{"api_version":"1","generated_at":"2026-07-24T20:08:20+00:00","cve":"CVE-2026-55991","urls":{"html":"https://cve.report/CVE-2026-55991","api":"https://cve.report/api/cve/CVE-2026-55991.json","docs":"https://cve.report/api","cve_org":"https://www.cve.org/CVERecord?id=CVE-2026-55991","nvd":"https://nvd.nist.gov/vuln/detail/CVE-2026-55991"},"summary":{"title":"Remote DNS-over-QUIC (DoQ) flow-control assertion failure in libngtcp2","description":"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.","state":"PUBLISHED","assigner":"NLnet Labs","published_at":"2026-07-22 14:17:21","updated_at":"2026-07-24 14:25:18"},"problem_types":["CWE-195","CWE-195 CWE-195: Signed to Unsigned Conversion Error"],"metrics":[{"version":"3.1","source":"sep@nlnetlabs.nl","type":"Secondary","score":"5.9","severity":"MEDIUM","vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H","data":{"version":"3.1","vectorString":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H","baseScore":5.9,"baseSeverity":"MEDIUM","attackVector":"NETWORK","attackComplexity":"HIGH","privilegesRequired":"NONE","userInteraction":"NONE","scope":"UNCHANGED","confidentialityImpact":"NONE","integrityImpact":"NONE","availabilityImpact":"HIGH"}},{"version":"3.1","source":"CNA","type":"CVSS","score":"5.9","severity":"MEDIUM","vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H","data":{"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"}}],"references":[{"url":"https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-55991.txt","name":"https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-55991.txt","refsource":"sep@nlnetlabs.nl","tags":["Vendor Advisory"],"title":"","mime":"","httpstatus":"","archivestatus":"0"},{"url":"https://www.cve.org/CVERecord?id=CVE-2026-55991","name":"CVE Program record","refsource":"CVE.ORG","tags":["canonical"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-55991","name":"NVD vulnerability detail","refsource":"NVD","tags":["canonical","analysis"]}],"affected":[{"source":"CNA","vendor":"NLnet Labs","product":"Unbound","version":"affected 1.22.0 1.25.2 semver","platforms":[]}],"timeline":[{"source":"CNA","time":"2026-05-27T00:00:00.000Z","lang":"en","value":"Issue reported by Qifan Zhang"},{"source":"CNA","time":"2026-06-15T00:00:00.000Z","lang":"en","value":"NLnet Labs shares patch with Qifan Zhang"},{"source":"CNA","time":"2026-06-17T00:00:00.000Z","lang":"en","value":"Qifan Zhang verifies patch"},{"source":"CNA","time":"2026-07-11T00:00:00.000Z","lang":"en","value":"Issue reported by Xuanchao Xie"},{"source":"CNA","time":"2026-07-12T00:00:00.000Z","lang":"en","value":"NLnet Labs shares patch with Xuanchao Xie"},{"source":"CNA","time":"2026-06-13T00:00:00.000Z","lang":"en","value":"Xuanchao Xie verifies patch"},{"source":"CNA","time":"2026-07-22T00:00:00.000Z","lang":"en","value":"Fixes released with version 1.25.2"}],"solutions":[{"source":"CNA","title":"","value":"This issue is fixed starting with version 1.25.2","time":"","lang":"en"}],"workarounds":[],"exploits":[],"credits":[{"source":"CNA","value":"Qifan Zhang (Palo Alto Networks)","lang":"en"},{"source":"CNA","value":"Xuanchao Xie","lang":"en"}],"nvd_cpes":[{"cve_year":"2026","cve_id":"55991","vulnerable":"1","versionEndIncluding":"","cpe1":"cpe","cpe2":"2.3","cpe3":"a","cpe4":"nlnetlabs","cpe5":"unbound","cpe6":"*","cpe7":"*","cpe8":"*","cpe9":"*","cpe10":"*","cpe11":"*","cpe12":"*","cpe13":"*"}],"vendor_comments":[],"enrichments":{"kev":null,"epss":null,"legacy_qids":[]},"source_records":{"cve_program":{"containers":{"adp":[{"metrics":[{"other":{"content":{"id":"CVE-2026-55991","options":[{"Exploitation":"none"},{"Automatable":"no"},{"Technical Impact":"partial"}],"role":"CISA Coordinator","timestamp":"2026-07-22T14:01:59.779165Z","version":"2.0.3"},"type":"ssvc"}}],"providerMetadata":{"dateUpdated":"2026-07-22T14:03:20.789Z","orgId":"134c704f-9b21-4f2e-91b3-4a467353bcc0","shortName":"CISA-ADP"},"title":"CISA ADP Vulnrichment"}],"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","type":"finder","value":"Xuanchao Xie"}],"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. 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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.","Type":"Description","Title":"Remote DNS-over-QUIC (DoQ) flow-control assertion failure in lib"}]}}}