CVE-2025-11711 Overview
CVE-2025-11711 is a medium-severity integrity vulnerability affecting Mozilla Firefox and Thunderbird. The flaw allows attackers to change the value of JavaScript Object properties that were designed to be non-writable, breaking a fundamental language invariant. This weakness is classified under [CWE-591] (Sensitive Data Storage in Improperly Locked Memory) and stems from incorrect enforcement of property attributes in the JavaScript engine.
Mozilla addressed the issue in Firefox 144, Firefox ESR 115.29, Firefox ESR 140.4, Thunderbird 144, and Thunderbird 140.4. Exploitation requires user interaction, typically by convincing a user to visit a malicious web page or open crafted HTML content.
Critical Impact
Attackers can bypass JavaScript property write protections to tamper with security-relevant object state, potentially undermining sandboxing assumptions and content isolation in the browser.
Affected Products
- Mozilla Firefox versions prior to 144
- Mozilla Firefox ESR versions prior to 115.29 and 140.4
- Mozilla Thunderbird versions prior to 144 and 140.4
Discovery Timeline
- 2025-10-14 - CVE-2025-11711 published to NVD
- 2026-06-17 - Last updated in NVD database
Technical Details for CVE-2025-11711
Vulnerability Analysis
The vulnerability resides in the SpiderMonkey JavaScript engine used by Firefox and Thunderbird. JavaScript defines property descriptors that specify whether a property is writable, configurable, or enumerable. When a property is marked non-writable through Object.defineProperty or Object.freeze, attempts to reassign its value must fail silently in non-strict mode or throw a TypeError in strict mode.
Under specific conditions, the engine failed to enforce the non-writable attribute, allowing modification of protected property values. This breaks integrity assumptions that JavaScript developers and internal browser code rely on to protect shared state, security tokens, and immutable configuration objects.
Root Cause
The root cause is improper enforcement of property write attributes within the SpiderMonkey engine. A code path in the property store mechanism bypassed the writability check, updating the underlying slot despite the descriptor indicating the property was locked. This aligns with [CWE-591], where data expected to be immutable is stored in a location that permits unintended writes.
Attack Vector
A remote attacker delivers malicious JavaScript through a web page or HTML email content rendered by the affected client. The user must interact with the attacker-controlled content for exploitation. Once executed, the script mutates non-writable properties on internal or third-party objects, corrupting integrity guarantees used by frameworks, content security policies enforced in JavaScript, or extension code. The vulnerability does not directly expose confidential data or crash the process, but it enables logic-level attacks against code that trusts frozen or sealed objects.
The vulnerability manifests in the property store path of the JavaScript engine. See Mozilla Bug Report #1989978 for engineering details.
Detection Methods for CVE-2025-11711
Indicators of Compromise
- Browser or Thunderbird processes running versions older than Firefox 144, Firefox ESR 115.29/140.4, or Thunderbird 144/140.4 after the patch release window.
- Unexpected mutations to frozen or sealed JavaScript objects observed in application logs or telemetry from JavaScript-heavy internal applications.
- Web content that attempts unusual patterns of Object.defineProperty reassignment followed by sensitive operations.
Detection Strategies
- Inventory endpoints for installed Firefox and Thunderbird versions and flag any running vulnerable builds.
- Monitor endpoint process telemetry for firefox.exe, thunderbird.exe, and their Linux and macOS equivalents alongside version metadata.
- Correlate browser child process spawning and unusual outbound network activity following visits to untrusted domains.
Monitoring Recommendations
- Ingest browser version and patch state into a centralized data lake for continuous compliance checks.
- Track EDR telemetry for anomalous script execution paths originating from browser renderer processes.
- Alert on Firefox and Thunderbird installations that remain unpatched beyond the organization's remediation SLA.
How to Mitigate CVE-2025-11711
Immediate Actions Required
- Upgrade all Firefox installations to version 144 or later.
- Upgrade Firefox ESR deployments to 115.29 or 140.4.
- Upgrade Thunderbird installations to 144 or 140.4.
- Apply distribution security updates on Debian and other Linux systems referenced in the Debian LTS Announcement.
Patch Information
Mozilla published fixes in the following advisories: MFSA 2025-81, MFSA 2025-82, MFSA 2025-83, MFSA 2025-84, and MFSA 2025-85. Administrators managing enterprise deployments should push updates through their configuration management systems and validate that all endpoints report the patched version.
Workarounds
- No official workaround exists; upgrading to a patched version is the only supported remediation.
- Restrict browsing to trusted sites through enterprise policy until patches are deployed.
- Disable JavaScript execution in Thunderbird for HTML mail rendering to reduce the attack surface for email-based delivery.
# Verify installed Firefox version on Linux endpoints
firefox --version
# Verify installed Thunderbird version on Linux endpoints
thunderbird --version
# Example Debian/Ubuntu upgrade command
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.
