CVE-2025-66563 is a cross-site scripting (XSS) vulnerability identified in 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 involving the quote.text and quote.source fields submitted by users. These inputs are inserted directly into the Document Object Model (DOM) without sufficient sanitization or encoding, allowing malicious HTML or JavaScript code to be rendered and executed in the context of any user viewing the malicious quote. Although some escaping is performed using quotes and textarea tags, it is insufficient to prevent script injection. The vulnerability does not require any authentication or privileges to exploit, but it does require user interaction—viewing the malicious quote. The CVSS 4.0 vector indicates network attack vector, low attack complexity, no privileges required, user interaction required, and high impact on confidentiality with limited impact on integrity and no impact on availability. No patches are currently linked, and no known exploits have been reported in the wild. This vulnerability could be leveraged to steal session tokens, perform actions on behalf of users, or deliver further malware payloads through the victim's browser. The flaw is categorized under CWE-79, which covers improper neutralization of input leading to XSS attacks. Organizations using Monkeytype, particularly in environments where user-generated content is shared or publicly accessible, are at risk. The vulnerability highlights the need for robust input validation and output encoding in web applications handling user content.

For European organizations, this vulnerability poses a significant risk to user confidentiality and session integrity. If exploited, attackers could execute arbitrary JavaScript in the context of affected users, potentially leading to session hijacking, credential theft, or unauthorized actions within the application. Educational institutions, training providers, and companies using Monkeytype for employee skill assessments or typing tests could see disruptions or data breaches. The impact extends beyond direct users if the application is embedded or integrated into larger platforms, potentially allowing lateral movement or further exploitation. Given the high CVSS score and the nature of XSS attacks, the vulnerability could also be used as a vector for phishing or delivering malware payloads. Although availability is not directly impacted, reputational damage and compliance issues related to data protection regulations such as GDPR could arise. The lack of current known exploits provides a window for mitigation, but the ease of exploitation and network accessibility make timely remediation critical.

1. Monitor Monkeytype official channels for patches addressing CVE-2025-66563 and apply updates promptly once available. 2. Until patches are released, restrict or disable user-generated quote submissions or limit their visibility to trusted users only. 3. Implement additional input validation and sanitization on the server side to strip or encode HTML tags and scripts from quote.text and quote.source fields. 4. Employ Content Security Policy (CSP) headers to restrict the execution of inline scripts and reduce the impact of potential XSS payloads. 5. Use web application firewalls (WAFs) with custom rules to detect and block malicious payloads targeting this vulnerability. 6. Educate users about the risks of clicking on untrusted links or viewing unverified content within the application. 7. Conduct security reviews and penetration testing focusing on user input handling in Monkeytype deployments. 8. Consider isolating Monkeytype instances in sandboxed environments to limit potential damage from exploitation.