CVE-2021-36395 in Moodle
Summary
by MITRE • 03/06/2023
In Moodle, the file repository's URL parsing required additional recursion handling to mitigate the risk of recursion denial of service.
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Analysis
by VulDB Data Team • 07/05/2025
The vulnerability identified as CVE-2021-36395 affects Moodle, a widely used learning management system that serves educational institutions globally. This issue resides within the file repository component of Moodle's architecture, specifically concerning how the system processes and parses URLs. The flaw represents a recursion denial of service vulnerability that can be exploited by malicious actors to disrupt normal system operations through carefully crafted input sequences. The vulnerability is particularly concerning given Moodle's deployment across thousands of educational institutions, making it a prime target for attackers seeking to disrupt learning environments.
The technical implementation of this vulnerability stems from inadequate recursion handling within Moodle's URL parsing mechanism. When the system processes file repository URLs, it fails to properly validate or limit recursive parsing operations that could occur when encountering certain URL structures or nested references. This deficiency allows an attacker to craft malicious URLs that trigger excessive recursive processing within the application's file repository subsystem. The vulnerability operates at the application layer and can be classified under CWE-674, which specifically addresses uncontrolled recursion in software systems. The flaw demonstrates a classic lack of input validation and resource limitation controls that are fundamental to preventing denial of service attacks.
The operational impact of this vulnerability extends beyond simple service disruption to potentially compromise the entire learning management system's availability. An attacker exploiting this vulnerability can cause the system to consume excessive CPU resources and memory through recursive URL processing, leading to denial of service conditions that affect legitimate users attempting to access course materials, submit assignments, or participate in online learning activities. The attack vector is particularly insidious because it can be executed through seemingly benign file repository operations, making detection more difficult. This vulnerability aligns with ATT&CK technique T1499.004, which covers network denial of service attacks through resource exhaustion, and demonstrates how application-level flaws can be leveraged to achieve system-wide availability impacts.
Mitigation strategies for CVE-2021-36395 should focus on implementing proper recursion depth limits within the URL parsing functionality of Moodle's file repository component. System administrators should ensure that all Moodle installations are updated to the latest patched versions that address this specific vulnerability. Additional protective measures include implementing rate limiting on repository access operations, adding input validation checks to prevent malformed URL sequences, and monitoring system resources for unusual patterns that might indicate exploitation attempts. Organizations should also consider implementing network-level controls to restrict access to file repository endpoints and establish logging mechanisms that can detect anomalous URL parsing behavior. The fix typically involves adding recursion depth counters and maximum iteration limits to the URL parsing algorithm, preventing unlimited recursive operations while maintaining legitimate functionality. Security teams should perform regular vulnerability assessments and penetration testing to ensure that similar recursion issues do not exist in other components of the Moodle system or related applications.