CVE-2026-102581 in Moodle
Summary
by MITRE • 09/30/2026
A flaw was found in Moodle. Insufficient output escaping in templates used to display forum posts enables a stored cross-site scripting (XSS) vulnerability. An attacker can inject malicious content into a forum post, which then executes arbitrary script code in the browser of another user viewing the affected post.
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Analysis
by VulDB Data Team • 09/30/2026
The identified vulnerability resides within the Moodle learning management system and specifically affects the template rendering engine responsible for displaying forum posts. This flaw is classified as an insufficient output escaping issue, which falls under the CWE-79 category known as Improper Neutralization of Input During Web Page Generation or Cross-site Scripting. In a properly secured web application, any data originating from user input must be strictly sanitized and encoded before being rendered in the browser to prevent it from being interpreted as executable code. However, due to this deficiency, malicious scripts embedded within forum posts are not neutralized by the server-side processing logic. This allows an attacker who has access to post content to inject arbitrary JavaScript or other client-side script payloads directly into the database records that store these messages.
The operational impact of this vulnerability is significant because it facilitates a stored cross-site scripting attack, which differs from reflected XSS in that the malicious payload persists on the target server rather than being delivered via a crafted link. When an unsuspecting user with valid credentials navigates to a forum thread containing the injected content, their browser will automatically execute the embedded script upon rendering the page. This execution occurs within the security context of the Moodle application and the authenticated session of the victim. Consequently, attackers can perform actions such as stealing session cookies or authentication tokens, hijacking user sessions, performing unauthorized administrative actions on behalf of the victim, defacing web pages, or redirecting users to malicious external sites designed for phishing or further malware distribution.
From a threat modeling perspective aligned with the MITRE ATT&CK framework, this vulnerability enables techniques associated with Collection and Credential Access, specifically targeting browser-based information gathering through stored script execution. The persistence of the payload means that every user viewing the compromised post becomes a potential victim without requiring any additional social engineering or interaction beyond normal browsing behavior. This makes the attack particularly dangerous in educational environments where Moodle is widely deployed, as it can affect large numbers of students and faculty members who trust the integrity of the forum content they view daily.
To mitigate this risk, immediate remediation involves applying the official security patches provided by the Moodle project for the affected versions. Administrators should ensure that their instances are updated to a version where the template engine correctly escapes all dynamic output variables before rendering them in HTML contexts. In addition to patching, organizations should implement Content Security Policy headers to restrict the sources from which scripts can be loaded and executed, thereby reducing the impact of any potential XSS flaws even if they exist. Regular security audits focusing on input validation and output encoding practices within custom plugins or themes are also recommended to prevent similar vulnerabilities in other parts of the application ecosystem.