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

Quick assessment

Affected
NLnet Labs Unbound
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

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 (C

CVSS 3.7 · Low
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I. Basic Information for CVE-2026-77860

Vulnerability Information

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Vulnerability Title
'serve-expired' can bypass Unbound 'wait-limit'
Source: CVE Program / CVE List V5
Vulnerability 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.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L
Source: CVE Program / CVE List V5
Vulnerability Type
对资源的重复操作
Source: CVE Program / CVE List V5

Affected Products

Vendor Product Affected Versions CPE Subscribe
NLnet Labs Unbound 1.20.0 ~ 1.26.1 -

II. Public POCs for CVE-2026-77860

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III. Intelligence Information for CVE-2026-77860

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Other References for CVE-2026-77860 (1)

Same Patch Batch · NLnet Labs · 2026-09-16 · 9 CVEs total

CVE-2026-81642 9.1 CRITICAL Heap buffer overflow and possible Remote Code Execution when digesting DNSKEY
CVE-2026-82717 8.4 HIGH CNAME synthesis could lead to heap corruption
CVE-2026-81634 7.5 HIGH Possible heap buffer overflow during DNSSEC canonicalization
CVE-2026-78227 6.5 MEDIUM Use-after-free in DoQ stream output buffer on reset re-transmission
CVE-2026-82720 5.9 MEDIUM Use-after-free in DoH stream cleanup code path
CVE-2026-85501 5.3 MEDIUM Retrap: Novel Vulnerabilities to launch Algorithmic Complexity Attacks on DNSSEC
CVE-2026-80225 5.3 MEDIUM Possible degradation of service from continuous queries on the same TCP/DoT connection
CVE-2026-77955 4.4 MEDIUM Possible ZONEMD verification bypass window

IV. Related Vulnerabilities

Same product: Unbound
Same vendor: NLnet Labs
Same weakness: CWE-675

V. Comments for CVE-2026-77860

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