Vulnerability-Lookup

FKIE_CVE-2026-55717

Vulnerability from fkie_nvd - Published: 2026-07-22 14:17 - Updated: 2026-07-24 14:24

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 1.10.0 up to and including 1.25.1, when 'serve-expired: yes' is set together with a 'response-ip: <net> redirect' /'response-ip-data: <net> CNAME <target>' rule (or the RPZ 'rpz-cname-override' equivalent), a remote client who controls any delegated domain can crash the daemon. The serve-expired-client-timeout callback runs a two-pass loop to chase the respip-generated CNAME alias; on the second pass it resets 'alias_rrset' but not 'partial_rep'. Later, this inconsistency leads to a NULL pointer dereference and an eventual crash. A malicious actor can exploit the vulnerability by controlling any zone that replies with an A/AAAA record that falls inside the configured response-ip/rpz subnet. By delaying the answer when the previous record has expired, the vulnerable path of 'serve-expired-client-timeout' is taken leading to denial of service via the server crash.

References

	URL	Tags
	sep@nlnetlabs.nl	https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-55717.txt	Vendor Advisory

Impacted products

	Vendor	Product	Version
	nlnetlabs	unbound	*

JSON

To clipboard

{
  "affected": [
    {
      "affectedData": [
        {
          "defaultStatus": "unaffected",
          "product": "Unbound",
          "vendor": "NLnet Labs",
          "versions": [
            {
              "lessThan": "1.25.2",
              "status": "affected",
              "version": "1.10.0",
              "versionType": "semver"
            }
          ]
        }
      ],
      "source": "sep@nlnetlabs.nl"
    }
  ],
  "configurations": [
    {
      "nodes": [
        {
          "cpeMatch": [
            {
              "criteria": "cpe:2.3:a:nlnetlabs:unbound:*:*:*:*:*:*:*:*",
              "matchCriteriaId": "4B6C880D-74C8-4AA4-AA06-CF117E87BA82",
              "versionEndExcluding": "1.25.2",
              "versionStartIncluding": "1.10.0",
              "vulnerable": true
            }
          ],
          "negate": false,
          "operator": "OR"
        }
      ]
    }
  ],
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "In NLnet Labs Unbound 1.10.0 up to and including 1.25.1, when \u0027serve-expired: yes\u0027 is set together with a \u0027response-ip: \u003cnet\u003e redirect\u0027 /\u0027response-ip-data: \u003cnet\u003e CNAME \u003ctarget\u003e\u0027 rule (or the RPZ \u0027rpz-cname-override\u0027 equivalent), a remote client who controls any delegated domain can crash the daemon. The serve-expired-client-timeout callback runs a two-pass loop to chase the respip-generated CNAME alias; on the second pass it resets \u0027alias_rrset\u0027 but not \u0027partial_rep\u0027. Later, this inconsistency leads to a NULL pointer dereference and an eventual crash. A malicious actor can exploit the vulnerability by controlling any zone that replies with an A/AAAA record that falls inside the configured response-ip/rpz subnet. By delaying the answer when the previous record has expired, the vulnerable path of \u0027serve-expired-client-timeout\u0027 is taken leading to denial of service via the server crash."
    }
  ],
  "id": "CVE-2026-55717",
  "lastModified": "2026-07-24T14:24:16.293",
  "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-2026-55717",
          "options": [
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              "exploitation": "none"
            },
            {
              "automatable": "no"
            },
            {
              "technicalImpact": "partial"
            }
          ],
          "role": "CISA Coordinator",
          "timestamp": "2026-07-22T14:07:12.831423Z",
          "version": "2.0.3"
        }
      }
    ]
  },
  "published": "2026-07-22T14:17:21.537",
  "references": [
    {
      "source": "sep@nlnetlabs.nl",
      "tags": [
        "Vendor Advisory"
      ],
      "url": "https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-55717.txt"
    }
  ],
  "sourceIdentifier": "sep@nlnetlabs.nl",
  "vulnStatus": "Analyzed",
  "weaknesses": [
    {
      "description": [
        {
          "lang": "en",
          "value": "CWE-476"
        }
      ],
      "source": "sep@nlnetlabs.nl",
      "type": "Secondary"
    }
  ]
}

