CVE-2025-11711 is a security vulnerability discovered in Mozilla Firefox and Thunderbird that permits modification of JavaScript Object properties that are supposed to be non-writable. In JavaScript, certain object properties are defined with attributes that prevent their values from being changed after initialization to maintain data integrity and expected behavior. This vulnerability breaks that model by enabling attackers to alter these protected properties, potentially undermining the security assumptions of web applications or browser extensions relying on these properties. The flaw affects Firefox versions earlier than 144, Firefox ESR versions earlier than 115.29 and 140.4, and Thunderbird versions earlier than 144 and 140.4. The vulnerability was publicly disclosed on October 14, 2025, but no CVSS score has been assigned yet, and no known exploits have been reported in the wild. The absence of a CVSS score suggests that the vulnerability is still under evaluation, but the ability to modify non-writable properties could allow attackers to bypass security controls, manipulate application logic, or escalate privileges within the browser context. Since the vulnerability affects core browser and email client components, it could be leveraged in drive-by attacks, malicious web content, or crafted email messages that execute JavaScript. The flaw does not require user authentication, and exploitation likely does not require user interaction beyond visiting a malicious webpage or opening a malicious email. This increases the risk profile, especially for organizations with high exposure to web-based threats. The vulnerability impacts the integrity of JavaScript execution, which could lead to further exploitation chains, including remote code execution or data leakage, depending on how the altered properties are used by applications or extensions.

For European organizations, this vulnerability poses a significant risk to the integrity of their web browsing and email communication environments. Since Firefox is a widely used browser across Europe, and Thunderbird is a popular open-source email client, many enterprises, government agencies, and critical infrastructure operators could be exposed. The ability to modify non-writable object properties could allow attackers to bypass security mechanisms implemented in web applications or browser extensions, potentially leading to unauthorized actions, data manipulation, or privilege escalation within the browser context. This could facilitate further attacks such as phishing, malware delivery, or lateral movement within corporate networks. The vulnerability could also undermine trust in web applications that rely on the immutability of certain JavaScript properties for security. Organizations in sectors with high regulatory requirements for data integrity and confidentiality, such as finance, healthcare, and public administration, could face compliance risks if exploited. Additionally, the lack of known exploits currently provides a window for proactive patching and mitigation before widespread attacks occur.

European organizations should prioritize updating Mozilla Firefox and Thunderbird to versions 144 or later for Firefox and Thunderbird, and ESR versions 115.29 or 140.4 and above as soon as patches are released by Mozilla. Until patches are available, organizations should consider implementing application whitelisting and restricting the execution of untrusted JavaScript code through browser security policies or extensions that limit script execution. Network-level protections such as web filtering to block access to known malicious sites and email filtering to detect and quarantine suspicious messages can reduce exposure. Security teams should monitor Mozilla security advisories and threat intelligence feeds for updates or exploit reports related to CVE-2025-11711. Conducting internal audits of browser extensions and web applications to identify reliance on non-writable JavaScript properties may help assess exposure. User awareness training to recognize phishing and malicious web content remains critical. Organizations should also consider sandboxing or isolating browser processes to limit the impact of potential exploitation. Finally, integrating this vulnerability into vulnerability management and incident response workflows will ensure timely detection and remediation.