CVE-2026-77860 (GCVE-0-2026-77860)

Vulnerability from cvelistv5 – Published: 2026-09-16 08:29 – Updated: 2026-09-16 14:45
VLAI
Title
'serve-expired' can bypass Unbound 'wait-limit'
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 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.
Severity
3.7 (Low)
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L
SSVC
Exploitation: none Automatable: no Technical Impact: partial
CISA Coordinator · CISA-ADP (v2.0.3)
Decision recorded 2026-09-16 14:45 UTC
CWE
  • CWE-675 - Multiple Operations on Resource in Single-Operation Context
Assigner
NLnet Labs CNA
CNA scorecard B 82/100 over 76 records in the last 180 days details
References
Impacted products
Vendor Product Version CPE status
NLnet Labs Unbound Affected: 1.20.0 , < 1.26.1 (semver)
guessed Create a notification for this product.
Date Public
2026-09-16 00:00
Credits
Xuanchao Xie (University of Science and Technology of China) Lutong Chen (University of Science and Technology of China) Kaiping Xue (University of Science and Technology of China)
Show details on NVD website
To clipboard 

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Solutions

CNA · NLnet Labs
This issue is fixed starting with version 1.26.1

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