CVE-2025-1935 is a vulnerability classified under CWE-79 (Improper Neutralization of Input During Web Page Generation, commonly known as Cross-Site Scripting or XSS-related issues) that affects Mozilla Firefox and Thunderbird before versions 136 and ESR versions before 128.8. The issue arises from the way the browser handles the registerProtocolHandler API, which allows websites to register themselves as handlers for custom URL protocols. An attacker can craft a malicious web page that uses clickjacking techniques to trick a user into accepting the registration of the attacker’s site as the default handler for a custom protocol. This is done by overlaying or hiding the info-bar prompt that Firefox displays when a site requests to register as a protocol handler, thereby misleading the user into consenting unknowingly. The vulnerability requires no privileges or authentication but does require user interaction (clicking or accepting the prompt). The CVSS v3.1 score is 4.3 (medium), reflecting the network attack vector, low complexity, no privileges required, but user interaction needed, and limited confidentiality impact without integrity or availability consequences. The vulnerability does not currently have known exploits in the wild and no official patches are linked yet, but affected users should anticipate updates from Mozilla. The exploitation could lead to phishing or redirect attacks by hijacking protocol handlers, potentially exposing users to further social engineering or malicious payloads.

For European organizations, the primary impact of CVE-2025-1935 is the potential compromise of user trust and confidentiality. If an attacker successfully tricks users into registering malicious protocol handlers, they could redirect or intercept specific URL schemes, leading to phishing, data leakage, or unauthorized actions initiated by protocol links. This can be particularly harmful in sectors relying heavily on secure communications and custom protocols, such as finance, government, and critical infrastructure. Although the vulnerability does not directly affect system integrity or availability, the indirect consequences of social engineering attacks enabled by this flaw could lead to credential theft or further exploitation. Organizations with large user bases using Firefox or Thunderbird on desktops are at risk, especially if users are not trained to recognize suspicious prompts. The lack of known exploits reduces immediate risk but does not eliminate the threat, especially as attackers may develop exploits once patches are released. The vulnerability’s network-based attack vector means it can be exploited remotely via malicious web content, increasing the attack surface for European enterprises with internet-facing users.

European organizations should implement the following specific mitigations: 1) Promptly update Mozilla Firefox and Thunderbird to versions 136 or ESR 128.8 or later once patches are released to eliminate the vulnerability. 2) Until patches are available, consider deploying browser policies or extensions that disable or restrict the registerProtocolHandler API to prevent unauthorized protocol registrations. 3) Conduct user awareness training focused on recognizing and rejecting suspicious protocol handler prompts and clickjacking attempts. 4) Employ web content filtering and endpoint security solutions to block or flag malicious web pages attempting to exploit this vulnerability. 5) Monitor network traffic for unusual protocol handler registrations or unexpected protocol invocations that could indicate exploitation attempts. 6) For organizations with custom protocol handlers, review and harden their registration and invocation mechanisms to prevent misuse. 7) Engage with Mozilla’s security advisories and update management processes to ensure timely patch deployment. These measures go beyond generic advice by focusing on controlling the specific attack vector and user interaction element unique to this vulnerability.