CVE-2021-23953

Name: CVE-2021-23953
Creator: NVD / NIST
Published: 2021-02-26T03:15:13.433+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

MEDIUMCVSS 4.3/10EPSS 1.06%

Last modified Jun 17, 2026

CVE-2021-23953 is a medium-severity vulnerability rated 4.3/10 on the CVSS scale. If a user clicked into a specifically crafted PDF, the PDF reader could be confused into leaking cross-origin information, when said information is served as chunked data. This vulnerability affects Firefox < 85, Thunderbird < 78.7, and Firefox ESR < 78.7.. EPSS estimates a 1.06% chance of exploitation in the next 30 days.

Description

If a user clicked into a specifically crafted PDF, the PDF reader could be confused into leaking cross-origin information, when said information is served as chunked data. This vulnerability affects Firefox < 85, Thunderbird < 78.7, and Firefox ESR < 78.7.

Metrics

CVSS 3.1

4.3/10

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

EPSS Probability

1.06%

60.1th percentile

Probability of exploitation in the next 30 days. Learn more

Affected Software

Vendor	Product	Versions
Mozilla	Firefox	< 85.0
Mozilla	Firefox Esr	< 78.7
Mozilla	Thunderbird	< 78.7

References

https://bugzilla.mozilla.org/show_bug.cgi?id=1683940Issue Tracking, Permissions Required, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-03/Release Notes, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-04/Release Notes, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-05/Release Notes, Vendor Advisory
https://bugzilla.mozilla.org/show_bug.cgi?id=1683940Issue Tracking, Permissions Required, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-03/Release Notes, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-04/Release Notes, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-05/Release Notes, Vendor Advisory

Timeline

Published: Feb 26, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-23953?

How severe is CVE-2021-23953?

CVE-2021-23953 has a CVSS score of 4.3/10 (MEDIUM severity). The EPSS model estimates a 1.06% probability of exploitation in the next 30 days.

How do I fix CVE-2021-23953?

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-2021-23953?

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

Scan your code now

Source: NVD / NIST