What’s Vulnerable › Vulnerabilities

CVE-2021-23984

• Severity: medium (CVSS 6.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 1.1% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2021-03-31 · updated 2026-06-17
• Weakness: CWE-290

Description

A malicious extension could have opened a popup window lacking an address bar. The title of the popup lacking an address bar should not be fully controllable, but in this situation was. This could have been used to spoof a website and attempt to trick the user into providing credentials. This vulnerability affects Firefox ESR < 78.9, Firefox < 87, and Thunderbird < 78.9.

Affected products

• Mozilla Firefox: before 87.0 — fixed in 87.0
• Mozilla Firefox ESR: before 78.9 — fixed in 78.9
• Mozilla Thunderbird: before 78.9 — fixed in 78.9

How to fix

Upgrade to a fixed version: Mozilla Firefox 87.0; Mozilla Firefox ESR 78.9; Mozilla Thunderbird 78.9. Check the vendor advisory for mitigations.

References

• https://bugzilla.mozilla.org/show_bug.cgi?id=1693664
• https://www.mozilla.org/security/advisories/mfsa2021-10/
• https://www.mozilla.org/security/advisories/mfsa2021-11/
• https://www.mozilla.org/security/advisories/mfsa2021-12/
• https://bugzilla.mozilla.org/show_bug.cgi?id=1693664
• https://www.mozilla.org/security/advisories/mfsa2021-10/
• https://www.mozilla.org/security/advisories/mfsa2021-11/
• https://www.mozilla.org/security/advisories/mfsa2021-12/

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