CVE-2026-55991

{"dataType": "CVE_RECORD", "dataVersion": "5.2", "cveMetadata": {"cveId": "CVE-2026-55991", "assignerOrgId": "206fc3a0-e175-490b-9eaa-a5738056c9f6", "state": "PUBLISHED", "assignerShortName": "NLnet Labs", "dateReserved": "2026-06-22T10:11:10.505Z", "datePublished": "2026-07-22T13:10:13.235Z", "dateUpdated": "2026-09-16T14:24:57.805Z"}, "containers": {"cna": {"affected": [{"defaultStatus": "unaffected", "product": "Unbound", "vendor": "NLnet Labs", "versions": [{"lessThan": "1.25.2", "status": "affected", "version": "1.22.0", "versionType": "semver"}]}], "credits": [{"lang": "en", "type": "finder", "value": "Qifan Zhang (Palo Alto Networks)"}, {"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"}], "datePublic": "2026-07-22T00:00:00.000Z", "descriptions": [{"lang": "en", "value": "In NLnet Labs Unbound 1.22.0 up to and including 1.25.1, a remote unauthenticated client can trigger a libngtcp2 assertion (if compiled with assertions on) and terminate the entire Unbound process using a single DNS-over-QUIC (DoQ) connection and one normal DNS query. This is caused by an erroneous error value passed to libngtcp2. When 'ngtcp2_conn_writev_stream()' returns 'NGTCP2_ERR_STREAM_DATA_BLOCKED', Unbound continues to call 'ngtcp2_ccerr_set_application_error()' with a '-1' error value. The 'int' literal '-1' is implicitly converted to the function's 'uint64_t error_code' parameter as '0xFFFFFFFFFFFFFFFF'. The follow-on 'ngtcp2_conn_write_connection_close()' serialises that value as a QUIC variable-length integer; because '2^64-1' exceeds the 62-bit varint ceiling, 'ngtcp2_put_uvarintlen()' fails 'assert(n < 4611686018427387904ULL)' and the whole resolver process aborts. A remote, unauthenticated DoQ client can trigger this deterministically with a single QUIC connection by advertising 'initial_max_stream_data_bidi_local = 1' in its transport parameters and sending one DoQ query without ever reading the stream."}], "metrics": [{"cvssV3_1": {"baseScore": 5.9, "baseSeverity": "MEDIUM", "vectorString": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H", "version": "3.1"}, "format": "CVSS", "scenarios": [{"lang": "en", "value": "Unbound compiled with DoQ support ('--with-libngtcp2')"}]}], "problemTypes": [{"descriptions": [{"cweId": "CWE-195", "description": "CWE-195: Signed to Unsigned Conversion Error", "lang": "en", "type": "CWE"}]}], "providerMetadata": {"orgId": "206fc3a0-e175-490b-9eaa-a5738056c9f6", "shortName": "NLnet Labs", "dateUpdated": "2026-09-16T14:24:57.805Z"}, "references": [{"tags": ["vendor-advisory"], "url": "https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-55991.txt"}], "solutions": [{"lang": "en", "value": "This issue is fixed starting with version 1.25.2"}], "timeline": [{"lang": "en", "time": "2026-05-27T00:00:00.000Z", "value": "Issue reported by Qifan Zhang"}, {"lang": "en", "time": "2026-06-15T00:00:00.000Z", "value": "NLnet Labs shares patch with Qifan Zhang"}, {"lang": "en", "time": "2026-06-17T00:00:00.000Z", "value": "Qifan Zhang verifies patch"}, {"lang": "en", "time": "2026-07-11T00:00:00.000Z", "value": "Issue reported by Xuanchao Xie"}, {"lang": "en", "time": "2026-07-12T00:00:00.000Z", "value": "NLnet Labs shares patch with Xuanchao Xie"}, {"lang": "en", "time": "2026-06-13T00:00:00.000Z", "value": "Xuanchao Xie verifies patch"}, {"lang": "en", "time": "2026-07-22T00:00:00.000Z", "value": "Fixes released with version 1.25.2"}], "title": "Remote DNS-over-QUIC (DoQ) flow-control assertion failure in libngtcp2", "x_generator": {"engine": "cvelib 1.8.0"}}, "adp": [{"metrics": [{"other": {"type": "ssvc", "content": {"timestamp": "2026-07-22T14:01:59.779165Z", "id": "CVE-2026-55991", "options": [{"Exploitation": "none"}, {"Automatable": "no"}, {"Technical Impact": "partial"}], "role": "CISA Coordinator", "version": "2.0.3"}}}], "title": "CISA ADP Vulnrichment", "providerMetadata": {"orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0", "shortName": "CISA-ADP", "dateUpdated": "2026-07-22T14:03:20.789Z"}}]}}

