CVE-2025-10859: Data stored in cookies for non-HTML content while browsing Incognito could be viewed after closing private tabs

Severity: medium (CVSS 4.0 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.1% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2025-09-30 · updated 2026-10-05
Weakness: CWE-359

Description

Cookie storage for non-HTML temporary documents was being shared incorrectly with normal browsing content, allowing information from private tabs to escape Incognito mode even after the user closed all tabs. This vulnerability was fixed in Firefox for iOS 143.1.

Affected products

Mozilla Firefox: before 143.1.0 — fixed in 143.1.0

How to fix

Upgrade to a fixed version: Mozilla Firefox 143.1.0. Check the vendor advisory for mitigations.

References

https://bugzilla.mozilla.org/show_bug.cgi?id=1684624
https://www.mozilla.org/security/advisories/mfsa2025-79/

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