CVE-2025-11712 is a vulnerability identified in Mozilla Firefox and Thunderbird before versions 144 and ESR 140.4, respectively. The issue arises from the handling of the OBJECT HTML tag's type attribute, which can be manipulated by a malicious web page to override the browser's default behavior when encountering web resources served without a content-type header. Normally, browsers rely on the content-type header to determine how to process and render resources. However, if a server fails to specify this header, Firefox's behavior can be influenced by the OBJECT tag's type attribute, potentially allowing an attacker to execute arbitrary scripts in the context of the victim's browser. This results in a cross-site scripting (XSS) vulnerability, specifically linked to CWE-116 (Improper Encoding or Escaping of Output). The vulnerability requires no privileges and can be exploited remotely over the network, but it does require user interaction, such as visiting a crafted malicious web page. The scope is 'changed' because the vulnerability can affect the confidentiality and integrity of data within the browser context, potentially allowing theft of sensitive information or session hijacking. The CVSS v3.1 score is 6.1 (medium severity), reflecting the ease of exploitation and the impact on confidentiality and integrity but no impact on availability. No patches or exploits are currently publicly available, but the vulnerability is published and should be addressed promptly. This vulnerability is particularly relevant for web applications or sites that serve files without proper content-type headers, as they increase the attack surface for XSS via this flaw.

For European organizations, the primary impact of CVE-2025-11712 lies in the potential compromise of user confidentiality and integrity of data processed within Firefox or Thunderbird clients. Attackers could exploit this vulnerability to execute malicious scripts, leading to session hijacking, theft of sensitive information such as credentials or personal data, and unauthorized actions performed on behalf of the user. This is especially critical for organizations handling sensitive or regulated data, including financial institutions, healthcare providers, and government agencies. The vulnerability does not affect availability, so denial-of-service is not a concern here. However, the exploitation requires user interaction, meaning phishing or social engineering campaigns could be used to lure users to malicious pages. European organizations with web servers that do not enforce proper content-type headers are at higher risk, as this misconfiguration directly contributes to the exploitability of the vulnerability. The medium severity rating suggests a moderate but significant risk, warranting timely remediation to prevent potential data breaches or compromise of user accounts.

1. Upgrade affected Mozilla Firefox and Thunderbird clients to version 144 or ESR 140.4 and above as soon as updates become available. 2. Audit and enforce proper server-side configurations to ensure all web resources are served with explicit and correct content-type headers, eliminating the conditions that enable 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. Educate users about the risks of interacting with untrusted or suspicious web pages, emphasizing caution with links received via email or messaging platforms. 5. Monitor network traffic and browser logs for unusual activity that could indicate exploitation attempts. 6. Employ web application firewalls (WAFs) configured to detect and block XSS payloads and anomalous OBJECT tag usage. 7. Coordinate with IT and security teams to prioritize patch management and vulnerability scanning focused on browser and email client versions in use.