CVE-2026-55717 (GCVE-0-2026-55717)

Vulnerability from cvelistv5 – Published: 2026-07-22 13:09 – Updated: 2026-07-22 14:07

Title

'serve-expired-client-timeout' and 'response-ip' CNAME redirect could lead to a crash

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 14:07 UTC

CWE

CWE-476 - NULL Pointer Dereference

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: 1.10.0 , < 1.25.2 (semver)

Date Public

2026-07-22 00:00

Credits

Qifan Zhang (Palo Alto Networks) Xin Wang (Northwestern Polytechnical University) Jiapeng Li (Northwestern Polytechnical University) Jiajia Liu (Northwestern Polytechnical University)

Show details on NVD website

JSON

To clipboard

{
  "containers": {
    "adp": [
      {
        "metrics": [
          {
            "other": {
              "content": {
                "id": "CVE-2026-55717",
                "options": [
                  {
                    "Exploitation": "none"
                  },
                  {
                    "Automatable": "no"
                  },
                  {
                    "Technical Impact": "partial"
                  }
                ],
                "role": "CISA Coordinator",
                "timestamp": "2026-07-22T14:07:12.831423Z",
                "version": "2.0.3"
              },
              "type": "ssvc"
            }
          }
        ],
        "providerMetadata": {
          "dateUpdated": "2026-07-22T14:07:31.075Z",
          "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
          "shortName": "CISA-ADP"
        },
        "title": "CISA ADP Vulnrichment"
      }
    ],
    "cna": {
      "affected": [
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          "product": "Unbound",
          "vendor": "NLnet Labs",
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        },
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          "lang": "en",
          "type": "finder",
          "value": "Jiapeng Li (Northwestern Polytechnical University)"
        },
        {
          "lang": "en",
          "type": "finder",
          "value": "Jiajia Liu (Northwestern Polytechnical University)"
        }
      ],
      "datePublic": "2026-07-22T00:00:00.000Z",
      "descriptions": [
        {
          "lang": "en",
          "value": "In NLnet Labs Unbound 1.10.0 up to and including 1.25.1, when \u0027serve-expired: yes\u0027 is set together with a \u0027response-ip: \u003cnet\u003e redirect\u0027 /\u0027response-ip-data: \u003cnet\u003e CNAME \u003ctarget\u003e\u0027 rule (or the RPZ \u0027rpz-cname-override\u0027 equivalent), a remote client who controls any delegated domain can crash the daemon. The serve-expired-client-timeout callback runs a two-pass loop to chase the respip-generated CNAME alias; on the second pass it resets \u0027alias_rrset\u0027 but not \u0027partial_rep\u0027. Later, this inconsistency leads to a NULL pointer dereference and an eventual crash. A malicious actor can exploit the vulnerability by controlling any zone that replies with an A/AAAA record that falls inside the configured response-ip/rpz subnet. By delaying the answer when the previous record has expired, the vulnerable path of \u0027serve-expired-client-timeout\u0027 is taken leading to denial of service via the server crash."
        }
      ],
      "metrics": [
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            "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"
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          "scenarios": [
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              "lang": "en",
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          ]
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      "problemTypes": [
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      },
      "references": [
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        }
      ],
      "solutions": [
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          "value": "This issue is fixed starting with version 1.25.2"
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      ],
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          "time": "2026-05-05T00:00:00.000Z",
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          "time": "2026-05-27T00:00:00.000Z",
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        },
        {
          "lang": "en",
          "time": "2026-05-27T00:00:00.000Z",
          "value": "NLnet Labs shares patch with Xin Wang"
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          "time": "2026-05-27T00:00:00.000Z",
          "value": "Xin Wang verifies patch"
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    "dateReserved": "2026-06-22T12:27:22.813Z",
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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.
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.
Browse all ATT&CK techniques and the vulnerabilities related to each.

Related by attack behaviour

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