CVE-2025-4094: Digits WordPress OTP Bypass Vulnerability

CVE-2025-4094 Overview

CVE-2025-4094 is a critical authentication bypass vulnerability affecting the DIGITS: WordPress Mobile Number Signup and Login plugin. The vulnerability exists in versions prior to 8.4.6.1 and stems from the absence of rate limiting on One-Time Password (OTP) validation attempts. This security flaw allows attackers to perform brute force attacks against OTP codes, potentially gaining unauthorized access to user accounts.

Critical Impact

Attackers can systematically brute force OTP codes without restriction, enabling complete authentication bypass and unauthorized account access on WordPress sites using the DIGITS plugin.

Affected Products

• DIGITS: WordPress Mobile Number Signup and Login plugin versions before 8.4.6.1
• WordPress installations using the vulnerable DIGITS plugin for mobile authentication
• Sites relying on OTP-based authentication through UnitedOver DIGITS

Discovery Timeline

• 2025-05-21 - CVE-2025-4094 published to NVD
• 2025-06-09 - Last updated in NVD database

Technical Details for CVE-2025-4094

Vulnerability Analysis

This vulnerability represents a fundamental weakness in the authentication mechanism of the DIGITS WordPress plugin. The plugin implements OTP-based authentication for mobile number signup and login functionality but fails to implement proper rate limiting controls on OTP validation endpoints. Without rate limiting, the system accepts an unlimited number of OTP validation attempts within any given timeframe.

OTP codes typically consist of 4-6 digit numeric values, providing a relatively small keyspace (10,000 to 1,000,000 possible combinations). When combined with the absence of rate limiting, an attacker can exhaustively test all possible OTP combinations in a matter of minutes or hours, depending on network conditions and server response times.

Root Cause

The root cause of this vulnerability is the missing implementation of rate limiting mechanisms on the OTP validation API endpoint. Proper secure OTP implementations should include:

• Maximum attempt thresholds per OTP code
• Account lockout mechanisms after repeated failures
• Progressive delays between validation attempts
• IP-based rate limiting
• OTP expiration after a limited number of failed attempts

The DIGITS plugin prior to version 8.4.6.1 lacks these essential security controls, allowing unlimited validation attempts against any given OTP code.

Attack Vector

The attack is network-based and requires no prior authentication or user interaction. An attacker targeting a WordPress site with the vulnerable DIGITS plugin can:

1. Initiate an OTP request for a target phone number (either their own or a victim's)
2. Capture the OTP validation endpoint and request format
3. Deploy an automated script to systematically submit all possible OTP combinations
4. Upon successful validation, gain access to the associated user account

The attack complexity is low since standard brute force tooling can be adapted for this purpose. For a 6-digit OTP, an attacker making 100 requests per second could theoretically exhaust the entire keyspace in approximately 2.7 hours.

Detection Methods for CVE-2025-4094

Indicators of Compromise

• High volume of failed OTP validation attempts from single IP addresses or user agents
• Rapid sequential requests to OTP validation endpoints (e.g., /wp-admin/admin-ajax.php with DIGITS-related actions)
• Unusual patterns of account access following multiple failed OTP attempts
• Server logs showing burst traffic to authentication-related endpoints

Detection Strategies

• Implement Web Application Firewall (WAF) rules to detect and block rapid OTP validation attempts
• Configure server-side logging to capture all OTP validation requests with timestamps and source IPs
• Set up alerts for authentication anomalies, particularly high-frequency requests to DIGITS plugin endpoints
• Monitor for patterns consistent with automated tooling, such as sequential OTP submissions

Monitoring Recommendations

• Enable verbose logging on WordPress AJAX handlers, particularly for authentication-related actions
• Deploy real-time log analysis to identify brute force patterns against OTP endpoints
• Monitor network traffic for unusual request volumes to /wp-admin/admin-ajax.php
• Track failed authentication metrics and establish baseline thresholds for alerting

How to Mitigate CVE-2025-4094

Immediate Actions Required

• Update the DIGITS plugin to version 8.4.6.1 or later immediately
• Audit authentication logs for signs of past exploitation attempts
• Consider temporarily disabling OTP-based authentication until the patch is applied
• Implement additional WAF rules to rate limit OTP validation requests at the network layer

Patch Information

UnitedOver has addressed this vulnerability in DIGITS plugin version 8.4.6.1. Organizations should update to this version or later through the WordPress plugin update mechanism. For detailed vulnerability information, refer to the WPScan Vulnerability Analysis.

Workarounds

• Implement server-level rate limiting using .htaccess, Nginx configuration, or a reverse proxy to restrict requests to OTP validation endpoints
• Deploy a Web Application Firewall with rules specifically targeting brute force attacks on authentication endpoints
• Consider implementing CAPTCHA or additional verification challenges before OTP validation
• If immediate patching is not possible, temporarily disable the mobile number authentication feature until the update can be applied

# Example Nginx rate limiting configuration for WordPress AJAX
# Add to nginx.conf or site configuration

limit_req_zone $binary_remote_addr zone=otp_limit:10m rate=5r/m;

location /wp-admin/admin-ajax.php {
    limit_req zone=otp_limit burst=10 nodelay;
    # Existing PHP-FPM configuration
    include fastcgi_params;
    fastcgi_pass php-fpm;
}