CVE-2021-26720

Status: ModifiedHigh (7.8)—

avahi-daemon-check-dns.sh in the Debian avahi package through 0.8-4 is executed as root via /etc/network/if-up.d/avahi-daemon, and allows a local attacker to cause a denial of service or create arbitrary empty files via a symlink attack on files under /run/avahi-daemon. NOTE: this only affects the packaging for Debian GNU/Linux (used indirectly by SUSE), not the upstream Avahi product.

CVSS

Version: 3.1
Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Base score: 7.8

Exploitation probability (EPSS)

Probability of exploitation in the next 30 days: 0.40%
Percentile among all scored CVEs: 32
Score date: 10/14/2026

EPSS (Exploit Prediction Scoring System, FIRST) estimates how likely a vulnerability is to be exploited in the wild within 30 days. It complements CVSS (impact) and CISA KEV (confirmed exploitation).

Affected technologies (2)

Avahi — Avahi Debian — Debian Linux

CWEs

CWE-59

References

Raw JSON (NVD)

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{
  "id": "CVE-2021-26720",
  "cveTags": [],
  "metrics": {
    "cvssMetricV2": [
      {
        "type": "Primary",
        "source": "nvd@nist.gov",
        "cvssData": {
          "version": "2.0",
          "baseScore": 4.6,
          "accessVector": "LOCAL",
          "vectorString": "AV:L/AC:L/Au:N/C:P/I:P/A:P",
          "authentication": "NONE",
          "integrityImpact": "PARTIAL",
          "accessComplexity": "LOW",
          "availabilityImpact": "PARTIAL",
          "confidentialityImpact": "PARTIAL"
        },
        "acInsufInfo": false,
        "impactScore": 6.4,
        "baseSeverity": "MEDIUM",
        "obtainAllPrivilege": false,
        "exploitabilityScore": 3.9,
        "obtainUserPrivilege": false,
        "obtainOtherPrivilege": false,
        "userInteractionRequired": false
      }
    ],
    "cvssMetricV31": [
      {
        "type": "Primary",
        "source": "nvd@nist.gov",
        "cvssData": {
          "scope": "UNCHANGED",
          "version": "3.1",
          "baseScore": 7.8,
          "attackVector": "LOCAL",
          "baseSeverity": "HIGH",
          "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H",
          "integrityImpact": "HIGH",
          "userInteraction": "NONE",
          "attackComplexity": "LOW",
          "availabilityImpact": "HIGH",
          "privilegesRequired": "LOW",
          "confidentialityImpact": "HIGH"
        },
        "impactScore": 5.9,
        "exploitabilityScore": 1.8
      }
    ]
  },
  "affected": [
    {
      "source": "cve@mitre.org",
      "affectedData": [
        {
          "vendor": "n/a",
          "product": "n/a",
          "versions": [
            {
              "status": "affected",
              "version": "n/a"
            }
          ]
        }
      ]
    }
  ],
  "published": "2021-02-17T22:15:12.710",
  "references": [
    {
      "url": "https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=982796",
      "tags": [
        "Mailing List",
        "Third Party Advisory"
      ],
      "source": "cve@mitre.org"
    },
    {
      "url": "https://bugs.launchpad.net/ubuntu/+source/avahi/+bug/1870824",
      "tags": [
        "Issue Tracking",
        "Third Party Advisory"
      ],
      "source": "cve@mitre.org"
    },
    {
      "url": "https://bugzilla.suse.com/show_bug.cgi?id=1180827",
      "tags": [
        "Issue Tracking",
        "Third Party Advisory"
      ],
      "source": "cve@mitre.org"
    },
    {
      "url": "https://lists.debian.org/debian-lts-announce/2022/06/msg00009.html",
      "tags": [
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        "Third Party Advisory"
      ],
      "source": "cve@mitre.org"
    },
    {
      "url": "https://metadata.ftp-master.debian.org/changelogs/main/a/avahi/avahi_0.8-4_changelog",
      "tags": [
        "Broken Link",
        "Third Party Advisory"
      ],
      "source": "cve@mitre.org"
    },
    {
      "url": "https://packages.debian.org/bullseye/avahi-daemon",
      "tags": [
        "Product",
        "Third Party Advisory"
      ],
      "source": "cve@mitre.org"
    },
    {
      "url": "https://packages.debian.org/buster/avahi-daemon",
      "tags": [
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        "Third Party Advisory"
      ],
      "source": "cve@mitre.org"
    },
    {
      "url": "https://packages.debian.org/sid/avahi-daemon",
      "tags": [
        "Product",
        "Third Party Advisory"
      ],
      "source": "cve@mitre.org"
    },
    {
      "url": "https://security-tracker.debian.org/tracker/CVE-2021-26720",
      "tags": [
        "Third Party Advisory"
      ],
      "source": "cve@mitre.org"
    },
    {
