{"id":"CVE-2026-55991","title":"Remote DNS-over-QUIC (DoQ) flow-control assertion failure in libngtcp2","summary":"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) conne…","severity":"medium","cvss":5.9,"cvssVector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H","cvssSource":"cna","cwe":["CWE-195"],"vendor":"NLnet Labs","product":"Unbound","affected":["Unbound >= 1.22.0 < 1.25.2"],"ssvc":{"exploitation":"none","automatable":"no","technicalImpact":"partial","timestamp":"2026-07-22T14:01:59.779165Z"},"published":"2026-07-22","updated":"2026-09-16","sourceUpdated":"2026-09-16T14:24:57.805Z","source":"CVEORG","sourceUrl":"https://www.cve.org/CVERecord?id=CVE-2026-55991","references":[{"url":"https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-55991.txt"}],"tags":["cve.org"],"epss":0.00365,"epssPercentile":0.27579,"ingestedAt":"2026-09-16T14:57:28.053Z","slug":"CVE-2026-55991","body":"## Overview\n\nIn 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.\n\n## Affected\n\n- `Unbound >= 1.22.0 < 1.25.2`\n\n## Remediation\n\nThis issue is fixed starting with version 1.25.2","depth":"sunlit","depthScore":33,"depthScoreParts":{"impact":32.5,"likelihood":0.1,"exploitation":0,"ransomware":0},"changes":[]}