CVE-2026-102586 in Moodle
Summary
by MITRE • 09/30/2026
A flaw was found in Moodle. Insufficient sanitization of username input on the password reset page allows a remote attacker to conduct a cross-site scripting (XSS) attack. By convincing an unauthenticated user to access a specially crafted password reset link, an attacker could execute arbitrary script in the victim's browser.
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Analysis
by VulDB Data Team • 09/30/2026
The vulnerability identified in Moodle represents a critical security flaw rooted in insufficient input validation and sanitization mechanisms within the application's authentication subsystem. Specifically, the defect resides on the password reset page where user-supplied data, particularly the username field, is processed without adequate filtering before being reflected back to the client side or used in subsequent operations that influence browser rendering. This lack of rigorous encoding allows special characters typically associated with HTML and JavaScript payloads to pass through validation checks intact. In a typical web application security model, input from untrusted sources must be strictly validated against expected formats and sanitized using context-aware escaping techniques before any data is incorporated into the output stream sent to the user's browser. The failure to implement these controls creates an opening for malicious actors to inject executable code directly into the page content served by the server.
From a technical perspective, this flaw facilitates a stored or reflected cross-site scripting attack depending on how the username is persisted and subsequently displayed during the password reset workflow. When an unauthenticated user interacts with the specially crafted link provided by the attacker, their browser parses the injected script as part of the legitimate HTML structure of the Moodle interface. Because the execution context remains within the same origin as the vulnerable application, the malicious scripts inherit all privileges associated with that domain. This includes access to session cookies, local storage data, and the ability to perform actions on behalf of the authenticated user if they are logged in at the time of exploitation. The attack vector relies heavily on social engineering tactics, requiring the attacker to deceive a victim into clicking a manipulated URL that triggers the vulnerable code path within Moodle's password recovery module.
The operational impact of this vulnerability is severe due to its potential for account compromise and data exfiltration. An attacker leveraging this flaw can steal sensitive information such as session tokens, which could lead to full account takeover even if multi-factor authentication is enabled in some configurations where the token bypasses secondary checks. Furthermore, the injected scripts can redirect users to phishing sites designed to harvest credentials or install malware on the victim's device through drive-by download techniques. In enterprise environments running Moodle for learning management systems, this vulnerability undermines trust and exposes student records, instructor materials, and administrative data to unauthorized access. The ability to execute arbitrary JavaScript also allows attackers to deface web pages or manipulate form submissions, potentially altering grades or enrollment statuses without proper authorization.
To mitigate this risk, immediate remediation efforts should focus on implementing robust input validation and output encoding strategies across the Moodle codebase. Developers must ensure that all user-supplied inputs are validated against a strict whitelist of allowed characters before processing and encoded using context-specific methods such as HTML entity encoding when rendering data in web pages. Upgrading to patched versions of Moodle where this issue has been addressed is the primary defense mechanism recommended by vendors. Additionally, deploying Web Application Firewalls can provide an additional layer of protection by detecting and blocking common XSS payloads based on signature-based rules while organizations work toward permanent code fixes. Security headers such as Content-Security-Policy should also be configured to restrict script execution sources and mitigate the impact if exploitation occurs despite preventive measures. Regular security audits and penetration testing focused on authentication flows will help identify similar weaknesses in other parts of the application infrastructure before they can be exploited by malicious actors seeking unauthorized access or data theft.