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    "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"
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                                    {
                                        "Technical Impact": "partial"
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                                "role": "CISA Coordinator",
                                "version": "2.0.3"
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                "title": "CISA ADP Vulnrichment",
                "providerMetadata": {
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                    "shortName": "CISA-ADP",
                    "dateUpdated": "2026-07-22T14:03:20.789Z"
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}