-23'
sourceUpdated: '2026-09-23T20:21:00.757'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-77860'
references:
  - url: 'https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-77860.txt'
    label: sep@nlnetlabs.nl
  - url: >-
      https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-77860.json
  - url: 'https://access.redhat.com/security/cve/CVE-2026-77860'
  - url: 'https://bugzilla.redhat.com/show_bug.cgi?id=2535050'
  - url: 'https://www.cve.org/CVERecord?id=CVE-2026-77860'
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2026-77860'
  - url: 'https://access.redhat.com/errata/RHSA-2026:68590'
tags:
  - nvd
  - cve.org
  - csaf
  - vex
  - red-hat
epss: 0.00327
epssPercentile: 0.23044
ssvc:
  exploitation: none
  automatable: 'no'
  technicalImpact: partial
  timestamp: '2026-09-16T14:45:26.867772Z'
ingestedAt: '2026-09-16T08:52:29.599Z'
---

## Overview

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

- `unbound >= 1.20.0, < 1.26.1`

## Remediation

Upgrade past the affected range:

- `unbound 1.26.1`

## Vendor advisories

- **Red Hat VEX** · Low · affected: Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 9, Red Hat OpenShift Container Platform 4 · no fix planned: Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 9, Red Hat OpenShift Container Platform 4 · updated 2026-09-21 · [vex](https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-77860.json)
- **RHSA-2026:68590** · Red Hat · fixed in: Red Hat Hardened Images · released 2026-09-17 · [advisory](https://access.redhat.com/errata/RHSA-2026:68590)