What is CVE-2021-29958?

CVE-2021-29958 is a medium-severity vulnerability in Mozilla Firefox, classified under Missing Authorization. CVSS score: 4.3/10. Published 2021-06-24.

How severe is CVE-2021-29958?

Medium severity. CVSS v3 base score is 4.3 out of 10.

Auth bypass in Mozilla Firefox

CVE-2021-29958

When a download was initiated, the client did not check whether it was in normal or private browsing mode, which led to private mode cookies being shared in normal browsing mode. This vulnerability affects Firefox for iOS < 34.

Vulnerability class: Broken Access Control

Published 2021-06-24 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 4.3 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:N/A:N.

EPSS exploit prediction

EPSS: 0.007 (50.1th percentile), scored 2026-10-01.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 50th percentile — 50.1% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.007 · newest: 0.007 · change: 0.000

Affected products

Mozilla Firefox
Mozilla Firefox For iOS — versions unspecified

Weakness classification (CWE)

CWE-862 — Missing Authorization

References

www.mozilla.org/security/advisories/mfsa2021-25/ (Release Notes, Vendor Advisory)
bugzilla.mozilla.org/show_bug.cgi (Permissions Required, Vendor Advisory)

Frequently asked questions

What is CVE-2021-29958?: CVE-2021-29958 is a medium-severity vulnerability in Mozilla Firefox, classified under Missing Authorization. CVSS score: 4.3/10. Published 2021-06-24.
How severe is CVE-2021-29958?: Medium severity. CVSS v3 base score is 4.3 out of 10.