CVE-2025-5263: Mozilla Firefox Information Disclosure Flaw

CVE-2025-5263 Overview

CVE-2025-5263 is a same-origin policy weakness in Mozilla Firefox and Thunderbird. Error handling for script execution was incorrectly isolated from web content, breaking the boundary between origins. An attacker hosting a malicious page could leverage this gap to perform cross-origin leak attacks against another site loaded by the user.

The flaw maps to [CWE-346: Origin Validation Error]. Mozilla addressed the issue in Firefox 139, Firefox ESR 115.24, Firefox ESR 128.11, Thunderbird 139, and Thunderbird 128.11. Exploitation requires user interaction, such as visiting an attacker-controlled page, and yields confidentiality impact only.

Critical Impact
A remote attacker can infer cross-origin information by observing script error handling, bypassing same-origin isolation guarantees that browsers rely on to protect authenticated sessions.

Affected Products

Mozilla Firefox versions prior to 139
Mozilla Firefox ESR versions prior to 115.24 and 128.11
Mozilla Thunderbird versions prior to 139 and 128.11

Discovery Timeline

2025-05-27 - CVE-2025-5263 published to NVD
2026-04-13 - Last updated in NVD database

Technical Details for CVE-2025-5263

Vulnerability Analysis

The vulnerability resides in how Firefox's JavaScript engine surfaces script execution errors. Error objects generated during script processing were not properly compartmentalized from web content executing in different origins. As a result, error metadata that should remain hidden behind same-origin checks became observable to unrelated pages.

Cross-origin leak attacks abuse this kind of side channel to infer the state, contents, or existence of resources hosted on another origin. Common targets include authenticated user data, single sign-on tokens, and session-bound API responses. The leak does not return raw response bodies but reveals enough signal to reconstruct sensitive state through repeated probing.

Root Cause

The root cause is an origin isolation defect [CWE-346]. Script error propagation paths did not enforce the same-origin policy when surfacing exceptions to content scripts. Code loaded by one origin could therefore observe error behavior triggered by resources or scripts associated with another origin.

Attack Vector

Exploitation is network-based and requires user interaction. The victim must load an attacker-controlled page in a vulnerable Firefox or Thunderbird build. The page then issues cross-origin script loads or other operations and inspects the resulting error handling behavior to extract origin-bound information. No authentication or elevated privileges are required on the attacker side.

No public proof-of-concept exploit code is available. Technical details are referenced in Mozilla Bug Report #1960745 and the corresponding Mozilla Foundation Security Advisories.

Detection Methods for CVE-2025-5263

Indicators of Compromise

Outbound browser traffic to newly registered or low-reputation domains immediately followed by repeated cross-origin script load attempts targeting authenticated services
Browser telemetry showing unusual volumes of script error events tied to a single tab session
Firefox or Thunderbird version strings in user-agent logs matching unpatched builds prior to Firefox 139, ESR 115.24, ESR 128.11, or Thunderbird 128.11

Detection Strategies

Inventory endpoints for Firefox and Thunderbird versions and flag any below the fixed releases for prioritized remediation
Inspect web proxy and DNS logs for patterns consistent with cross-origin probing, such as scripted requests to authenticated SaaS endpoints from non-business origins
Correlate browser process activity with network telemetry to identify suspicious script-loading behavior during user sessions

Monitoring Recommendations

Centralize browser version and update status in asset management to detect drift from the patched baseline
Monitor authentication providers for anomalous session reuse or token replay patterns that could indicate leaked state
Track CVE feeds and the Mozilla Foundation security advisory page for related origin isolation issues that frequently cluster with this class of bug

How to Mitigate CVE-2025-5263

Immediate Actions Required

Update Firefox to version 139 or later on all managed endpoints
Upgrade Firefox ESR deployments to 115.24 or 128.11 depending on the supported branch
Update Thunderbird to 139 or 128.11 to address the matching Thunderbird advisories
Verify Linux distribution package versions, including Debian LTS builds referenced in the advisories, are at the fixed revisions

Patch Information

Mozilla published fixes in the following advisories: MFSA-2025-42, MFSA-2025-43, MFSA-2025-44, MFSA-2025-45, and MFSA-2025-46. Distribution-level updates are covered by Debian LTS Announcement #43 and Debian LTS Announcement #46.

Workarounds

Restrict browsing on unpatched hosts to trusted internal sites until updates are deployed
Enforce automatic browser update policies through enterprise configuration management
Deploy strict Content Security Policy and cross-origin headers on internal applications to reduce the value of any cross-origin leak

bash

# Configuration example: enforce Firefox auto-update via enterprise policies.json
# Place at /etc/firefox/policies/policies.json (Linux) or %ProgramFiles%\Mozilla Firefox\distribution\policies.json (Windows)
{
  "policies": {
    "DisableAppUpdate": false,
    "AppAutoUpdate": true,
    "ManualAppUpdate": false
  }
}