CVE-2025-59838 in monkeytype
Summary
by MITRE • 09/25/2025
Monkeytype is a minimalistic and customizable typing test. In versions 25.36.0 and prior, improper handling of user input when loading a saved custom text results in XSS. This issue has been fixed in version 25.44.0.
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Analysis
by VulDB Data Team • 10/29/2025
The vulnerability identified as CVE-2025-59838 affects Monkeytype, a popular minimalistic typing test application designed for customizable user experiences. This web-based application allows users to save and load custom text for typing practice, creating a personalized testing environment. The security flaw manifests specifically during the text loading process when users attempt to retrieve previously saved custom content, representing a critical weakness in the application's input validation and sanitization mechanisms.
The technical implementation flaw stems from inadequate sanitization of user-supplied data when processing saved custom text content. When users load their saved text, the application fails to properly escape or filter special characters that could contain malicious script code. This improper input handling creates an environment where attacker-controlled payloads can be executed within the context of a user's browser session. The vulnerability specifically targets the application's text rendering functionality, where user input is directly incorporated into the DOM without appropriate security measures. This type of flaw aligns with CWE-79, which categorizes cross-site scripting vulnerabilities as a result of insufficient input validation and output encoding.
The operational impact of this vulnerability extends beyond simple script execution, as it allows for complete browser session compromise and potential data exfiltration. An attacker could craft malicious custom text content containing JavaScript payloads that would execute whenever another user loads that content, potentially stealing cookies, session tokens, or other sensitive information. This vulnerability is particularly dangerous in collaborative environments where users share custom text files, as it enables privilege escalation through session hijacking and persistent malicious code delivery. The attack surface is amplified by the application's widespread use and the ease with which malicious content can be embedded within legitimate user-generated text.
Mitigation strategies for this vulnerability require immediate implementation of proper input sanitization and output encoding mechanisms. The application must implement comprehensive content security policies that prevent script execution within user-generated content, alongside robust input validation that strips or escapes dangerous characters such as angle brackets, script tags, and event handlers. Security patches should enforce strict sanitization of all user inputs before rendering, implementing library-based solutions such as DOMPurify or similar HTML sanitization tools. Additionally, the application should adopt a defense-in-depth approach by implementing CSP headers and ensuring that all user-generated content is treated as untrusted. This vulnerability demonstrates the critical importance of input validation in web applications and aligns with ATT&CK technique T1211, which covers exploitation of input validation vulnerabilities for code execution within user contexts. The fix implemented in version 25.44.0 addresses these concerns through comprehensive sanitization of user inputs and improved handling of custom text loading operations.