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CVE-2021-23987

Status: ModifiedHigh (8.8)—

Mozilla developers and community members reported memory safety bugs present in Firefox 86 and Firefox ESR 78.8. Some of these bugs showed evidence of memory corruption and we presume that with enough effort some of these could have been exploited to run arbitrary code. This vulnerability affects Firefox ESR < 78.9, Firefox < 87, and Thunderbird < 78.9.

CVSS

Exploitation probability (EPSS)

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 (3)

Mozilla — FirefoxMozilla — Firefox ESRMozilla — Thunderbird

CWEs

References

Raw JSON (NVD)

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{
  "id": "CVE-2021-23987",
  "cveTags": [],
  "metrics": {
    "cvssMetricV2": [
      {
        "type": "Primary",
        "source": "nvd@nist.gov",
        "cvssData": {
          "version": "2.0",
          "baseScore": 6.8,
          "accessVector": "NETWORK",
          "vectorString": "AV:N/AC:M/Au:N/C:P/I:P/A:P",
          "authentication": "NONE",
          "integrityImpact": "PARTIAL",
          "accessComplexity": "MEDIUM",
          "availabilityImpact": "PARTIAL",
          "confidentialityImpact": "PARTIAL"
        },
        "acInsufInfo": false,
        "impactScore": 6.4,
        "baseSeverity": "MEDIUM",
        "obtainAllPrivilege": false,
        "exploitabilityScore": 8.6,
        "obtainUserPrivilege": false,
        "obtainOtherPrivilege": false,
        "userInteractionRequired": true
      }
    ],
    "cvssMetricV31": [
      {
        "type": "Primary",
        "source": "nvd@nist.gov",
        "cvssData": {
          "scope": "UNCHANGED",
          "version": "3.1",
          "baseScore": 8.8,
          "attackVector": "NETWORK",
          "baseSeverity": "HIGH",
          "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H",
          "integrityImpact": "HIGH",
          "userInteraction": "REQUIRED",
          "attackComplexity": "LOW",
          "availabilityImpact": "HIGH",
          "privilegesRequired": "NONE",
          "confidentialityImpact": "HIGH"
        },
        "impactScore": 5.9,
        "exploitabilityScore": 2.8
      }
    ]
  },
  "affected": [
    {
      "source": "security@mozilla.org",
      "affectedData": [
        {
          "vendor": "Mozilla",
          "product": "Firefox ESR",
          "versions": [
            {
              "status": "affected",
              "version": "unspecified",
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            }
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        },
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          "product": "Firefox",
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              "version": "unspecified",
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              "versionType": "custom"
            }
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        },
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          "product": "Thunderbird",
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              "version": "unspecified",
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              "versionType": "custom"
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  ],
  "published": "2021-03-31T14:15:19.420",
  "references": [
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      "url": "https://bugzilla.mozilla.org/buglist.cgi?bug_id=1513519%2C1683439%2C1690169%2C1690718",
      "tags": [
        "Issue Tracking",
        "Patch",
        "Vendor Advisory"
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      "source": "security@mozilla.org"
    },
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      "url": "https://www.mozilla.org/security/advisories/mfsa2021-10/",
      "tags": [
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      "source": "security@mozilla.org"
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      "tags": [
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      "source": "security@mozilla.org"
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      "url": "https://bugzilla.mozilla.org/buglist.cgi?bug_id=1513519%2C1683439%2C1690169%2C1690718",
      "tags": [
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      "tags": [
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      "url": "https://www.mozilla.org/security/advisories/mfsa2021-11/",
      "tags": [
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      ],
      "source": "af854a3a-2127-422b-91ae-364da2661108"
    },
    {
      "url": "https://www.mozilla.org/security/advisories/mfsa2021-12/",
      "tags": [
        "Vendor Advisory"
      ],
      "source": "af854a3a-2127-422b-91ae-364da2661108"
    }
  ],
  "vulnStatus": "Modified",
  "weaknesses": [
    {
      "type": "Primary",
      "source": "nvd@nist.gov",
      "description": [
        {
          "lang": "en",
          "value": "CWE-787"
        }
      ]
    }
  ],
  "descriptions": [
    {
      "lang": "en",
      "value": "Mozilla developers and community members reported memory safety bugs present in Firefox 86 and Firefox ESR 78.8. Some of these bugs showed evidence of memory corruption and we presume that with enough effort some of these could have been exploited to run arbitrary code. This vulnerability affects Firefox ESR < 78.9, Firefox < 87, and Thunderbird < 78.9."
    },
    {
      "lang": "es",
      "value": "Los desarrolladores y miembros de la comunidad de Mozilla informaron de fallos de seguridad de memoria presentes en Firefox 86 y Firefox ESR 78.8. Algunos de estos fallos mostraban evidencias de corrupción de memoria y suponemos que con suficiente esfuerzo algunos de ellos podrían haber sido explotados para ejecutar código arbitrario. Esta vulnerabilidad afecta a Firefox ESR versiones anteriores a 78.9, Firefox versiones anteriores a 87 y Thunderbird versiones anteriores a 78.9"
    }
  ],
  "lastModified": "2026-06-17T03:39:07.077",
  "configurations": [
    {
      "nodes": [
        {
          "negate": false,
          "cpeMatch": [
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              "vulnerable": true,
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              "versionEndExcluding": "87.0"
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  "sourceIdentifier": "security@mozilla.org"
}