A recent security audit has exposed critical vulnerabilities in Moodle, the open-source learning management system (LMS) used by millions of educational institutions and organizations worldwide.
Researchers discovered that a logical flaw in Moodle’s core allows attackers to bypass key security restrictions, opening the door to potentially devastating attacks such as Server-Side Request Forgery (SSRF) and, in certain cloud environments, even remote code execution (RCE).
The vulnerabilities in Moodle version 4.4.3 and earlier stem from a subtle but impactful Time-of-Check to Time-of-Use (TOC-TOU) bug. ‘
This flaw affects all features that accept user-supplied URLs, including calendar imports and the File Picker’s URL Downloader.
The issue is particularly severe for Moodle instances hosted on cloud platforms like AWS, where exploitation could escalate to full system compromise.
How Attackers Bypass Security: The TOC-TOU Flaw
At the core of the vulnerability is a mismatch between when Moodle checks a user-supplied URL and when it uses it.
Moodle first validates the URL by resolving its hostname and checking it against a blocklist to prevent access to restricted addresses (such as localhost or AWS metadata endpoints).
However, the network request is made in a separate step, creating a window of opportunity for attackers to manipulate DNS responses.
By deploying a malicious DNS server, an attacker can respond with a benign IP address during the initial check, convincing Moodle that the URL is safe.
When the actual request is made, the attacker’s DNS server then returns a forbidden address, such as 127.0.0.1 or a sensitive cloud metadata endpoint.
This allows the attacker to bypass SSRF protections and target internal resources that should be off-limits.
Proof-of-concept exploits demonstrate that this technique can be used via features like calendar synchronization and file uploads.
In the File Picker, for example, an attacker can craft an HTML file with image tags pointing to attacker-controlled domains.
When Moodle parses the file, it fetches the images, unknowingly making requests to internal or sensitive endpoints.
Impact, Exploitation Scenarios, and Mitigation
The impact of these vulnerabilities is far-reaching.
Attackers exploiting the SSRF flaw can:
- Access internal network resources behind firewalls
- Steal sensitive data from cloud metadata endpoints (such as AWS IMDSv1)
- Potentially escalate to remote code execution if the Moodle instance is improperly configured in the cloud
Key features affected include calendar imports, the File Picker’s URL Downloader, and any functionality that processes external URLs.
The vulnerabilities are especially dangerous for cloud-hosted Moodle deployments that have not disabled legacy metadata services.
Mitigation steps include:
- Applying patches as soon as they become available from the Moodle security team
- Restricting outbound network access from Moodle servers
- Disabling IMDSv1 and enforcing IMDSv2 on AWS-hosted Moodle instances
This incident highlights the complexity of securing applications that process user-supplied URLs and underscores the need for continuous security reviews, especially for widely deployed platforms like Moodle.
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