{"dataType": "CVE_RECORD", "containers": {"adp": [{"title": "CISA ADP Vulnrichment", "metrics": [{"other": {"type": "ssvc", "content": {"id": "CVE-2026-55991", "role": "CISA Coordinator", "options": [{"Exploitation": "none"}, {"Automatable": "no"}, {"Technical Impact": "partial"}], "version": "2.0.3", "timestamp": "2026-07-22T14:01:59.779165Z"}}}], "providerMetadata": {"orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0", "shortName": "CISA-ADP", "dateUpdated": "2026-07-22T14:03:16.153Z"}}], "cna": {"title": "Remote DNS-over-QUIC (DoQ) flow-control assertion failure in libngtcp2", "credits": [{"lang": "en", "type": "finder", "value": "Qifan Zhang (Palo Alto Networks)"}, {"lang": "en", "type": "finder", "value": "Xuanchao Xie (University of Science and Technology of China)"}, {"lang": "en", "type": "finder", "value": "Lutong Chen (University of Science and Technology of China)"}, {"lang": "en", "type": "finder", "value": "Kaiping Xue (University of Science and Technology of China)"}], "metrics": [{"format": "CVSS", "cvssV3_1": {"version": "3.1", "baseScore": 5.9, "baseSeverity": "MEDIUM", "vectorString": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H"}, "scenarios": [{"lang": "en", "value": "Unbound compiled with DoQ support ('--with-libngtcp2')"}]}], "affected": [{"vendor": "NLnet Labs", "product": "Unbound", "versions": [{"status": "affected", "version": "1.22.0", "lessThan": "1.25.2", "versionType": "semver"}], "defaultStatus": "unaffected"}], "timeline": [{"lang": "en", "time": "2026-05-27T00:00:00.000Z", "value": "Issue reported by Qifan Zhang"}, {"lang": "en", "time": "2026-06-15T00:00:00.000Z", "value": "NLnet Labs shares patch with Qifan Zhang"}, {"lang": "en", "time": "2026-06-17T00:00:00.000Z", "value": "Qifan Zhang verifies patch"}, {"lang": "en", "time": "2026-07-11T00:00:00.000Z", "value": "Issue reported by Xuanchao Xie"}, {"lang": "en", "time": "2026-07-12T00:00:00.000Z", "value": "NLnet Labs shares patch with Xuanchao Xie"}, {"lang": "en", "time": "2026-06-13T00:00:00.000Z", "value": "Xuanchao Xie verifies patch"}, {"lang": "en", "time": "2026-07-22T00:00:00.000Z", "value": "Fixes released with version 1.25.2"}], "solutions": [{"lang": "en", "value": "This issue is fixed starting with version 1.25.2"}], "datePublic": "2026-07-22T00:00:00.000Z", "references": [{"url": "https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-55991.txt", "tags": ["vendor-advisory"]}], "x_generator": {"engine": "cvelib 1.8.0"}, "descriptions": [{"lang": "en", "value": "In NLnet Labs Unbound 1.22.0 up to and including 1.25.1, a remote unauthenticated client can trigger a libngtcp2 assertion (if compiled with assertions on) and terminate the entire Unbound process using a single DNS-over-QUIC (DoQ) connection and one normal DNS query. This is caused by an erroneous error value passed to libngtcp2. When 'ngtcp2_conn_writev_stream()' returns 'NGTCP2_ERR_STREAM_DATA_BLOCKED', Unbound continues to call 'ngtcp2_ccerr_set_application_error()' with a '-1' error value. The 'int' literal '-1' is implicitly converted to the function's 'uint64_t error_code' parameter as '0xFFFFFFFFFFFFFFFF'. The follow-on 'ngtcp2_conn_write_connection_close()' serialises that value as a QUIC variable-length integer; because '2^64-1' exceeds the 62-bit varint ceiling, 'ngtcp2_put_uvarintlen()' fails 'assert(n < 4611686018427387904ULL)' and the whole resolver process aborts. A remote, unauthenticated DoQ client can trigger this deterministically with a single QUIC connection by advertising 'initial_max_stream_data_bidi_local = 1' in its transport parameters and sending one DoQ query without ever reading the stream."}], "problemTypes": [{"descriptions": [{"lang": "en", "type": "CWE", "cweId": "CWE-195", "description": "CWE-195: Signed to Unsigned Conversion Error"}]}], "providerMetadata": {"orgId": "206fc3a0-e175-490b-9eaa-a5738056c9f6", "shortName": "NLnet Labs", "dateUpdated": "2026-09-16T14:24:57.805Z"}}}, "cveMetadata": {"cveId": "CVE-2026-55991", "state": "PUBLISHED", "dateUpdated": "2026-09-16T14:24:57.805Z", "dateReserved": "2026-06-22T10:11:10.505Z", "assignerOrgId": "206fc3a0-e175-490b-9eaa-a5738056c9f6", "datePublished": "2026-07-22T13:10:13.235Z", "assignerShortName": "NLnet Labs"}, "dataVersion": "5.2"}

