{"dataType": "CVE_RECORD", "dataVersion": "5.2", "cveMetadata": {"cveId": "CVE-2026-77860", "assignerOrgId": "206fc3a0-e175-490b-9eaa-a5738056c9f6", "state": "PUBLISHED", "assignerShortName": "NLnet Labs", "dateReserved": "2026-09-07T14:06:21.971Z", "datePublished": "2026-09-16T08:29:23.501Z", "dateUpdated": "2026-09-16T08:29:23.501Z"}, "containers": {"cna": {"title": "'serve-expired' can bypass Unbound 'wait-limit'", "datePublic": "2026-09-16T00:00:00.000Z", "affected": [{"vendor": "NLnet Labs", "product": "Unbound", "versions": [{"version": "1.20.0", "status": "affected", "lessThan": "1.26.1", "versionType": "semver"}], "defaultStatus": "unaffected"}], "descriptions": [{"lang": "en", "value": "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."}], "metrics": [{"format": "CVSS", "scenarios": [{"lang": "en", "value": "'serve-expired' configuration option used"}], "cvssV3_1": {"version": "3.1", "baseScore": 3.7, "baseSeverity": "LOW", "vectorString": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L"}}], "problemTypes": [{"descriptions": [{"lang": "en", "cweId": "CWE-675", "description": "CWE-675: Multiple Operations on Resource in Single-Operation Context", "type": "CWE"}]}], "solutions": [{"lang": "en", "value": "This issue is fixed starting with version 1.26.1"}], "timeline": [{"time": "2026-07-30T00:00:00.000Z", "lang": "en", "value": "Issue reported by Xuanchao Xie"}, {"time": "2026-07-30T00:00:00.000Z", "lang": "en", "value": "NLnet Labs shares patch"}, {"time": "2026-07-30T00:00:00.000Z", "lang": "en", "value": "Xuanchao Xie verifies patch"}, {"time": "2026-09-16T00:00:00.000Z", "lang": "en", "value": "Fixes released with version 1.26.1"}], "credits": [{"lang": "en", "value": "Xuanchao Xie (University of Science and Technology of China)", "type": "finder"}, {"lang": "en", "value": "Lutong Chen (University of Science and Technology of China)", "type": "finder"}, {"lang": "en", "value": "Kaiping Xue (University of Science and Technology of China)", "type": "finder"}], "references": [{"url": "https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-77860.txt", "tags": ["vendor-advisory"]}], "providerMetadata": {"orgId": "206fc3a0-e175-490b-9eaa-a5738056c9f6", "shortName": "NLnet Labs", "dateUpdated": "2026-09-16T08:29:23.501Z"}, "x_generator": {"engine": "cvelib 1.8.0"}}}}

{}

{"affected": [{"affectedData": [{"defaultStatus": "unaffected", "product": "Unbound", "vendor": "NLnet Labs", "versions": [{"lessThan": "1.26.1", "status": "affected", "version": "1.20.0", "versionType": "semver"}]}], "source": "sep@nlnetlabs.nl"}], "cveTags": [], "descriptions": [{"lang": "en", "value": "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."}], "id": "CVE-2026-77860", "lastModified": "2026-09-16T09:17:05.623", "metrics": {"cvssMetricV31": [{"cvssData": {"attackComplexity": "HIGH", "attackVector": "NETWORK", "availabilityImpact": "LOW", "baseScore": 3.7, "baseSeverity": "LOW", "confidentialityImpact": "NONE", "integrityImpact": "NONE", "privilegesRequired": "NONE", "scope": "UNCHANGED", "userInteraction": "NONE", "vectorString": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L", "version": "3.1"}, "exploitabilityScore": 2.2, "impactScore": 1.4, "source": "sep@nlnetlabs.nl", "type": "Secondary"}]}, "published": "2026-09-16T09:17:05.623", "references": [{"source": "sep@nlnetlabs.nl", "url": "https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-77860.txt"}], "sourceIdentifier": "sep@nlnetlabs.nl", "vulnStatus": "Received", "weaknesses": [{"description": [{"lang": "en", "value": "CWE-675"}], "source": "sep@nlnetlabs.nl", "type": "Secondary"}]}

{"bugzilla": {"description": "unbound: Unbound: Denial of Service via 'serve-expired' code path bypass", "id": "2535050", "url": "https://bugzilla.redhat.com/show_bug.cgi?id=2535050"}, "csaw": false, "cvss3": {"cvss3_base_score": "3.7", "cvss3_scoring_vector": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L", "status": "draft"}, "cwe": "CWE-911", "details": ["A flaw was found in Unbound. A remote attacker can exploit a vulnerability in the 'serve-expired' code path by sending specially crafted DNS queries. This manipulation causes a double decrement of a security counter, effectively bypassing a protection mechanism designed to limit pending queries. Consequently, an attacker can hold an arbitrary number of pending queries from a single source, leading to a Denial of Service (DoS) condition."], "name": "CVE-2026-77860", "package_state": [{"cpe": "cpe:/a:redhat:openshift:4", "fix_state": "Fix deferred", "package_name": "openshift/ose-rhel-coreos-8", "product_name": "Red Hat OpenShift Container Platform 4"}, {"cpe": "cpe:/a:redhat:openshift:4", "fix_state": "Fix deferred", "package_name": "openshift/ose-rhel-coreos-9", "product_name": "Red Hat OpenShift Container Platform 4"}], "public_date": "2026-09-16T08:29:23Z", "references": ["https://www.cve.org/CVERecord?id=CVE-2026-77860\nhttps://nvd.nist.gov/vuln/detail/CVE-2026-77860\nhttps://www.nlnetlabs.nl/downloads/unbound/CVE-2026-77860.txt"], "threat_severity": "Low"}

{}