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CVE-2020-26951

Status: ModifiedMedium (6.1)—

A parsing and event loading mismatch in Firefox's SVG code could have allowed load events to fire, even after sanitization. An attacker already capable of exploiting an XSS vulnerability in privileged internal pages could have used this attack to bypass our built-in sanitizer. This vulnerability affects Firefox < 83, Firefox ESR < 78.5, and Thunderbird < 78.5.

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-2020-26951",
  "cveTags": [],
  "metrics": {
    "cvssMetricV2": [
      {
        "type": "Primary",
        "source": "nvd@nist.gov",
        "cvssData": {
          "version": "2.0",
          "baseScore": 4.3,
          "accessVector": "NETWORK",
          "vectorString": "AV:N/AC:M/Au:N/C:N/I:P/A:N",
          "authentication": "NONE",
          "integrityImpact": "PARTIAL",
          "accessComplexity": "MEDIUM",
          "availabilityImpact": "NONE",
          "confidentialityImpact": "NONE"
        },
        "acInsufInfo": false,
        "impactScore": 2.9,
        "baseSeverity": "MEDIUM",
        "obtainAllPrivilege": false,
        "exploitabilityScore": 8.6,
        "obtainUserPrivilege": false,
        "obtainOtherPrivilege": false,
        "userInteractionRequired": true
      }
    ],
    "cvssMetricV31": [
      {
        "type": "Primary",
        "source": "nvd@nist.gov",
        "cvssData": {
          "scope": "CHANGED",
          "version": "3.1",
          "baseScore": 6.1,
          "attackVector": "NETWORK",
          "baseSeverity": "MEDIUM",
          "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N",
          "integrityImpact": "LOW",
          "userInteraction": "REQUIRED",
          "attackComplexity": "LOW",
          "availabilityImpact": "NONE",
          "privilegesRequired": "NONE",
          "confidentialityImpact": "LOW"
        },
        "impactScore": 2.7,
        "exploitabilityScore": 2.8
      }
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  "affected": [
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      "source": "security@mozilla.org",
      "affectedData": [
        {
          "vendor": "Mozilla",
          "product": "Firefox",
          "versions": [
            {
              "status": "affected",
              "version": "< 83"
            }
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        },
        {
          "vendor": "Mozilla",
          "product": "Firefox ESR",
          "versions": [
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            }
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          "vendor": "Mozilla",
          "product": "Thunderbird",
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  "published": "2020-12-09T01:15:12.643",
  "references": [
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      "url": "https://bugzilla.mozilla.org/show_bug.cgi?id=1667113",
      "tags": [
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        "Permissions Required",
        "Vendor Advisory"
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      "source": "security@mozilla.org"
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    {
      "url": "https://www.mozilla.org/security/advisories/mfsa2020-50/",
      "tags": [
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      "source": "security@mozilla.org"
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      "url": "https://www.mozilla.org/security/advisories/mfsa2020-51/",
      "tags": [
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      "source": "security@mozilla.org"
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    {
      "url": "https://www.mozilla.org/security/advisories/mfsa2020-52/",
      "tags": [
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      "source": "security@mozilla.org"
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      "url": "https://bugzilla.mozilla.org/show_bug.cgi?id=1667113",
      "tags": [
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        "Vendor Advisory"
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      "source": "af854a3a-2127-422b-91ae-364da2661108"
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    },
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      "tags": [
        "Vendor Advisory"
      ],
      "source": "af854a3a-2127-422b-91ae-364da2661108"
    }
  ],
  "vulnStatus": "Modified",
  "weaknesses": [
    {
      "type": "Primary",
      "source": "nvd@nist.gov",
      "description": [
        {
          "lang": "en",
          "value": "CWE-79"
        }
      ]
    }
  ],
  "descriptions": [
    {
      "lang": "en",
      "value": "A parsing and event loading mismatch in Firefox's SVG code could have allowed load events to fire, even after sanitization. An attacker already capable of exploiting an XSS vulnerability in privileged internal pages could have used this attack to bypass our built-in sanitizer. This vulnerability affects Firefox < 83, Firefox ESR < 78.5, and Thunderbird < 78.5."
    },
    {
      "lang": "es",
      "value": "Un desajuste en el análisis y la carga de eventos en el código SVG de Firefox podría haber permitido a unos eventos de carga dispararse, incluso después del saneamiento. Un atacante ya capaz de explotar una vulnerabilidad de tipo XSS en páginas internas privilegiadas podría haber usado este ataque para omitir nuestro sanitizador incorporado. Esta vulnerabilidad afecta a Firefox versiones anteriores a 83, Firefox ESR versiones anteriores a  78.5, y Thunderbird versiones anteriores a 78.5"
    }
  ],
  "lastModified": "2026-06-17T03:08:36.320",
  "configurations": [
    {
      "nodes": [
        {
          "negate": false,
          "cpeMatch": [
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              "vulnerable": true,
              "matchCriteriaId": "9385C808-43DD-4D02-B1A9-89A2E3986DF2",
              "versionEndExcluding": "83.0"
            },
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  "sourceIdentifier": "security@mozilla.org"
}