What is CVE-2026-53900?

CVE-2026-53900 is a medium-severity vulnerability in Mozilla Firefox For iOS, classified under Insufficient Verification of Data Authenticity. CVSS score: 4.3/10. Published 2026-06-16.

How severe is CVE-2026-53900?

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

Vulnerability in Mozilla Firefox For iOS

CVE-2026-53900

Firefox for iOS preserved cookies set on the initial PDF request across cross-origin HTTP redirects in TemporaryDocument, allowing a malicious site to inject arbitrary cookies into requests to an unrelated target domain. This vulnerability was fixed in Firefox for iOS 152.0.

Published 2026-06-16 · 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: 0.001 (3.1th percentile), scored 2026-09-29.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 3rd percentile — 3.1% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.001 · newest: 0.001 · change: +0.000

Affected products

Mozilla Firefox For iOS — versions 152.0
Mozilla Firefox Mobile

Weakness classification (CWE)

References

bugzilla.mozilla.org/show_bug.cgi (Permissions Required)
www.mozilla.org/security/advisories/mfsa2026-56/ (Vendor Advisory)

Frequently asked questions

What is CVE-2026-53900?: CVE-2026-53900 is a medium-severity vulnerability in Mozilla Firefox For iOS, classified under Insufficient Verification of Data Authenticity. CVSS score: 4.3/10. Published 2026-06-16.
How severe is CVE-2026-53900?: Medium severity. CVSS v3 base score is 4.3 out of 10.