{"id":"CVE-2026-55991","state":"PUBLISHED","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.","publishedAt":"2026-07-22T13:10:13.235Z","updatedAt":"2026-09-16T14:24:57.805Z","severity":{"score":5.9,"label":"Medium","version":"3.1","vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H","source":"NLnet Labs"},"assigner":"NLnet Labs","products":["NLnet Labs Unbound"],"affected":[{"vendor":"NLnet Labs","product":"Unbound","defaultStatus":"unaffected","versions":[{"version":"1.22.0","status":"affected","lessThan":"1.25.2","versionType":"semver"}]}],"references":["https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-55991.txt"],"weaknesses":["CWE-195"],"ingestedAt":"2026-10-29T20:12:24.861Z","source":{"repository":"CVEProject/cvelistV5","path":"cves/2026/55xxx/CVE-2026-55991.json","commit":"556c9d3a8cd17e218bb8e452ba7cd619a384ff47"},"raw":{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2026-55991","state":"PUBLISHED","datePublished":"2026-07-22T13:10:13.235Z","dateUpdated":"2026-09-16T14:24:57.805Z","assignerShortName":"NLnet Labs","assignerOrgId":"206fc3a0-e175-490b-9eaa-a5738056c9f6","dateReserved":"2026-06-22T10:11:10.505Z"},"containers":{"cna":{"title":"Remote DNS-over-QUIC (DoQ) flow-control assertion failure in libngtcp2","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."}],"affected":[{"vendor":"NLnet Labs","product":"Unbound","defaultStatus":"unaffected","versions":[{"version":"1.22.0","status":"affected","lessThan":"1.25.2","versionType":"semver"}]}],"references":[{"url":"https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-55991.txt","tags":["vendor-advisory"]}],"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":{"shortName":"NLnet Labs","orgId":"206fc3a0-e175-490b-9eaa-a5738056c9f6","dateUpdated":"2026-09-16T14:24:57.805Z"},"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","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"}],"x_generator":{"engine":"cvelib 1.8.0"}},"adp":[{"title":"CISA ADP Vulnrichment","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"}}}],"providerMetadata":{"shortName":"CISA-ADP","orgId":"134c704f-9b21-4f2e-91b3-4a467353bcc0","dateUpdated":"2026-07-22T14:03:20.789Z"}}]}}}