CVE-2023-6933: Better Search Replace RCE Vulnerability

CVE-2023-6933 Overview

The Better Search Replace plugin for WordPress contains a PHP Object Injection vulnerability in all versions up to, and including, 1.4.4. The vulnerability exists due to deserialization of untrusted input, which allows unauthenticated attackers to inject a PHP Object. While no POP (Property Oriented Programming) chain is present in the vulnerable plugin itself, if a POP chain exists via an additional plugin or theme installed on the target system, it could allow the attacker to delete arbitrary files, retrieve sensitive data, or execute arbitrary code.

Critical Impact

Unauthenticated attackers can exploit this PHP Object Injection vulnerability to potentially achieve remote code execution, delete arbitrary files, or exfiltrate sensitive data when combined with an available POP chain from other installed plugins or themes.

Affected Products

• WPEngine Better Search Replace versions up to and including 1.4.4
• WordPress installations using the Better Search Replace plugin
• Sites with additional plugins/themes containing exploitable POP chains

Discovery Timeline

• 2024-02-05 - CVE CVE-2023-6933 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2023-6933

Vulnerability Analysis

This vulnerability is classified as Insecure Deserialization (CWE-502), a dangerous class of vulnerabilities that occurs when untrusted data is deserialized without proper validation. The Better Search Replace plugin processes serialized PHP objects from user-controllable input without adequate security controls, enabling attackers to inject malicious PHP objects into the application.

PHP Object Injection vulnerabilities are particularly dangerous in WordPress environments due to the extensive plugin ecosystem. When a malicious object is deserialized, PHP automatically executes certain "magic methods" (such as __wakeup(), __destruct(), or __toString()) if they are defined in the object's class. Attackers can leverage these methods to chain together a series of operations (POP chains) that ultimately lead to code execution or other malicious outcomes.

The vulnerability affects the class-bsr-db.php file within the plugin, where the deserialization occurs without proper input sanitization. This allows unauthenticated remote attackers to submit crafted serialized data that, when processed, instantiates arbitrary PHP objects.

Root Cause

The root cause of this vulnerability lies in the improper handling of serialized data within the class-bsr-db.php file. The plugin deserializes user-supplied input without implementing proper validation or using safe deserialization practices. PHP's native unserialize() function is inherently dangerous when used with untrusted input, as it can instantiate arbitrary objects and trigger magic methods that may perform unintended operations.

Attack Vector

This is a network-based attack that can be exploited remotely without authentication. The attack flow involves:

1. An attacker identifies a WordPress installation running a vulnerable version of Better Search Replace
2. The attacker crafts a malicious serialized PHP object payload
3. If a suitable POP chain exists in other installed plugins or themes, the attacker includes gadgets that will chain together to achieve code execution
4. The attacker submits the crafted payload to the vulnerable endpoint
5. When the plugin deserializes the payload, the malicious object is instantiated and executes the attacker's intended operations

The vulnerability mechanism involves unsafe deserialization in the database handling class. For technical details on the vulnerable code path, see the WordPress Plugin Code Review and the Wordfence Vulnerability Report.

Detection Methods for CVE-2023-6933

Indicators of Compromise

• Unexpected serialized data patterns in HTTP request logs containing PHP object notation (e.g., O:XX:)
• Unusual file system modifications or deletions that cannot be attributed to legitimate administrative activity
• Web server logs showing requests to Better Search Replace endpoints with unusually long or encoded payloads
• Evidence of new files created in plugin directories or web root with suspicious content

Detection Strategies

• Monitor web application firewall (WAF) logs for serialized PHP object patterns in request parameters
• Implement file integrity monitoring on WordPress core files, plugins, and themes directories
• Review access logs for unauthenticated requests to Better Search Replace plugin endpoints
• Deploy endpoint detection rules targeting PHP deserialization attack patterns

Monitoring Recommendations

• Enable verbose logging for WordPress and PHP to capture deserialization events
• Configure alerting on web server logs for requests containing serialized PHP object strings
• Implement behavioral monitoring for WordPress processes that spawn unexpected child processes
• Monitor database query patterns for anomalous search and replace operations

How to Mitigate CVE-2023-6933

Immediate Actions Required

• Update Better Search Replace plugin immediately to version 1.4.5 or later
• Audit all installed WordPress plugins and themes for known POP chain gadgets
• Review web server logs for evidence of exploitation attempts
• Consider temporarily disabling the plugin if immediate update is not possible

Patch Information

WPEngine has released a security patch addressing this vulnerability. The fix is documented in the WordPress Changeset Report. Users should update to Better Search Replace version 1.4.5 or later, which implements proper input validation and safe deserialization practices.

Workarounds

• Temporarily deactivate the Better Search Replace plugin until updates can be applied
• Implement web application firewall rules to block requests containing serialized PHP object patterns
• Restrict access to WordPress admin areas using IP allowlisting where feasible
• Remove unnecessary plugins and themes that may contain exploitable POP chains

# Configuration example
# Disable Better Search Replace plugin via WP-CLI until patched
wp plugin deactivate better-search-replace

# Update to patched version
wp plugin update better-search-replace

# Verify plugin version after update
wp plugin get better-search-replace --field=version