What is CVE-2021-23958?

CVE-2021-23958 is a medium-severity vulnerability in Mozilla Firefox, classified under Exposure of Resource to Wrong Sphere. CVSS score: 6.5/10. Published 2021-02-26.

How severe is CVE-2021-23958?

Medium severity. CVSS v3 base score is 6.5 out of 10.

Vulnerability in Mozilla Firefox

CVE-2021-23958

The browser could have been confused into transferring a screen sharing state into another tab, which would leak unintended information. This vulnerability affects Firefox < 85.

Published 2021-02-26 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 6.5 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N.

EPSS exploit prediction

EPSS: 0.009 (57.5th percentile), scored 2026-10-01.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 58th percentile — 57.5% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.009 · newest: 0.009 · change: -0.000

Affected products

Mozilla Firefox — versions < 85

Weakness classification (CWE)

CWE-668 — Exposure of Resource to Wrong Sphere

References

www.mozilla.org/security/advisories/mfsa2021-03/ (Release Notes, Vendor Advisory)
bugzilla.mozilla.org/show_bug.cgi (Permissions Required, Issue Tracking, Vendor Advisory)

Frequently asked questions

What is CVE-2021-23958?: CVE-2021-23958 is a medium-severity vulnerability in Mozilla Firefox, classified under Exposure of Resource to Wrong Sphere. CVSS score: 6.5/10. Published 2021-02-26.
How severe is CVE-2021-23958?: Medium severity. CVSS v3 base score is 6.5 out of 10.