CVE-2026-102577 in Moodle
Summary
by MITRE • 09/30/2026
A flaw was found in Moodle. Incorrect handling of IPv4-mapped IPv6 addresses within the URL downloader's host-blocking logic allows an authenticated remote user to bypass blocked-host restrictions. By supplying a crafted URL, an attacker can induce the server to make requests to restricted destinations, leading to Server-Side Request Forgery (SSRF).
Be aware that VulDB is the high quality source for vulnerability data.
Analysis
by VulDB Data Team • 09/30/2026
The vulnerability identified in Moodle represents a significant security flaw within its URL downloader component, specifically concerning how the application processes and validates hostnames during outbound request operations. This issue stems from an incorrect handling of IPv4-mapped IPv6 addresses, which are special IPv6 address formats used to represent IPv4 addresses within an IPv6 context. In standard networking stacks, these mapped addresses allow software designed for IPv6 to interact with legacy IPv4 systems without requiring separate code paths. However, the host-blocking logic implemented in Moodle fails to properly normalize or recognize these specific address representations when evaluating whether a destination is permitted or restricted by administrative policies. This oversight creates a logical gap that allows an authenticated remote user to circumvent security controls designed to prevent unauthorized outbound network connections.
From a technical perspective, this flaw enables Server-Side Request Forgery attacks against internal resources that would otherwise be protected by Moodle's built-in allow-listing mechanisms. When a user submits a crafted URL containing an IPv4-mapped IPv6 address representation of a blocked host, the application's validation routine does not correctly identify it as belonging to the restricted domain or IP range. Consequently, the server proceeds to initiate an HTTP request to this destination on behalf of the attacker. This behavior effectively bypasses the intended access control restrictions, allowing the authenticated user to leverage Moodle’s server-side processing capabilities to interact with internal services, databases, or other sensitive infrastructure components that are not exposed directly to the public internet but are accessible from within the network segment where Moodle resides.
The operational impact of this vulnerability is substantial for organizations relying on Moodle's URL downloader feature while maintaining strict outbound traffic policies. An attacker who has obtained valid credentials can exploit this flaw to perform internal port scanning, exfiltrate data from internal APIs, or interact with backend services such as database management systems or administrative interfaces that are not intended to be accessed via the web application layer. This type of attack falls under the category of Server-Side Request Forgery and is classified in the MITRE ATT&CK framework as T1571, which involves establishing a connection back to an internal server through a vulnerable application. The ability to bypass host-blocking restrictions undermines the principle of least privilege and can lead to further compromise if internal services have weaker authentication mechanisms or lack proper segmentation from critical data stores.
To mitigate this risk, administrators should ensure that Moodle is updated to the latest version where this validation logic has been corrected to properly handle IPv4-mapped IPv6 addresses during host verification processes. In environments where immediate patching is not feasible, network-level controls such as firewall rules or proxy configurations can be employed to restrict outbound traffic from the web server IP address to only explicitly trusted destinations, thereby providing a compensating control that does not rely solely on application-layer filtering. Additionally, implementing strict input validation and normalization routines for all URL inputs before processing them through request libraries is essential to prevent similar bypass techniques in other components of the platform. Regular security audits focusing on outbound connection handling and adherence to industry standards such as CWE-918 regarding Server-Side Request Forgery will help maintain robust defense-in-depth strategies against this class of vulnerabilities.