CVE-2025-11711 is a vulnerability discovered in Mozilla Firefox and Thunderbird that allows an attacker to change the value of JavaScript object properties that are intended to be non-writeable. This issue arises from improper enforcement of property attributes in the JavaScript engine, specifically violating the expected immutability of certain object properties. Affected versions include Firefox prior to 144, Firefox ESR versions before 115.29 and 140.4, and Thunderbird versions before 144 and 140.4. The vulnerability can be exploited remotely by an unauthenticated attacker who entices a user to visit a malicious website or open a crafted email containing malicious JavaScript code. Exploitation requires user interaction but no prior privileges. The impact primarily concerns the integrity of the JavaScript environment, potentially allowing attackers to manipulate application logic, bypass security restrictions, or facilitate further exploitation chains. There is no direct impact on confidentiality or availability. The CVSS v3.1 score is 6.5 (medium), reflecting network attack vector, low attack complexity, no privileges required, user interaction needed, unchanged scope, no confidentiality impact, high integrity impact, and no availability impact. No public exploits are known at this time, and Mozilla has not yet published patches, though the vulnerability is officially assigned and disclosed. The CWE classification is CWE-591, indicating improper enforcement of property attributes. This vulnerability underscores the importance of strict adherence to JavaScript property semantics to prevent manipulation of browser behavior.

For European organizations, this vulnerability poses a risk primarily to the integrity of web applications and browser-based security controls. Attackers could exploit this flaw to alter JavaScript object properties that should be immutable, potentially enabling bypass of client-side security mechanisms, manipulation of web application logic, or facilitation of further attacks such as cross-site scripting (XSS) or privilege escalation within the browser context. Although there is no direct confidentiality or availability impact, the integrity compromise can lead to data manipulation or unauthorized actions within web applications. Organizations relying heavily on Firefox or Thunderbird for daily operations, especially those handling sensitive data or critical services, may face increased risk of targeted attacks leveraging this vulnerability. The requirement for user interaction means phishing or social engineering campaigns could be used to trigger exploitation. Given the widespread use of Firefox and Thunderbird in Europe, particularly in government, education, and enterprise sectors, the threat could have broad implications if exploited at scale.

1. Monitor Mozilla security advisories closely and apply official patches or updates as soon as they are released to affected Firefox and Thunderbird versions. 2. Until patches are available, consider deploying browser configuration policies that restrict JavaScript execution from untrusted sources, such as enabling strict Content Security Policy (CSP) headers on internal and external web applications. 3. Educate users about the risks of interacting with untrusted websites or email content to reduce the likelihood of triggering exploitation. 4. Employ network-level protections such as web filtering and email security gateways to block access to known malicious domains or phishing campaigns. 5. Use endpoint detection and response (EDR) solutions to monitor for anomalous browser behavior that could indicate exploitation attempts. 6. For organizations with custom web applications, conduct thorough security reviews to ensure client-side code does not rely on assumptions about JavaScript property immutability that could be violated by this vulnerability. 7. Consider temporary use of alternative browsers for critical users until patches are applied, if feasible.