Vulnerability-Lookup

GHSA-J6HJ-WVX2-8WQP

Vulnerability from github – Published: 2026-07-22 15:31 – Updated: 2026-07-22 15:31

Details

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.

Severity

5.9 (Medium)


                  
                    CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

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{
  "affected": [],
  "aliases": [
    "CVE-2026-55991"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-195"
    ],
    "github_reviewed": false,
    "github_reviewed_at": null,
    "nvd_published_at": "2026-07-22T14:17:21Z",
    "severity": "MODERATE"
  },
  "details": "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": "GHSA-j6hj-wvx2-8wqp",
  "modified": "2026-07-22T15:31:25Z",
  "published": "2026-07-22T15:31:25Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-55991"
    },
    {
      "type": "WEB",
      "url": "https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-55991.txt"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H",
      "type": "CVSS_V3"
    }
  ]
}

CVE-2026-55991 (GCVE-0-2026-55991)

Vulnerability from cvelistv5 – Published: 2026-07-22 13:10 – Updated: 2026-07-22 14:03

Title

Remote DNS-over-QUIC (DoQ) flow-control assertion failure in libngtcp2

Summary

Severity

5.9 (Medium)


                        
                          CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

SSVC

Exploitation: none Automatable: no Technical Impact: partial

CISA Coordinator (v2.0.3)

CWE

CWE-195 - Signed to Unsigned Conversion Error

Assigner

NLnet Labs

References

1 reference

URL	Tags
https://www.nlnetlabs.nl/downloads/unbound/CVE-20…	vendor-advisory

Impacted products

1 product

Vendor	Product	Version
NLnet Labs	Unbound	Affected: 1.22.0 , < 1.25.2 (semver)

Date Public

2026-07-22 00:00

Credits

Qifan Zhang (Palo Alto Networks) Xuanchao Xie

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JSON

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{
  "containers": {
    "adp": [
      {
        "metrics": [
          {
            "other": {
              "content": {
                "id": "CVE-2026-55991",
                "options": [
                  {
                    "Exploitation": "none"
                  },
                  {
                    "Automatable": "no"
                  },
                  {
                    "Technical Impact": "partial"
                  }
                ],
                "role": "CISA Coordinator",
                "timestamp": "2026-07-22T14:01:59.779165Z",
                "version": "2.0.3"
              },
              "type": "ssvc"
            }
          }
        ],
        "providerMetadata": {
          "dateUpdated": "2026-07-22T14:03:20.789Z",
          "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
          "shortName": "CISA-ADP"
        },
        "title": "CISA ADP Vulnrichment"
      }
    ],
    "cna": {
      "affected": [
        {
          "defaultStatus": "unaffected",
          "product": "Unbound",
          "vendor": "NLnet Labs",
          "versions": [
            {
              "lessThan": "1.25.2",
              "status": "affected",
              "version": "1.22.0",
              "versionType": "semver"
            }
          ]
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      ],
      "credits": [
        {
          "lang": "en",
          "type": "finder",
          "value": "Qifan Zhang (Palo Alto Networks)"
        },
        {
          "lang": "en",
          "type": "finder",
          "value": "Xuanchao Xie"
        }
      ],
      "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 \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."
        }
      ],
      "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 (\u0027--with-libngtcp2\u0027)"
            }
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        }
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        }
      ],
      "solutions": [
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          "lang": "en",
          "value": "This issue is fixed starting with version 1.25.2"
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      ],
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          "lang": "en",
          "time": "2026-05-27T00:00:00.000Z",
          "value": "Issue reported by Qifan Zhang"
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          "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"
        },
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          "time": "2026-07-12T00:00:00.000Z",
          "value": "NLnet Labs shares patch with Xuanchao Xie"
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          "time": "2026-06-13T00:00:00.000Z",
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          "lang": "en",
          "time": "2026-07-22T00:00:00.000Z",
          "value": "Fixes released with version 1.25.2"
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      "title": "Remote DNS-over-QUIC (DoQ) flow-control assertion failure in libngtcp2",
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        "engine": "cvelib 1.8.0"
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  "cveMetadata": {
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    "assignerShortName": "NLnet Labs",
    "cveId": "CVE-2026-55991",
    "datePublished": "2026-07-22T13:10:13.235Z",
    "dateReserved": "2026-06-22T10:11:10.505Z",
    "dateUpdated": "2026-07-22T14:03:20.789Z",
    "state": "PUBLISHED"
  },
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}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

Detection rules are retrieved from Rulezet.

The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.

Action not permitted

GHSA-J6HJ-WVX2-8WQP

CVE-2026-55991 (GCVE-0-2026-55991)

Tags

Sightings

Nomenclature