CVE-2025-11712 is a security vulnerability discovered in Mozilla Firefox and Thunderbird products, specifically affecting Firefox versions prior to 144 and Firefox ESR versions prior to 140.4, as well as Thunderbird versions prior to 144 and 140.4. The vulnerability arises from the way the browser handles the type attribute of an OBJECT HTML tag when encountering web resources that are served without a content-type HTTP header. Normally, browsers rely on the content-type header to determine how to process and render resources. However, in this case, the type attribute could override the default browser behavior, potentially allowing a malicious web page to manipulate how the browser interprets and executes content. This behavior can be exploited to facilitate cross-site scripting (XSS) attacks on websites that unsafely serve files without specifying a content-type header. XSS vulnerabilities allow attackers to inject malicious scripts into web pages viewed by other users, potentially leading to session hijacking, data theft, or other malicious activities. The vulnerability does not require user authentication but does require that the targeted site improperly serve resources without content-type headers, which is a misconfiguration or unsafe practice on the server side. There are no known exploits in the wild at the time of publication, but the vulnerability is publicly disclosed and thus could be targeted by attackers. The absence of a CVSS score means severity must be assessed based on impact and exploitability factors. Given the potential for script injection and the reliance on server misconfiguration, this vulnerability represents a significant risk to affected users. Mozilla has published the vulnerability details but no direct patch links were provided in the source data, indicating that users should update to Firefox 144 or ESR 140.4 or later once available to remediate the issue.

For European organizations, this vulnerability poses a risk primarily to web applications and services that rely on Firefox or Thunderbird clients and that may serve resources without proper content-type headers. Successful exploitation could lead to cross-site scripting attacks, compromising user sessions, stealing sensitive data, or enabling further attacks such as privilege escalation or malware delivery. The impact on confidentiality and integrity is high if exploited, as attackers can execute arbitrary scripts in the context of trusted websites. Availability impact is low as this is not a denial-of-service vulnerability. Organizations with legacy Firefox or Thunderbird deployments are at greater risk. Additionally, organizations hosting web resources with improper content-type headers increase their exposure. Given the widespread use of Firefox in Europe, especially in government, education, and enterprise sectors, the vulnerability could affect a broad user base. The lack of known exploits reduces immediate risk but the public disclosure increases the likelihood of future attacks. Therefore, European entities should consider this a high-priority issue to prevent potential compromise.

1. Upgrade all Mozilla Firefox installations to version 144 or later and Thunderbird to version 144 or later, or at minimum to Firefox ESR 140.4 or Thunderbird ESR 140.4 to ensure the vulnerability is patched. 2. Audit and correct web server configurations to ensure all served resources include accurate and appropriate content-type headers, eliminating the unsafe condition that enables this vulnerability. 3. Implement Content Security Policy (CSP) headers on web applications to restrict the execution of unauthorized scripts and reduce the impact of potential XSS attacks. 4. Conduct security reviews and penetration testing focused on XSS vulnerabilities, especially on sites that serve dynamic or user-generated content. 5. Educate developers and system administrators about the importance of proper HTTP header management and secure coding practices to prevent similar issues. 6. Monitor security advisories from Mozilla for any updates or patches related to this vulnerability. 7. Employ web application firewalls (WAFs) with rules to detect and block suspicious script injection attempts. These steps go beyond generic advice by focusing on both client-side patching and server-side configuration hardening.