CVE-2021-23984

Name: CVE-2021-23984
Creator: NVD / NIST
Published: 2021-03-31T14:15:19.17+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

MEDIUMCVSS 6.5/10EPSS 1.11%

Last modified Jun 17, 2026

CVE-2021-23984 is a medium-severity vulnerability rated 6.5/10 on the CVSS scale. 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. EPSS estimates a 1.11% chance of exploitation in the next 30 days.

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.

Metrics

CVSS 3.1

6.5/10

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N

EPSS Probability

1.11%

61.8th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-290

Affected Software

Vendor	Product	Versions
Mozilla	Firefox	< 87.0
Mozilla	Firefox Esr	< 78.9
Mozilla	Thunderbird	< 78.9

References

https://bugzilla.mozilla.org/show_bug.cgi?id=1693664Issue Tracking, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-10/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-11/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-12/Vendor Advisory
https://bugzilla.mozilla.org/show_bug.cgi?id=1693664Issue Tracking, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-10/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-11/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-12/Vendor Advisory

Timeline

Published: Mar 31, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-23984?

How severe is CVE-2021-23984?

CVE-2021-23984 has a CVSS score of 6.5/10 (MEDIUM severity). The EPSS model estimates a 1.11% probability of exploitation in the next 30 days.

How do I fix CVE-2021-23984?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

Are you affected by CVE-2021-23984?

Run a free Strix scan to check your systems for this vulnerability.

Scan your code now

Source: NVD / NIST