CVE-2025-67853: Moodle Auth Bypass Vulnerability

CVE-2025-67853 Overview

A security flaw was discovered in Moodle's confirmation email service that lacks proper rate limiting controls. This vulnerability allows remote attackers to exploit the email confirmation mechanism to enumerate or guess user credentials, significantly facilitating brute-force attacks against user accounts. The absence of rate limiting enables attackers to make unlimited authentication attempts without triggering any defensive measures.

Critical Impact

Remote attackers can perform credential enumeration and brute-force attacks against Moodle user accounts due to missing rate limiting in the email confirmation service, potentially leading to unauthorized access to educational platform accounts.

Affected Products

• Moodle (versions affected not specified in advisory)

Discovery Timeline

• 2026-02-03 - CVE-2025-67853 published to NVD
• 2026-02-03 - Last updated in NVD database

Technical Details for CVE-2025-67853

Vulnerability Analysis

This vulnerability falls under CWE-307 (Improper Restriction of Excessive Authentication Attempts), a common weakness that occurs when an application does not implement sufficient measures to prevent multiple failed authentication attempts. In the case of Moodle, the confirmation email service lacks proper rate limiting, which creates a significant security gap.

The vulnerability allows unauthenticated remote attackers to target the email confirmation service without any throttling mechanism to slow down or block their attempts. This means an attacker can repeatedly query the system to determine whether specific email addresses or usernames exist within the Moodle installation, and subsequently attempt to guess associated passwords through brute-force methods.

Root Cause

The root cause of this vulnerability is the absence of rate limiting controls in Moodle's email confirmation service. When a user requests an email confirmation, the application processes each request without tracking the frequency of requests from a particular source. This design flaw enables attackers to send unlimited requests to the service, which can be leveraged for user enumeration by observing response differences for valid versus invalid accounts.

Attack Vector

The attack is network-based and requires no authentication or user interaction. An attacker can remotely target the email confirmation service endpoint and systematically probe for valid user accounts. The attack progression typically involves:

1. Identifying the Moodle installation's email confirmation endpoint
2. Sending numerous requests with different email addresses or usernames
3. Analyzing response patterns to identify valid accounts
4. Launching brute-force password attacks against confirmed accounts

Since no rate limiting exists, attackers can automate this process using scripts to rapidly enumerate users and attempt credential guessing at scale. The vulnerability exposes confidentiality by revealing user existence and enabling subsequent unauthorized access attempts.

Detection Methods for CVE-2025-67853

Indicators of Compromise

• Unusually high volume of requests to the email confirmation service endpoint from single or multiple IP addresses
• Repeated failed authentication attempts against multiple user accounts in a short timeframe
• Sequential or patterned email address queries suggesting automated enumeration activity
• Abnormal traffic spikes to Moodle authentication-related URLs

Detection Strategies

• Implement web application firewall (WAF) rules to detect and alert on excessive requests to confirmation endpoints
• Monitor authentication logs for patterns indicative of brute-force attempts, such as multiple failed logins across different accounts
• Deploy anomaly detection systems to identify unusual request frequencies to Moodle services
• Configure SIEM alerts for high-volume requests from single IP addresses targeting user-related endpoints

Monitoring Recommendations

• Enable verbose logging on the Moodle email confirmation service to capture request metadata
• Set up real-time alerting for authentication failure thresholds that exceed normal baselines
• Monitor network traffic for automated tooling signatures commonly used in enumeration attacks
• Review access logs regularly for patterns of systematic account probing

How to Mitigate CVE-2025-67853

Immediate Actions Required

• Implement rate limiting on the email confirmation service to restrict the number of requests per IP address or session
• Enable CAPTCHA or similar challenge-response mechanisms for email confirmation requests
• Configure fail2ban or similar tools to automatically block IP addresses exhibiting brute-force behavior
• Review and restrict access to the email confirmation endpoint through network-level controls if possible

Patch Information

Consult the official Moodle security advisories for patch availability. Additional information can be found in the Red Hat CVE-2025-67853 Advisory and the Red Hat Bugzilla Report #2423847. Apply vendor-provided patches as soon as they become available and ensure your Moodle installation is updated to the latest secure version.

Workarounds

• Deploy a reverse proxy with rate limiting capabilities in front of the Moodle installation to throttle requests
• Implement IP-based access restrictions to limit confirmation service access to trusted networks where feasible
• Use web application firewall rules to block requests that match enumeration or brute-force patterns
• Consider temporarily disabling the email confirmation feature if not critical to operations until a patch is applied

# Example nginx rate limiting configuration
# Add to nginx server block configuration
limit_req_zone $binary_remote_addr zone=moodle_confirm:10m rate=5r/m;

location /login/confirm.php {
    limit_req zone=moodle_confirm burst=3 nodelay;
    # Additional proxy settings
}