What is CVE-2026-53899?

CVE-2026-53899 is a medium-severity vulnerability in Mozilla Firefox For iOS, classified under Insufficient Verification of Data Authenticity. CVSS score: 6.5/10. Published 2026-06-16.

How severe is CVE-2026-53899?

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

Vulnerability in Mozilla Firefox For iOS

CVE-2026-53899

Firefox for iOS used partial domain matching when attaching cookies to PDF requests, allowing a malicious site on a suffix domain to receive cookies belonging to the target site. 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 6.5 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N.

EPSS exploit prediction

EPSS: 0.001 (2.5th percentile), scored 2026-09-30.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 3rd percentile — 2.5% 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)

CWE-345 — Insufficient Verification of Data Authenticity

References

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

Frequently asked questions

What is CVE-2026-53899?: CVE-2026-53899 is a medium-severity vulnerability in Mozilla Firefox For iOS, classified under Insufficient Verification of Data Authenticity. CVSS score: 6.5/10. Published 2026-06-16.
How severe is CVE-2026-53899?: Medium severity. CVSS v3 base score is 6.5 out of 10.