      "url": "https://www.openwall.com/lists/oss-security/2021/02/15/2",
      "tags": [
        "Mailing List",
        "Third Party Advisory"
      ],
      "source": "cve@mitre.org"
    },
    {
      "url": "https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=982796",
      "tags": [
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        "Third Party Advisory"
      ],
      "source": "af854a3a-2127-422b-91ae-364da2661108"
    },
    {
      "url": "https://bugs.launchpad.net/ubuntu/+source/avahi/+bug/1870824",
      "tags": [
        "Issue Tracking",
        "Third Party Advisory"
      ],
      "source": "af854a3a-2127-422b-91ae-364da2661108"
    },
    {
      "url": "https://bugzilla.suse.com/show_bug.cgi?id=1180827",
      "tags": [
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        "Third Party Advisory"
      ],
      "source": "af854a3a-2127-422b-91ae-364da2661108"
    },
    {
      "url": "https://lists.debian.org/debian-lts-announce/2022/06/msg00009.html",
      "tags": [
        "Mailing List",
        "Third Party Advisory"
      ],
      "source": "af854a3a-2127-422b-91ae-364da2661108"
    },
    {
      "url": "https://metadata.ftp-master.debian.org/changelogs/main/a/avahi/avahi_0.8-4_changelog",
      "tags": [
        "Broken Link",
        "Third Party Advisory"
      ],
      "source": "af854a3a-2127-422b-91ae-364da2661108"
    },
    {
      "url": "https://packages.debian.org/bullseye/avahi-daemon",
      "tags": [
        "Product",
        "Third Party Advisory"
      ],
      "source": "af854a3a-2127-422b-91ae-364da2661108"
    },
    {
      "url": "https://packages.debian.org/buster/avahi-daemon",
      "tags": [
        "Product",
        "Third Party Advisory"
      ],
      "source": "af854a3a-2127-422b-91ae-364da2661108"
    },
    {
      "url": "https://packages.debian.org/sid/avahi-daemon",
      "tags": [
        "Product",
        "Third Party Advisory"
      ],
      "source": "af854a3a-2127-422b-91ae-364da2661108"
    },
    {
      "url": "https://security-tracker.debian.org/tracker/CVE-2021-26720",
      "tags": [
        "Third Party Advisory"
      ],
      "source": "af854a3a-2127-422b-91ae-364da2661108"
    },
    {
      "url": "https://www.openwall.com/lists/oss-security/2021/02/15/2",
      "tags": [
        "Mailing List",
        "Third Party Advisory"
      ],
      "source": "af854a3a-2127-422b-91ae-364da2661108"
    }
  ],
  "vulnStatus": "Modified",
  "weaknesses": [
    {
      "type": "Primary",
      "source": "nvd@nist.gov",
      "description": [
        {
          "lang": "en",
          "value": "CWE-59"
        }
      ]
    }
  ],
  "descriptions": [
    {
      "lang": "en",
      "value": "avahi-daemon-check-dns.sh in the Debian avahi package through 0.8-4 is executed as root via /etc/network/if-up.d/avahi-daemon, and allows a local attacker to cause a denial of service or create arbitrary empty files via a symlink attack on files under /run/avahi-daemon. NOTE: this only affects the packaging for Debian GNU/Linux (used indirectly by SUSE), not the upstream Avahi product."
    },
    {
      "lang": "es",
      "value": "El archivo avahi-daemon-check-dns.sh en el paquete Debian avahi a versiones hasta 0.8-4, es ejecutado como root por medio de /etc/network/if-up.d/avahi-daemon, y permite a un atacante local causar una denegación de servicio o crear archivos vacíos arbitrarios por medio de un ataque de tipo symlink en archivos bajo /run/avahi-daemon.&#xa0;NOTA: esto solo afecta al empaquetado de Debian GNU/Linux (usado indirectamente por SUSE), no al producto de Avahi previo"
    }
  ],
  "lastModified": "2026-06-17T03:43:43.297",
  "configurations": [
    {
      "nodes": [
        {
          "negate": false,
          "cpeMatch": [
            {
              "criteria": "cpe:2.3:a:avahi:avahi:*:*:*:*:*:*:*:*",
              "vulnerable": true,
              "matchCriteriaId": "7960274F-0D6F-4054-8236-145A6430B6CE",
              "versionEndIncluding": "0.8-4"
            }
          ],
          "operator": "OR"
        }
      ]
    },
    {
      "nodes": [
        {
          "negate": false,
          "cpeMatch": [
            {
              "criteria": "cpe:2.3:o:debian:debian_linux:9.0:*:*:*:*:*:*:*",
              "vulnerable": true,
              "matchCriteriaId": "DEECE5FC-CACF-4496-A3E7-164736409252"
            },
            {
              "criteria": "cpe:2.3:o:debian:debian_linux:10.0:*:*:*:*:*:*:*",
              "vulnerable": true,
              "matchCriteriaId": "07B237A9-69A3-4A9C-9DA0-4E06BD37AE73"
            }
          ],
          "operator": "OR"
        }
      ]
    }
  ],
  "sourceIdentifier": "cve@mitre.org"
}