A recent security audit has uncovered critical vulnerabilities within Moodle, the widely used open-source learning management system (LMS), exposing millions of instances to potential exploitation. These vulnerabilities allow attackers to evade core security mechanisms and exploit systems via Server-Side Request Forgery (SSRF). The flaws center around a subtle yet impactful Time-of-Check to Time-of-Use (TOC-TOU) bug that affects all Moodle features that accept user-supplied URLs, putting many educational and organizational Moodle instances at risk, particularly those hosted on cloud platforms like AWS.
1. Exploiting TOC-TOU in Moodle’s Core
The vulnerabilities were identified in Moodle version 4.4.3, the latest stable release at the time of the audit. Researchers found that Moodle’s process for validating URLs provided by users suffers from a logical flaw, allowing attackers to bypass SSRF restrictions and target internal network resources. This TOC-TOU bug arises due to a separation between the DNS resolution, which checks the hostname against a blocklist, and the actual network request. An attacker can manipulate DNS responses between these two steps, causing Moodle to believe a URL is safe during the check, but then making the actual request point to a forbidden address, such as localhost or sensitive AWS metadata endpoints.
The attack chain leverages Moodle’s Calendar synchronization and File Picker features, enabling users to import external resources via URLs. The exploit begins when a user supplies a controlled URL in the Calendar or File Picker feature. Moodle’s backend code first checks the hostname using DNS resolution, ensuring it’s not blacklisted. If this check passes, Moodle proceeds to fetch the resource using curl_exec(). At this point, the attacker changes the DNS record to resolve to an internal or sensitive address, resulting in an internal request and bypassing all intended restrictions. This exploit could lead to significant security risks for Moodle instances, particularly those with sensitive data.
2. Example Attack Scenario and Code Implications
A practical example of this TOC-TOU vulnerability can be demonstrated using the code handling the URL validation process. The user-supplied data is acquired and used in the URL validation chain, with curl being employed to fetch the resource. The separation between the hostname check and the network request allows attackers to change DNS responses during these steps. A simplified code excerpt highlights how this bug can be exploited:
$formdata = $form->get_data(); // User-controlled data// ...URL validation chain...$curl = new curl();$response = $curl->get($user_supplied_url);function url_is_blocked($url) { $host = gethostbynamel(parse_url($url)['host']); // Check if $host is in blocklist // Time passes... // Later, curl_exec() is called, which re-resolves the host}To manipulate responses based on the request order, an attacker could deploy a Python-based DNS server to return a benign IP during the initial check and a target IP during the fetch, enabling exploitation:
TOC_TOU_CHECK = 0def resolve_domain(name): global TOC_TOU_CHECK TOC_TOU_CHECK += 1 if TOC_TOU_CHECK % 2 == 0: return "127.0.0.1" else: return "203.0.113.1"This approach demonstrates the feasibility of exploiting this flaw within Moodle’s URL validation logic, highlighting the urgent need for mitigation measures to secure vulnerable systems from potential attacks.
3. Impact and Recommendations
The potential repercussions of this TOC-TOU vulnerability are severe, especially for Moodle instances hosted on AWS. If IMDSv1 is not disabled, attackers could escalate SSRF attacks to remote code execution, giving them control over the affected servers. The flaw impacts features like Calendar imports and the File Picker’s URL Downloader, necessitating immediate action from administrators to mitigate risks. The steps to protect Moodle instances include applying patches once available, restricting outbound network access, and transitioning from IMDSv1 to IMDSv2 on AWS-hosted instances.
The TOC-TOU vulnerability in Moodle underscores the complexities and dangers of SSRF, particularly in environments handling sensitive educational data. Institutions and organizations should prioritize addressing this issue and strengthen their deployments until official patches are released. Regular security audits and vigilant monitoring are essential in today’s threat landscape. By understanding the nature of this vulnerability and taking proactive measures, organizations can protect themselves from potential exploitation and ensure the safety of their systems and data.
4. Future Considerations
A recent security audit revealed critical flaws within Moodle, the highly popular open-source learning management system (LMS). These vulnerabilities place millions of instances at risk of exploitation. The discovered flaws enable attackers to bypass core security mechanisms and exploit systems using Server-Side Request Forgery (SSRF). Central to these vulnerabilities is a subtle yet significant Time-of-Check to Time-of-Use (TOC-TOU) bug. This bug affects all Moodle features that accept user-supplied URLs, increasing the risk for many educational and organizational Moodle instances. The risk is particularly heightened for those hosted on cloud platforms like AWS, as these environments can be more susceptible to such attacks. Consequently, this discovery has raised urgent security concerns for users and administrators who depend on Moodle for critical educational and organizational functions. Effective mitigation strategies and timely updates are essential to protect these systems from potential exploitation.
