CVE-2025-66563 is a cross-site scripting (XSS) vulnerability categorized under CWE-79, affecting the monkeytype typing test application versions 25.49.0 and earlier. The vulnerability stems from improper neutralization of user input during web page generation, specifically in the handling of quote submissions. The fields quote.text and quote.source accept user input that is inserted directly into the Document Object Model (DOM) without sufficient sanitization or escaping. Although some escaping is attempted using quotes and textarea tags, it is insufficient to prevent HTML tags from being rendered, allowing an attacker to inject malicious JavaScript code. When a victim views a maliciously crafted quote, the injected script executes in their browser context, potentially enabling session hijacking, theft of sensitive information, or other malicious activities. The vulnerability requires no authentication and no privileges, but does require user interaction to view the malicious quote. The CVSS 4.0 base score is 7.1, indicating a high severity level, with network attack vector, low attack complexity, no privileges required, user interaction required, and high impact on confidentiality. No known exploits have been reported in the wild as of the publication date. The vulnerability was publicly disclosed on December 4, 2025, and no official patches have been linked yet. The root cause is the failure to properly sanitize and encode user-supplied input before embedding it into the web page, a common issue in web applications that handle dynamic content. This vulnerability highlights the importance of rigorous input validation and output encoding in web development to prevent XSS attacks.

For European organizations, the impact of CVE-2025-66563 can be significant, especially for those using monkeytype in educational, training, or productivity environments. Successful exploitation can lead to the execution of arbitrary JavaScript in users' browsers, potentially resulting in session hijacking, theft of credentials or personal data, unauthorized actions performed on behalf of users, and distribution of malware. This can compromise the confidentiality and integrity of user data and disrupt availability if attackers leverage the vulnerability to launch further attacks such as phishing or drive-by downloads. Organizations relying on monkeytype for remote training or typing skill assessments may face reputational damage and operational disruption. Additionally, because the vulnerability requires user interaction, social engineering techniques could be employed to increase exploitation success. The absence of known exploits currently provides a window for proactive mitigation. However, the widespread use of web browsers and the ease of injecting malicious scripts make this a high-risk vulnerability that could be leveraged in targeted attacks against European entities. Compliance with GDPR and other data protection regulations may also be impacted if personal data is compromised due to this vulnerability.

To mitigate CVE-2025-66563, organizations should first verify if they are using monkeytype version 25.49.0 or earlier and plan immediate upgrades once patches are available. In the absence of official patches, implement strict input validation and sanitization on all user-submitted content, especially quote.text and quote.source fields, ensuring that HTML tags and scripts are properly escaped or removed before insertion into the DOM. Employ robust output encoding techniques such as context-aware encoding (e.g., HTML entity encoding) to prevent script execution. Deploy Content Security Policy (CSP) headers to restrict the execution of inline scripts and limit the sources of executable code. Educate users to be cautious when interacting with user-generated content and to report suspicious quotes. Monitor web application logs for unusual activity or injection attempts. If possible, isolate or sandbox the quote rendering component to minimize the impact of potential script execution. Regularly review and update web application security practices to include automated scanning for XSS vulnerabilities. Finally, coordinate with the monkeytype vendor or community to track patch releases and apply them promptly.