CVE-2021-23999

Name: CVE-2021-23999
Creator: NVD / NIST
Published: 2021-06-24T14:15:09.477+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

HIGHCVSS 8.8/10EPSS 1.28%

Last modified Jun 17, 2026

CVE-2021-23999 is a high-severity vulnerability rated 8.8/10 on the CVSS scale. If a Blob URL was loaded through some unusual user interaction, it could have been loaded by the System Principal and granted additional privileges that should not be granted to web content. This vulnerability affects Firefox ESR < 78.10, Thunderbird < 78.10, and Firefox < 88.. EPSS estimates a 1.28% chance of exploitation in the next 30 days.

Description

If a Blob URL was loaded through some unusual user interaction, it could have been loaded by the System Principal and granted additional privileges that should not be granted to web content. This vulnerability affects Firefox ESR < 78.10, Thunderbird < 78.10, and Firefox < 88.

Metrics

CVSS 3.1

8.8/10

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

EPSS Probability

1.28%

66.3th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

Affected Software

Vendor	Product	Versions
Mozilla	Firefox	< 88.0
Mozilla	Firefox Esr	< 78.10
Mozilla	Thunderbird	< 78.10

References

https://bugzilla.mozilla.org/show_bug.cgi?id=1691153Exploit, Issue Tracking, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-14/Release Notes, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-15/Release Notes, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-16/Release Notes, Vendor Advisory
https://bugzilla.mozilla.org/show_bug.cgi?id=1691153Exploit, Issue Tracking, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-14/Release Notes, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-15/Release Notes, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-16/Release Notes, Vendor Advisory

Timeline

Published: Jun 24, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-23999?

How severe is CVE-2021-23999?

CVE-2021-23999 has a CVSS score of 8.8/10 (HIGH severity). The EPSS model estimates a 1.28% probability of exploitation in the next 30 days.

How do I fix CVE-2021-23999?

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-2021-23999?

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

Scan your code now

Source: NVD / NIST