What is CVE-2021-29971?

CVE-2021-29971 is a critical-severity vulnerability in Mozilla Firefox, classified under Improper Preservation of Permissions. CVSS score: 9.8/10. Published 2021-08-05.

How severe is CVE-2021-29971?

Critical severity. CVSS v3 base score is 9.8 out of 10.

Vulnerability in Mozilla Firefox

CVE-2021-29971

If a user had granted a permission to a webpage and saved that grant, any webpage running on the same host - irrespective of scheme or port - would be granted that permission. *This bug only affects Firefox for Android. Other operating systems are unaffected.*. This vulnerability affects Firefox < 90.

Published 2021-08-05 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 9.8 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H.

EPSS exploit prediction

EPSS: 0.010 (62.1th percentile), scored 2026-10-01.

Low probability of exploitation in the next 30 days, but still tracked. 62nd percentile — 62.1% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.010 · newest: 0.010 · change: 0.000

Affected products

Mozilla Firefox — versions unspecified

Weakness classification (CWE)

CWE-281 — Improper Preservation of Permissions

References

www.mozilla.org/security/advisories/mfsa2021-28/ (Vendor Advisory)
bugzilla.mozilla.org/show_bug.cgi (Permissions Required, Issue Tracking)

Frequently asked questions

What is CVE-2021-29971?: CVE-2021-29971 is a critical-severity vulnerability in Mozilla Firefox, classified under Improper Preservation of Permissions. CVSS score: 9.8/10. Published 2021-08-05.
How severe is CVE-2021-29971?: Critical severity. CVSS v3 base score is 9.8 out of 10.