CVE-2026-27397: Really Simple Security Pro Auth Bypass

CVE-2026-27397 Overview

CVE-2026-27397 is an Authorization Bypass Through User-Controlled Key vulnerability (CWE-639) affecting Really Simple Security Pro, a popular WordPress security plugin developed by Really Simple Plugins B.V. This Insecure Direct Object Reference (IDOR) vulnerability allows attackers to exploit incorrectly configured access control security levels, potentially enabling unauthorized access to resources that should be protected.

The vulnerability exists in versions of Really Simple Security Pro through 9.5.4.0, allowing unauthenticated attackers to manipulate user-controlled keys to bypass authorization mechanisms. This type of flaw occurs when an application uses user-supplied input to directly access objects without proper validation of the user's authorization to access those objects.

Critical Impact

Unauthenticated attackers can bypass authorization controls by manipulating user-controlled keys, potentially gaining unauthorized access to protected resources and compromising WordPress site integrity.

Affected Products

• Really Simple Security Pro versions through 9.5.4.0
• WordPress sites running vulnerable versions of the Really Simple Security Pro plugin

Discovery Timeline

• 2026-03-19 - CVE-2026-27397 published to NVD
• 2026-03-19 - Last updated in NVD database

Technical Details for CVE-2026-27397

Vulnerability Analysis

This vulnerability is classified as CWE-639 (Authorization Bypass Through User-Controlled Key), commonly referred to as an Insecure Direct Object Reference (IDOR) vulnerability. The flaw allows attackers to manipulate identifiers or keys that the application uses to reference internal objects, bypassing the intended access control mechanisms.

In the context of Really Simple Security Pro, the vulnerability stems from the plugin's failure to properly validate whether a user is authorized to access specific resources before granting access based on user-controlled input. Attackers can exploit this by modifying request parameters to reference objects or resources they should not have permission to access.

The network-based attack vector means exploitation can occur remotely without requiring authentication or user interaction, though the impact is limited to integrity and availability concerns rather than confidentiality breaches.

Root Cause

The root cause of this vulnerability lies in insufficient authorization validation within the Really Simple Security Pro plugin. When processing requests containing user-controlled identifiers, the application fails to verify that the requesting user has legitimate access rights to the referenced resource. Instead of implementing proper access control checks, the plugin trusts user-supplied input to determine which objects to access or modify.

This pattern of trusting user input for authorization decisions without server-side validation is a common source of IDOR vulnerabilities in web applications and WordPress plugins.

Attack Vector

The attack vector for CVE-2026-27397 is network-based, requiring no authentication or user interaction. An attacker can exploit this vulnerability by:

1. Identifying endpoints in the Really Simple Security Pro plugin that accept user-controlled identifiers
2. Crafting malicious requests with modified identifier values to reference objects belonging to other users or administrative resources
3. Submitting these requests to bypass authorization checks and gain unauthorized access to protected functionality

The exploitation does not require local access to the target system, making it exploitable by remote attackers across the network. The vulnerability impacts both the integrity and availability of affected WordPress installations, potentially allowing unauthorized modifications or disruption of security features.

Detection Methods for CVE-2026-27397

Indicators of Compromise

• Unusual access patterns to Really Simple Security Pro plugin endpoints with sequential or predictable identifier values
• Log entries showing requests with modified user identifiers accessing resources belonging to different users
• Unexpected changes to security configurations or user settings without corresponding administrator actions
• Multiple requests from single IP addresses attempting to enumerate object identifiers

Detection Strategies

• Monitor web server access logs for requests to Really Simple Security Pro plugin endpoints with unusual parameter patterns
• Implement Web Application Firewall (WAF) rules to detect and block IDOR exploitation attempts targeting the plugin
• Review WordPress audit logs for unauthorized configuration changes or access attempts
• Deploy runtime application security monitoring to detect authorization bypass attempts

Monitoring Recommendations

• Enable comprehensive logging for all Really Simple Security Pro plugin activities and API requests
• Configure alerting for failed authorization attempts and unusual access patterns
• Implement rate limiting on plugin endpoints to slow enumeration attacks
• Monitor for indicators of automated scanning or fuzzing targeting the vulnerable plugin endpoints

How to Mitigate CVE-2026-27397

Immediate Actions Required

• Update Really Simple Security Pro to the latest patched version immediately
• Audit WordPress site for any unauthorized changes made during the vulnerable period
• Review access logs to identify potential exploitation attempts
• Temporarily disable the Really Simple Security Pro plugin if an immediate update is not possible

Patch Information

The vendor has addressed this vulnerability in versions after 9.5.4.0. Administrators should update to the latest available version of Really Simple Security Pro through the WordPress plugin management interface or by downloading the updated plugin directly from the vendor. For detailed patch information and the security advisory, refer to the Patchstack Vulnerability Report.

Workarounds

• Implement a Web Application Firewall (WAF) to filter malicious requests targeting the vulnerable plugin functionality
• Restrict access to the WordPress admin area and plugin endpoints using IP allowlisting where feasible
• Monitor and audit all plugin activity until the patch can be applied
• Consider temporarily deactivating the plugin if it is not critical to site operations and a patch cannot be immediately deployed

# WordPress CLI command to check plugin version
wp plugin list --name=really-simple-ssl-pro --fields=name,version,update_version

# Update the plugin to the latest version
wp plugin update really-simple-ssl-pro

# If deactivation is necessary as a temporary workaround
wp plugin deactivate really-simple-ssl-pro