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CVE-2026-77860

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

This page contains facts from public vulnerability sources. Check the original advisory before updating.

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 3.7 · Low · EPSS 0.3% · Fix available

Published
Updated

Stack: system

Affected products and versions

  • unbound

    Versions
    • ≥ 1.20.0, < 1.26.1: Affected (semver)

    Update to 1.26.1

  • unbound Debian:12

    Versions
    • 1.17.1-2
    • 1.17.1-2+deb12u1
    • 1.17.1-2+deb12u2
    • 1.17.1-2+deb12u3
    • 1.17.1-2+deb12u4
    • 1.17.1-2+loong64
    • 1.18.0-1
    • 1.18.0-2
    • 1.19.1-1
    • 1.19.2-1
    • 1.20.0-1
    • 1.21.1-1
    • 1.22.0-1
    • 1.22.0-2
    • 1.23.1-1
    • 1.24.0-1
    • 1.24.0-2
    • 1.24.1-1
    • 1.24.1-2
    • 1.24.2-1
    • 1.25.0-1
    • 1.25.1-1
    • 1.25.2-1
    • 1.26.0-1
    • 1.26.0-2
    • 1.26.1-1
    Affected ranges
    • ECOSYSTEM: (−∞, +∞)

    No fix published

  • unbound Debian:13

    Versions
    • 1.22.0-2
    • 1.22.0-2+deb13u1
    • 1.22.0-2+deb13u2
    • 1.22.0-2+deb13u3
    • 1.23.1-1
    • 1.24.0-1
    • 1.24.0-2
    • 1.24.1-1
    • 1.24.1-2
    • 1.24.2-1
    • 1.25.0-1
    • 1.25.1-1
    • 1.25.2-1
    • 1.26.0-1
    • 1.26.0-2
    Affected ranges
    • ECOSYSTEM: < 1.26.1-0+deb13u1

    Update to 1.26.1-0+deb13u1

  • unbound Debian:14

    Versions
    • 1.22.0-2
    • 1.23.1-1
    • 1.24.0-1
    • 1.24.0-2
    • 1.24.1-1
    • 1.24.1-2
    • 1.24.2-1
    • 1.25.0-1
    • 1.25.1-1
    • 1.25.2-1
    • 1.26.0-1
    • 1.26.0-2
    Affected ranges
    • ECOSYSTEM: < 1.26.1-1

    Update to 1.26.1-1

  • unbound Ubuntu:Pro:14.04:LTS

    Versions
    • 1.4.20-1
    • 1.4.21-1
    • 1.4.21-1ubuntu1
    • 1.4.21-1ubuntu2
    • 1.4.22-1ubuntu1
    • 1.4.22-1ubuntu2
    • 1.4.22-1ubuntu3
    • 1.4.22-1ubuntu4
    • 1.4.22-1ubuntu4.14.04.1
    • 1.4.22-1ubuntu4.14.04.2
    • 1.4.22-1ubuntu4.14.04.3
    • 1.4.22-1ubuntu4.14.04.3+esm1
    • 1.4.22-1ubuntu4.14.04.3+esm2
    • 1.4.22-1ubuntu4.14.04.3+esm3
    • 1.4.22-1ubuntu4.14.04.3+esm4
    • 1.4.22-1ubuntu4.14.04.3+esm5
    Affected ranges
    • ECOSYSTEM: (−∞, +∞)

    No fix published

  • unbound Ubuntu:Pro:16.04:LTS

    Versions
    • 1.4.22-1ubuntu6
    • 1.5.7-1ubuntu1
    • 1.5.7-1ubuntu2
    • 1.5.8-1ubuntu1
    • 1.5.8-1ubuntu1.1
    • 1.5.8-1ubuntu1.1+esm1
    • 1.5.8-1ubuntu1.1+esm2
    • 1.5.8-1ubuntu1.1+esm3
    • 1.5.8-1ubuntu1.1+esm4
    • 1.5.8-1ubuntu1.1+esm5
    Affected ranges
    • ECOSYSTEM: (−∞, +∞)

