FKIE_CVE-2026-55991

Vulnerability from fkie_nvd - Published: 2026-07-22 14:17 - Updated: 2026-07-24 14:25
Severity
5.9 (Medium) - CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
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) 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.
References
sep@nlnetlabs.nlhttps://www.nlnetlabs.nl/downloads/unbound/CVE-2026-55991.txtVendor Advisory
Impacted products
Vendor Product Version
nlnetlabs unbound *
To clipboard 

{
  "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 \u0027ngtcp2_conn_writev_stream()\u0027 returns \u0027NGTCP2_ERR_STREAM_DATA_BLOCKED\u0027, Unbound continues to call \u0027ngtcp2_ccerr_set_application_error()\u0027 with a \u0027-1\u0027 error value. The \u0027int\u0027 literal \u0027-1\u0027 is implicitly converted to the function\u0027s \u0027uint64_t error_code\u0027 parameter as \u00270xFFFFFFFFFFFFFFFF\u0027. The follow-on \u0027ngtcp2_conn_write_connection_close()\u0027 serialises that value as a QUIC variable-length integer; because \u00272^64-1\u0027 exceeds the 62-bit varint ceiling, \u0027ngtcp2_put_uvarintlen()\u0027 fails \u0027assert(n \u003c 4611686018427387904ULL)\u0027 and the whole resolver process aborts. A remote, unauthenticated DoQ client can trigger this deterministically with a single QUIC connection by advertising \u0027initial_max_stream_data_bidi_local = 1\u0027 in its transport parameters and sending one DoQ query without ever reading the stream."
    }
  ],
  "id": "CVE-2026-55991",
  "lastModified": "2026-07-24T14:25:18.750",
  "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"
    }
  ]
}



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