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: 2026-09-16 14:24

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 · CISA-ADP (v2.0.3)

Decision recorded 2026-07-22 14:01 UTC

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 (University of Science and Technology of China) Lutong Chen (University of Science and Technology of China) Kaiping Xue (University of Science and Technology of China)

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{
  "containers": {
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      {
        "metrics": [
          {
            "other": {
              "content": {
                "id": "CVE-2026-55991",
                "options": [
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                  },
                  {
                    "Automatable": "no"
                  },
                  {
                    "Technical Impact": "partial"
                  }
                ],
                "role": "CISA Coordinator",
                "timestamp": "2026-07-22T14:01:59.779165Z",
                "version": "2.0.3"
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        "providerMetadata": {
          "dateUpdated": "2026-07-22T14:03:20.789Z",
          "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
          "shortName": "CISA-ADP"
        },
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    "cna": {
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          "vendor": "NLnet Labs",
          "versions": [
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              "status": "affected",
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            }
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      "credits": [
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          "type": "finder",
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        },
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          "lang": "en",
          "type": "finder",
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        },
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          "type": "finder",
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        }
      ],
      "datePublic": "2026-07-22T00:00:00.000Z",
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        }
      ],
      "metrics": [
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        }
      ],
      "solutions": [
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          "lang": "en",
          "value": "This issue is fixed starting with version 1.25.2"
        }
      ],
      "timeline": [
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          "lang": "en",
          "time": "2026-05-27T00:00:00.000Z",
          "value": "Issue reported by Qifan Zhang"
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          "time": "2026-06-15T00:00:00.000Z",
          "value": "NLnet Labs shares patch with Qifan Zhang"
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          "time": "2026-06-17T00:00:00.000Z",
          "value": "Qifan Zhang verifies patch"
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          "time": "2026-07-11T00:00:00.000Z",
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          "time": "2026-06-13T00:00:00.000Z",
          "value": "Xuanchao Xie verifies patch"
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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": {
    "assignerOrgId": "206fc3a0-e175-490b-9eaa-a5738056c9f6",
    "assignerShortName": "NLnet Labs",
    "cveId": "CVE-2026-55991",
    "datePublished": "2026-07-22T13:10:13.235Z",
    "dateReserved": "2026-06-22T10:11:10.505Z",
    "dateUpdated": "2026-09-16T14:24:57.805Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2"
}

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.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
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