CVE-2026-3658: WordPress Simply Schedule Appointments SQLi

CVE-2026-3658 Overview

The Appointment Booking Calendar — Simply Schedule Appointments Booking Plugin for WordPress contains a SQL Injection vulnerability in the fields parameter affecting all versions up to and including 1.6.10.0. The vulnerability exists due to insufficient escaping on user-supplied input and lack of proper SQL query preparation. Unauthenticated attackers can exploit this flaw to append malicious SQL queries to existing database queries, enabling extraction of sensitive information including usernames, email addresses, and password hashes.

Critical Impact

Unauthenticated SQL Injection allows attackers to extract sensitive user data from the WordPress database without any credentials, potentially compromising all site users and administrators.

Affected Products

• Simply Schedule Appointments Booking Plugin for WordPress versions ≤ 1.6.10.0
• WordPress installations using vulnerable plugin versions
• Websites with publicly accessible appointment booking functionality

Discovery Timeline

• 2026-03-19 - CVE-2026-3658 published to NVD
• 2026-03-19 - Last updated in NVD database

Technical Details for CVE-2026-3658

Vulnerability Analysis

This SQL Injection vulnerability (CWE-89) exists in the Simply Schedule Appointments plugin's handling of the fields parameter. The vulnerability is classified as an unauthenticated attack, meaning no user credentials or privileges are required for exploitation. The attack can be conducted remotely over the network, and successful exploitation results in unauthorized access to confidential database information while maintaining the integrity and availability of the system.

The vulnerability spans multiple code files within the plugin architecture, including the appointment type model, API model, and database model classes. The lack of proper input sanitization and parameterized queries allows malicious SQL code to be injected through the fields parameter.

Root Cause

The root cause of this vulnerability is insufficient input validation and improper SQL query construction. The plugin fails to properly escape user-supplied data in the fields parameter before incorporating it into SQL queries. Additionally, the existing SQL queries lack prepared statements with parameterized inputs, which is the recommended approach for preventing SQL Injection attacks. The vulnerable code paths can be traced through the Appointment Type Model, TD API Model, and TD DB Model.

Attack Vector

The attack is conducted over the network without requiring authentication. An attacker can craft malicious HTTP requests containing SQL injection payloads in the fields parameter. When the vulnerable plugin processes these requests, the injected SQL commands are executed against the WordPress database.

The exploitation involves manipulating the fields parameter to append additional SQL statements (such as UNION-based queries) to extract data from tables containing sensitive information. Attackers can systematically enumerate database structure and exfiltrate usernames, email addresses, and password hashes from the wp_users table and other sensitive tables.

For detailed technical analysis, refer to the Wordfence Vulnerability Intelligence Report.

Detection Methods for CVE-2026-3658

Indicators of Compromise

• Unusual or malformed requests to appointment booking endpoints containing SQL syntax characters (single quotes, UNION, SELECT, etc.)
• Database query logs showing unexpected queries against user tables or information_schema
• Web server access logs with abnormally long query strings targeting the Simply Schedule Appointments plugin
• Failed login attempts using credentials not previously seen, indicating potential credential harvesting

Detection Strategies

• Deploy Web Application Firewall (WAF) rules to detect SQL injection patterns in the fields parameter
• Enable WordPress database query logging to monitor for suspicious SELECT statements against sensitive tables
• Implement intrusion detection rules for common SQL injection payloads (UNION SELECT, INFORMATION_SCHEMA, etc.)
• Monitor for unusual data access patterns from the WordPress application

Monitoring Recommendations

• Configure real-time alerting for SQL injection signature matches in web traffic
• Review WordPress plugin error logs for database-related exceptions
• Audit database access logs for bulk data retrieval operations
• Set up honeypot fields in forms to detect automated exploitation attempts

How to Mitigate CVE-2026-3658

Immediate Actions Required

• Update Simply Schedule Appointments plugin to the latest patched version immediately
• If immediate patching is not possible, temporarily disable the plugin until an update can be applied
• Review database access logs for signs of prior exploitation
• Reset all user passwords if exploitation is suspected, prioritizing administrator accounts

Patch Information

A security patch has been released to address this vulnerability. The fix can be reviewed in WordPress Changeset #3485143. Site administrators should update the Simply Schedule Appointments plugin through the WordPress admin dashboard or by downloading the latest version from the WordPress plugin repository.

Workarounds

• Implement WAF rules to block requests containing SQL injection patterns in the fields parameter
• Restrict access to appointment booking endpoints via IP allowlisting if feasible
• Use a WordPress security plugin to add additional input validation layers
• Consider temporarily disabling public-facing appointment booking functionality until patching is complete

# Example WAF rule for ModSecurity to block SQL injection attempts
SecRule ARGS:fields "@detectSQLi" \
    "id:100001,\
    phase:2,\
    deny,\
    status:403,\
    log,\
    msg:'SQL Injection attempt blocked in fields parameter - CVE-2026-3658',\
    tag:'attack-sqli'"