    No fix published

  • unbound Ubuntu:Pro:18.04:LTS

    Versions
    • 1.6.5-1
    • 1.6.7-1
    • 1.6.7-1build1
    • 1.6.7-1ubuntu1
    • 1.6.7-1ubuntu2
    • 1.6.7-1ubuntu2.1
    • 1.6.7-1ubuntu2.2
    • 1.6.7-1ubuntu2.3
    • 1.6.7-1ubuntu2.4
    • 1.6.7-1ubuntu2.5
    • 1.6.7-1ubuntu2.6
    • 1.6.7-1ubuntu2.6+esm2
    • 1.6.7-1ubuntu2.6+esm3
    • 1.6.7-1ubuntu2.6+esm4
    • 1.6.7-1ubuntu2.6+esm5
    • 1.6.7-1ubuntu2.6+esm6
    Affected ranges
    • ECOSYSTEM: (−∞, +∞)

    No fix published

  • unbound Ubuntu:Pro:20.04:LTS

    Versions
    • 1.9.0-2ubuntu1
    • 1.9.0-2ubuntu2
    • 1.9.4-2
    • 1.9.4-2ubuntu1
    • 1.9.4-2ubuntu1.1
    • 1.9.4-2ubuntu1.2
    • 1.9.4-2ubuntu1.3
    • 1.9.4-2ubuntu1.4
    • 1.9.4-2ubuntu1.5
    • 1.9.4-2ubuntu1.6
    • 1.9.4-2ubuntu1.8
    • 1.9.4-2ubuntu1.9
    • 1.9.4-2ubuntu1.11
    • 1.9.4-2ubuntu1.11+esm1
    • 1.9.4-2ubuntu1.11+esm2
    • 1.9.4-2ubuntu1.11+esm3
    Affected ranges
    • ECOSYSTEM: (−∞, +∞)

    No fix published

  • unbound Ubuntu:22.04:LTS

    Versions
    • 1.13.1-1ubuntu1
    • 1.13.1-1ubuntu3
    • 1.13.1-1ubuntu5
    • 1.13.1-1ubuntu5.1
    • 1.13.1-1ubuntu5.2
    • 1.13.1-1ubuntu5.3
    • 1.13.1-1ubuntu5.4
    • 1.13.1-1ubuntu5.5
    • 1.13.1-1ubuntu5.7
    • 1.13.1-1ubuntu5.8
    • 1.13.1-1ubuntu5.10
    • 1.13.1-1ubuntu5.11
    • 1.13.1-1ubuntu5.12
    • 1.13.1-1ubuntu5.13
    • 1.13.1-1ubuntu5.14
    • 1.13.1-1ubuntu5.15
    • 1.13.1-1ubuntu5.16
    • 1.13.1-1ubuntu5.17
    Affected ranges
    • ECOSYSTEM: (−∞, +∞)

    No fix published

  • unbound Ubuntu:24.04:LTS

    Versions
    • 1.17.1-2
    • 1.18.0-2ubuntu1
    • 1.18.0-2ubuntu2
    • 1.19.1-1ubuntu1
    • 1.19.2-1ubuntu1
    • 1.19.2-1ubuntu3
    • 1.19.2-1ubuntu3.1
    • 1.19.2-1ubuntu3.2
    • 1.19.2-1ubuntu3.3
    • 1.19.2-1ubuntu3.4
    • 1.19.2-1ubuntu3.5
    • 1.19.2-1ubuntu3.6
    • 1.19.2-1ubuntu3.7
    • 1.19.2-1ubuntu3.8
    • 1.19.2-1ubuntu3.9
    • 1.19.2-1ubuntu3.10
    • 1.19.2-1ubuntu3.12
    Affected ranges
    • ECOSYSTEM: (−∞, +∞)

    No fix published

  • unbound Ubuntu:26.10

    Versions
    • 1.24.2-1ubuntu2
    • 1.25.1-1ubuntu1
    • 1.25.1-1ubuntu3
    Affected ranges
    • ECOSYSTEM: (−∞, +∞)

    No fix published

  • unbound Ubuntu:26.04:LTS

    Versions
    • 1.22.0-2ubuntu2
    • 1.22.0-2ubuntu4
    • 1.24.2-1ubuntu1
    • 1.24.2-1ubuntu2
    • 1.24.2-1ubuntu2.1
    • 1.24.2-1ubuntu2.2
    • 1.24.2-1ubuntu2.3
    • 1.24.2-1ubuntu2.4
    Affected ranges
    • ECOSYSTEM: (−∞, +∞)

    No fix published

Original sources

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