What’s Vulnerable › Vulnerabilities

CVE-2020-26953

• Severity: medium (CVSS 4.3 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 1.3% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2020-12-09 · updated 2026-06-17
• Weakness: CWE-1021

Description

It was possible to cause the browser to enter fullscreen mode without displaying the security UI; thus making it possible to attempt a phishing attack or otherwise confuse the user. This vulnerability affects Firefox < 83, Firefox ESR < 78.5, and Thunderbird < 78.5.

Affected products

• Mozilla Firefox: before 83.0 — fixed in 83.0
• Mozilla Firefox ESR: before 78.5 — fixed in 78.5
• Mozilla Thunderbird: before 78.5 — fixed in 78.5

How to fix

Upgrade to a fixed version: Mozilla Firefox 83.0; Mozilla Firefox ESR 78.5; Mozilla Thunderbird 78.5. Check the vendor advisory for mitigations.

References

• https://bugzilla.mozilla.org/show_bug.cgi?id=1656741
• https://www.mozilla.org/security/advisories/mfsa2020-50/
• https://www.mozilla.org/security/advisories/mfsa2020-51/
• https://www.mozilla.org/security/advisories/mfsa2020-52/
• https://bugzilla.mozilla.org/show_bug.cgi?id=1656741
• https://www.mozilla.org/security/advisories/mfsa2020-50/
• https://www.mozilla.org/security/advisories/mfsa2020-51/
• https://www.mozilla.org/security/advisories/mfsa2020-52/

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