CVE-2025-5266 Overview
CVE-2025-5266 is an information disclosure vulnerability in Mozilla Firefox and Thunderbird. Script elements loading cross-origin resources generated load and error events that leaked information about the loaded resource. Attackers can abuse this behavior to conduct Cross-Site Leak (XS-Leaks) attacks against users visiting a malicious page. The issue is classified under [CWE-200] Information Exposure. Mozilla addressed the flaw in Firefox 139, Firefox ESR 128.11, Thunderbird 139, and Thunderbird 128.11.
Critical Impact
Attackers can infer cross-origin resource state from a user's browsing session by observing load and error events fired by cross-origin script requests, enabling XS-Leaks reconnaissance against authenticated users.
Affected Products
- Mozilla Firefox versions prior to 139
- Mozilla Firefox ESR versions prior to 128.11
- Mozilla Thunderbird versions prior to 139 and 128.11
Discovery Timeline
- 2025-05-27 - CVE-2025-5266 published to NVD
- 2026-06-17 - Last updated in NVD database
Technical Details for CVE-2025-5266
Vulnerability Analysis
The vulnerability resides in how Firefox handles <script> elements that reference cross-origin resources. When a script element requests an external resource, the browser fires either a load or error event depending on the response. These events remained observable to the embedding page even when the resource crossed origin boundaries. Attackers can register handlers on these events to differentiate between successful and failed cross-origin loads. This side channel enables XS-Leaks attacks that reveal user-specific state on third-party sites, such as authentication status or search results tied to the victim's session.
Root Cause
The root cause is an information exposure weakness [CWE-200] in the Firefox script loader. The load and error event dispatch logic did not sufficiently restrict information from cross-origin script fetches. The browser exposed distinguishable outcomes to script consumers regardless of the response origin.
Attack Vector
Exploitation requires user interaction, typically visiting a malicious or attacker-controlled web page. The attacker embeds <script> tags pointing at sensitive cross-origin endpoints. Event handlers observe whether each request succeeded or failed. The attacker aggregates these signals to infer content or state on the targeted origin. No credentials or elevated privileges are required on the attacker side.
No verified proof-of-concept code is publicly available. See the Mozilla Bug Report #1965628 for the technical discussion.
Detection Methods for CVE-2025-5266
Indicators of Compromise
- Web requests from browsers to third-party origins containing repeated <script> element references to sensitive endpoints such as authenticated APIs or search URLs.
- Anomalous JavaScript on untrusted pages registering onload and onerror handlers against cross-origin script sources.
- Client telemetry showing outdated Firefox or Thunderbird build IDs below the fixed versions.
Detection Strategies
- Inventory browser versions across managed endpoints and flag any Firefox instance below 139 or ESR below 128.11.
- Inspect proxy or DNS logs for patterns consistent with XS-Leaks probing, such as bursts of cross-origin script requests followed by outbound telemetry beacons.
- Correlate web filtering categories with browser process telemetry to surface users loading untrusted content in vulnerable browser versions.
Monitoring Recommendations
- Enforce browser version reporting through endpoint management and alert when hosts run unpatched Mozilla builds.
- Monitor egress traffic for suspicious script-loading sequences from newly registered or low-reputation domains.
- Track Mozilla security advisories MFSA-2025-42 and MFSA-2025-44 for related updates.
How to Mitigate CVE-2025-5266
Immediate Actions Required
- Upgrade Firefox to version 139 or later on all managed workstations.
- Upgrade Firefox ESR deployments to version 128.11 or later.
- Upgrade Thunderbird to version 139 or Thunderbird ESR 128.11.
- Apply the Debian LTS updates referenced in the Debian LTS Announcement May 2025 #43 and Debian LTS Announcement May 2025 #46.
Patch Information
Mozilla fixed the vulnerability in Firefox 139, Firefox ESR 128.11, Thunderbird 139, and Thunderbird 128.11. Details are documented in the Mozilla Security Advisory MFSA-2025-42, MFSA-2025-44, MFSA-2025-45, and MFSA-2025-46.
Workarounds
- Restrict browsing to trusted sites through web filtering while patches are rolled out.
- Deploy Content Security Policy headers on sensitive first-party applications to reduce the value of any successful XS-Leaks probe.
- Consider disabling third-party scripts through browser policies until endpoints are updated.
# Verify Firefox version on Linux endpoints
firefox --version
# Example Debian update commands
sudo apt update
sudo apt install --only-upgrade firefox-esr thunderbird
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
