CVE-2026-74962: Mozilla Firefox Information Disclosure Flaw

CVE-2026-74962 Overview

CVE-2026-74962 is a site isolation vulnerability in the Networking: Cookies component of Mozilla Firefox and Thunderbird. The flaw stems from improper origin validation [CWE-346], allowing a network-based attacker to bypass same-site isolation boundaries when a user interacts with a crafted page. Successful exploitation can compromise confidentiality and integrity of cookie data across origins.

Mozilla addressed the issue in Firefox 154, Firefox ESR 140.14, Firefox ESR 153.1, Thunderbird 154, Thunderbird 140.14, and Thunderbird 153.1.

Critical Impact
An attacker can bypass site isolation for cookies over the network with only user interaction required, exposing session state and enabling cross-site tampering.

Affected Products

Mozilla Firefox versions prior to 154
Mozilla Firefox ESR versions prior to 140.14 and 153.1
Mozilla Thunderbird versions prior to 154, 140.14, and 153.1

Discovery Timeline

2026-08-18 - CVE-2026-74962 published to NVD
2026-08-19 - Last updated in NVD database

Technical Details for CVE-2026-74962

Vulnerability Analysis

The vulnerability resides in the Networking: Cookies component, which enforces site isolation for cookie storage and retrieval. Site isolation ensures that cookies belonging to one origin are inaccessible to content served by a different origin. In the affected builds, this boundary is not consistently enforced, resulting in an origin validation failure classified under [CWE-346].

Because the issue involves cookies, exploitation can expose authentication tokens, session identifiers, and other sensitive state used to authorize requests. The impact scope covers confidentiality and integrity of application state that relies on cookie separation between sites.

Root Cause

The root cause is improper origin validation within the cookie handling path. The component does not sufficiently verify that the requesting context matches the cookie's owning site before granting access, breaking the same-origin guarantee that site isolation is intended to preserve.

Attack Vector

Exploitation occurs over the network. An attacker hosts a crafted page or resource and induces the target user to visit or interact with it. User interaction is required, but no privileges or prior authentication to the browser are needed. Once triggered, the flaw allows cross-site access to cookie data that should be isolated.

No verified public proof-of-concept code is available for this issue. Refer to Mozilla Bug Report #2050425 and the associated advisories for technical detail.

Detection Methods for CVE-2026-74962

Indicators of Compromise

Firefox or Thunderbird processes at versions below the fixed builds (< 154, ESR < 140.14, ESR < 153.1) present in the environment.
Anomalous outbound connections from browser processes to newly registered or low-reputation domains that coincide with user browsing sessions.
Unexpected authenticated session activity from user accounts that recently browsed untrusted sites, suggesting cookie or session reuse.

Detection Strategies

Inventory installed browser and mail client versions across managed endpoints and flag hosts running vulnerable builds.
Correlate web proxy logs with endpoint browser telemetry to identify visits to suspicious pages followed by session anomalies for shared web applications.
Monitor for unusual cross-origin request patterns and cookie-bearing requests to sensitive internal applications originating from workstations running unpatched Firefox or Thunderbird.

Monitoring Recommendations

Track version compliance for Mozilla products using existing endpoint inventory sources.
Alert on authentication events for critical SaaS or internal apps that originate from atypical sessions after known risky browsing activity.
Log and review browser update failures to ensure automatic updates are reaching endpoints.

How to Mitigate CVE-2026-74962

Immediate Actions Required

Update Firefox to version 154 or Firefox ESR to 140.14 or 153.1 across all managed endpoints.
Update Thunderbird to version 154, 140.14, or 153.1 on all systems where it is deployed.
Verify that browser auto-update mechanisms are enabled and functioning after deployment.

Patch Information

Mozilla has released fixes in Firefox 154, Firefox ESR 140.14, Firefox ESR 153.1, Thunderbird 154, Thunderbird 140.14, and Thunderbird 153.1. See Mozilla Security Advisory MFSA 2026-74, MFSA 2026-76, MFSA 2026-77, MFSA 2026-78, MFSA 2026-79, and MFSA 2026-80 for build-specific fix details.

Workarounds

Restrict browsing to trusted sites where feasible until patches are deployed, since user interaction with attacker-controlled content is required for exploitation.
Enforce enterprise policies that block untrusted third-party content and limit cross-site cookie behavior in browser configuration.
Segment access to high-value web applications using additional authentication controls that do not rely solely on browser cookies.

bash

# Configuration example: verify Firefox version on Linux endpoints
firefox --version

# Windows PowerShell: query installed Mozilla product versions
Get-ItemProperty 'HKLM:\Software\Mozilla\Mozilla Firefox' | Select-Object CurrentVersion
Get-ItemProperty 'HKLM:\Software\Mozilla\Mozilla Thunderbird' | Select-Object CurrentVersion