CVE-2025-10529: Mozilla Firefox Auth Bypass Vulnerability

CVE-2025-10529 Overview

CVE-2025-10529 is a same-origin policy (SOP) bypass in the Layout component of Mozilla Firefox and Thunderbird. The flaw is classified under [CWE-942] (Permissive Cross-domain Policy with Untrusted Domains) and allows a remote attacker to circumvent origin isolation boundaries enforced by the browser rendering engine. Mozilla addressed the issue in Firefox 143, Firefox ESR 140.3, Thunderbird 143, and Thunderbird 140.3.

Exploitation requires no authentication and no user interaction beyond loading attacker-controlled web content. Successful abuse can lead to disclosure of cross-origin data and limited integrity impact on data accessible to the victim's browsing session.

Critical Impact
A remote, network-based attacker can bypass the browser's same-origin policy via the Layout component to read or influence data from other origins loaded in Firefox or Thunderbird.

Affected Products

Mozilla Firefox versions prior to 143
Mozilla Firefox ESR versions prior to 140.3
Mozilla Thunderbird versions prior to 143 and prior to 140.3

Discovery Timeline

2025-09-16 - CVE-2025-10529 published to the National Vulnerability Database (NVD)
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-10529

Vulnerability Analysis

The vulnerability resides in the Layout component of Gecko, the rendering engine shared by Firefox and Thunderbird. The Layout module is responsible for computing the visual arrangement of DOM elements, including cross-origin frames, images, and other embedded content. A logic flaw in this component permits an attacker-controlled document to observe or interact with resources served from a different origin.

Under the same-origin policy, scripts loaded from origin A must not be able to read content from origin B unless CORS grants access. This vulnerability breaks that boundary, exposing data such as document contents, computed layout information, or rendered pixel data that should remain isolated.

Because Thunderbird reuses the Gecko engine to render HTML email, the flaw extends to the mail client. Remote content loaded within a message context can serve as the attacker origin.

Root Cause

The root cause is a permissive cross-domain policy in layout-related code paths, aligned with [CWE-942]. Specific technical details are tracked in Mozilla Bug Report #1970490, which remains access-restricted. The advisories at MFSA-2025-73, MFSA-2025-75, MFSA-2025-77, and MFSA-2025-78 confirm the fix landed in Firefox 143 and equivalent ESR and Thunderbird builds.

Attack Vector

An attacker hosts a malicious web page or crafts HTML email content that triggers the vulnerable layout code path. When a victim visits the page in a vulnerable Firefox build or opens the message in a vulnerable Thunderbird build with remote content enabled, the attacker's script observes data from a cross-origin resource loaded in the same session. No privileges or user interaction beyond navigation are required.

The vulnerability manifests through interactions in the Layout component; consult the linked Mozilla Security Advisories for the technical fix details, as no verified public proof-of-concept is available.

Detection Methods for CVE-2025-10529

Indicators of Compromise

Firefox or Thunderbird processes running versions below 143 (or ESR 140.3) after the patch release date
Outbound requests from browser sessions to unfamiliar domains immediately following visits to untrusted pages containing suspicious iframes or cross-origin embeds
HTML email messages in Thunderbird that trigger remote content loads to attacker-controlled infrastructure

Detection Strategies

Inventory endpoint browser and mail client versions and flag any Firefox build below 143, Firefox ESR below 140.3, or Thunderbird below 143/140.3
Correlate web proxy logs for user visits to newly registered or low-reputation domains hosting iframes referencing sensitive internal applications
Review Thunderbird configurations enterprise-wide to confirm remote content blocking policies are enforced for untrusted senders

Monitoring Recommendations

Monitor DNS and HTTP telemetry for browser sessions requesting cross-origin resources from anomalous parent pages
Track software inventory feeds for Mozilla product versions and alert on non-compliant endpoints
Log Thunderbird message rendering events where remote content was permitted, particularly for external senders

How to Mitigate CVE-2025-10529

Immediate Actions Required

Upgrade Firefox to version 143 or later on all endpoints
Upgrade Firefox ESR to version 140.3 or later where ESR is used for enterprise standardization
Upgrade Thunderbird to version 143 or Thunderbird ESR 140.3 or later
Apply distribution-specific updates such as those announced in Debian LTS Announcement #20 and Debian LTS Announcement #26

Patch Information

Mozilla shipped the fix in Firefox 143, Firefox ESR 140.3, Thunderbird 143, and Thunderbird 140.3. Refer to MFSA-2025-73, MFSA-2025-75, MFSA-2025-77, and MFSA-2025-78 for release notes and full advisory content.

Workarounds

Disable JavaScript for untrusted origins using enterprise policy where operationally feasible until patching completes
Block remote content loading in Thunderbird for all external senders via the mailnews.message_display.disable_remote_image preference
Route browser traffic through an enterprise proxy that blocks known malicious domains and enforces strict Content Security Policies for internal applications
Deploy browser policies to restrict third-party frames on sensitive internal web applications

bash

# Enterprise policies.json example to enforce Firefox auto-update to a patched version
{
  "policies": {
    "AppAutoUpdate": true,
    "DisableAppUpdate": false,
    "OverrideFirstRunPage": "",
    "BlockAboutConfig": false
  }
}