{
  "cveId": "CVE-2026-55991",
  "url": "https://patchblox.com/security/cve/CVE-2026-55991",
  "summary": "CVE-2026-55991 is fixed in unbound on 1 platform, first by DEBIAN-CVE-2026-55991, released July 28, 2026. It is not reported as exploited. Patch Radar priority: Routine.",
  "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 \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.",
  "published": "2026-07-22T14:17:21Z",
  "severity": "Moderate",
  "cvss": 5.9,
  "cvssVector": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H",
  "cwe": "Signed to Unsigned Conversion Error",
  "epss": 0.00365,
  "epssPercentile": 0.30563,
  "priority": "Routine",
  "priorityTier": 4,
  "exploited": false,
  "vendorExploited": false,
  "disclosed": false,
  "vendorAdvisory": "https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-55991",
  "fixes": [
    {
      "platform": "linux",
      "product": "unbound",
      "release": "Debian 13.5 (trixie)",
      "fixedVersion": "1.26.1-0\u002Bdeb13u1",
      "fix": "DEBIAN-CVE-2026-55991",
      "url": "https://patchblox.com/security/fix/linux/debian-cve-2026-55991",
      "released": "2026-07-28T14:13:38Z"
    },
    {
      "platform": "linux",
      "product": "unbound",
      "release": "Debian 14",
      "fixedVersion": "1.25.2-1",
      "fix": "DEBIAN-CVE-2026-55991",
      "url": "https://patchblox.com/security/fix/linux/debian-cve-2026-55991",
      "released": "2026-07-28T14:13:38Z"
    },
    {
      "platform": "linux",
      "product": "unbound",
      "release": "opensuse-leap 16.0",
      "fixedVersion": "1.25.2-160000.1.1",
      "fix": "OPENSUSE-SU-2026:21550-1",
      "url": "https://patchblox.com/security/fix/linux/opensuse-su-2026-21550-1",
      "released": "2026-08-10T15:27:28Z"
    },
    {
      "platform": "linux",
      "product": "unbound",
      "release": "sles 16.0",
      "fixedVersion": "1.25.2-160000.1.1",
      "fix": "SUSE-SU-2026:23226-1",
      "url": "https://patchblox.com/security/fix/linux/suse-su-2026-23226-1",
      "released": "2026-08-10T15:26:22Z"
    }
  ],
  "noFixYet": [],
  "dataAsOf": "2026-11-09T13:31:49Z",
  "attribution": {
    "text": "Data: Patchblox Patch Radar",
    "url": "https://patchblox.com/security/patch-radar",
    "terms": "https://patchblox.com/security/patch-radar/use-the-data"
  }
}