{"affected": [{"affectedData": [{"defaultStatus": "unaffected", "product": "Unbound", "vendor": "NLnet Labs", "versions": [{"lessThan": "1.25.2", "status": "affected", "version": "1.22.0", "versionType": "semver"}]}], "source": "sep@nlnetlabs.nl"}], "configurations": [{"nodes": [{"cpeMatch": [{"criteria": "cpe:2.3:a:nlnetlabs:unbound:*:*:*:*:*:*:*:*", "matchCriteriaId": "0C48102E-1D81-4A87-898D-F1CE83C78D26", "versionEndExcluding": "1.25.2", "versionStartIncluding": "1.22.0", "vulnerable": true}], "negate": false, "operator": "OR"}]}], "cveTags": [], "descriptions": [{"lang": "en", "value": "In NLnet Labs Unbound 1.22.0 up to and including 1.25.1, a remote unauthenticated client can trigger a libngtcp2 assertion (if compiled with assertions on) and terminate the entire Unbound process using a single DNS-over-QUIC (DoQ) connection and one normal DNS query. This is caused by an erroneous error value passed to libngtcp2. When 'ngtcp2_conn_writev_stream()' returns 'NGTCP2_ERR_STREAM_DATA_BLOCKED', Unbound continues to call 'ngtcp2_ccerr_set_application_error()' with a '-1' error value. The 'int' literal '-1' is implicitly converted to the function's 'uint64_t error_code' parameter as '0xFFFFFFFFFFFFFFFF'. The follow-on 'ngtcp2_conn_write_connection_close()' serialises that value as a QUIC variable-length integer; because '2^64-1' exceeds the 62-bit varint ceiling, 'ngtcp2_put_uvarintlen()' fails 'assert(n < 4611686018427387904ULL)' and the whole resolver process aborts. A remote, unauthenticated DoQ client can trigger this deterministically with a single QUIC connection by advertising 'initial_max_stream_data_bidi_local = 1' in its transport parameters and sending one DoQ query without ever reading the stream."}, {"lang": "es", "value": "En NLnet Labs Unbound 1.22.0 hasta la versi\u00f3n 1.25.1 inclusive, un cliente remoto no autenticado puede activar una aserci\u00f3n de libngtcp2 (si se compila con aserciones activadas) y terminar todo el proceso de Unbound utilizando una \u00fanica conexi\u00f3n DNS-over-QUIC (DoQ) y una consulta DNS normal. Esto es causado por un valor de error err\u00f3neo pasado a libngtcp2. Cuando \u2019ngtcp2_conn_writev_stream()\u2019 devuelve \u2019NGTCP2_ERR_STREAM_DATA_BLOCKED\u2019, Unbound contin\u00faa llamando a \u2019ngtcp2_ccerr_set_application_error()\u2019 con un valor de error de \u2019-1\u2019. El literal \u2019int\u2019 \u2019-1\u2019 se convierte impl\u00edcitamente al par\u00e1metro \u2019uint64_t error_code\u2019 de la funci\u00f3n como \u20190xFFFFFFFFFFFFFFFF\u2019. La subsiguiente \u2019ngtcp2_conn_write_connection_close()\u2019 serializa ese valor como un entero de longitud variable QUIC; debido a que \u20192^64-1\u2019 excede el l\u00edmite de varint de 62 bits, \u2019ngtcp2_put_uvarintlen()\u2019 falla en \u2019assert(n < 4611686018427387904ULL)\u2019 y todo el proceso del resolvedor se aborta. Un cliente DoQ remoto no autenticado puede activar esto de forma determinista con una \u00fanica conexi\u00f3n QUIC al anunciar \u2019initial_max_stream_data_bidi_local = 1\u2019 en sus par\u00e1metros de transporte y enviando una consulta DoQ sin leer nunca el flujo."}], "id": "CVE-2026-55991", "lastModified": "2026-10-24T13:10:00.147", "metrics": {"cvssMetricV31": [{"cvssData": {"attackComplexity": "HIGH", "attackVector": "NETWORK", "availabilityImpact": "HIGH", "baseScore": 5.9, "baseSeverity": "MEDIUM", "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:H", "version": "3.1"}, "exploitabilityScore": 2.2, "impactScore": 3.6, "source": "sep@nlnetlabs.nl", "type": "Secondary"}], "ssvcV203": [{"source": "134c704f-9b21-4f2e-91b3-4a467353bcc0", "ssvcData": {"id": "CVE-2026-55991", "options": [{"exploitation": "none"}, {"automatable": "no"}, {"technicalImpact": "partial"}], "role": "CISA Coordinator", "timestamp": "2026-07-22T14:01:59.779165Z", "version": "2.0.3"}}]}, "published": "2026-07-22T14:17:21.957", "references": [{"source": "sep@nlnetlabs.nl", "tags": ["Vendor Advisory"], "url": "https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-55991.txt"}], "sourceIdentifier": "sep@nlnetlabs.nl", "vulnStatus": "Analyzed", "weaknesses": [{"description": [{"lang": "en", "value": "CWE-195"}], "source": "sep@nlnetlabs.nl", "type": "Secondary"}]}

{"affected_release": [{"advisory": "RHSA-2026:43588", "cpe": "cpe:/a:redhat:hummingbird:1", "package": "unbound-main-1.25.2-0.1.hum1", "product_name": "Red Hat Hardened Images", "release_date": "2026-07-22T00:00:00Z"}], "bugzilla": {"description": "unbound: Unbound: Denial of Service via crafted DNS-over-QUIC connection", "id": "2506147", "url": "https://bugzilla.redhat.com/show_bug.cgi?id=2506147"}, "csaw": false, "cvss3": {"cvss3_base_score": "5.9", "cvss3_scoring_vector": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H", "status": "verified"}, "cwe": "CWE-190", "details": ["A flaw was found in Unbound. A remote, unauthenticated client can exploit this vulnerability by sending a specially crafted DNS-over-QUIC (DoQ) connection and a single DNS query. This can trigger an assertion failure in the libngtcp2 library, leading to the termination of the entire Unbound process. This results in a Denial of Service (DoS), making the DNS resolver unavailable."], "mitigation": {"lang": "en:us", "value": "To prevent exploitation, disable DNS-over-QUIC (DoQ) functionality within Unbound if it is not actively used. This typically involves adjusting Unbound's configuration to remove or comment out DoQ-related listening directives. After modifying the configuration, restart the Unbound service for the changes to take effect."}, "name": "CVE-2026-55991", "package_state": [{"cpe": "cpe:/o:redhat:enterprise_linux:10", "fix_state": "Fix deferred", "package_name": "unbound", "product_name": "Red Hat Enterprise Linux 10"}, {"cpe": "cpe:/o:redhat:enterprise_linux:6", "fix_state": "Not affected", "package_name": "unbound", "product_name": "Red Hat Enterprise Linux 6"}, {"cpe": "cpe:/o:redhat:enterprise_linux:7", "fix_state": "Not affected", "package_name": "unbound", "product_name": "Red Hat Enterprise Linux 7"}, {"cpe": "cpe:/o:redhat:enterprise_linux:8", "fix_state": "Not affected", "package_name": "unbound", "product_name": "Red Hat Enterprise Linux 8"}, {"cpe": "cpe:/o:redhat:enterprise_linux:9", "fix_state": "Fix deferred", "package_name": "unbound", "product_name": "Red Hat Enterprise Linux 9"}, {"cpe": "cpe:/a:redhat:openshift:4", "fix_state": "Fix deferred", "package_name": "rhcos", "product_name": "Red Hat OpenShift Container Platform 4"}], "public_date": "2026-07-22T13:10:13Z", "references": ["https://www.cve.org/CVERecord?id=CVE-2026-55991\nhttps://nvd.nist.gov/vuln/detail/CVE-2026-55991\nhttps://www.nlnetlabs.nl/downloads/unbound/CVE-2026-55991.txt"], "statement": "This Moderate flaw in Unbound allows a remote, unauthenticated client to cause a denial of service. By sending a specially crafted DNS-over-QUIC (DoQ) connection and a single DNS query, an attacker can terminate the Unbound process, making the DNS resolver unavailable. This vulnerability affects Unbound instances configured to support DoQ.", "threat_severity": "Moderate"}

{"affected": [{"configurations": [{"platform": null, "status": "affected", "versions": {"scheme": "semver", "value": "[1.22.0,1.25.2)"}}], "enrichment": {"confidence": 97.0, "confidence_source": "matching", "scores": [{"score": 97.0, "source": "matching"}]}, "original": {"product": "Unbound", "source": "cna", "vendor": "NLnet Labs"}, "product": "unbound", "vendor": "nlnetlabs"}], "created": "2026-07-23T19:00:17.241183+00:00", "updated": "2026-08-04T15:45:03.827134+00:00", "vendors": ["nlnetlabs", "nlnetlabs$PRODUCT$unbound"]}