CVE-2019-9787 in WordPress
Summary
by MITRE
WordPress before 5.1.1 does not properly filter comment content, leading to Remote Code Execution by unauthenticated users in a default configuration. This occurs because CSRF protection is mishandled, and because Search Engine Optimization of A elements is performed incorrectly, leading to XSS. The XSS results in administrative access, which allows arbitrary changes to .php files. This is related to wp-admin/includes/ajax-actions.php and wp-includes/comment.php.
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Analysis
by VulDB Data Team • 08/27/2025
WordPress versions prior to 5.1.1 contained a critical vulnerability in the comment submission handling mechanism that enabled unauthenticated remote code execution through a sophisticated chain of exploits. The vulnerability stemmed from inadequate input validation and improper handling of cross-site request forgery protection measures within the comment processing pipeline. Attackers could leverage this flaw to inject malicious content that would bypass standard security filters, ultimately gaining administrative privileges through reflected cross-site scripting attacks.
The technical implementation of this vulnerability involved multiple layers of exploitation starting with the comment submission form in wp-includes/comment.php where user input was not properly sanitized before being stored in the database. The mishandling of CSRF protection meant that malicious actors could craft requests that appeared legitimate to the WordPress system, allowing them to submit comments containing malicious payloads. Additionally, the search engine optimization functionality for anchor elements in the codebase performed incorrect HTML sanitization, creating persistent XSS vulnerabilities that could be triggered when comments were displayed on the site.
The operational impact of this vulnerability was severe as it allowed attackers to execute arbitrary code on vulnerable WordPress installations without requiring authentication. Once an attacker gained administrative access through the XSS vector, they could modify any PHP files on the server, effectively taking complete control of the website. This level of access enabled attackers to install backdoors, steal sensitive data, deface websites, or use the compromised server for further attacks. The default configuration of WordPress installations made this vulnerability particularly dangerous as many sites were running vulnerable versions without proper security hardening measures.
The attack chain began with an unauthenticated user submitting a comment containing malicious JavaScript code that would execute in the browser of administrators who viewed the comment. This reflected XSS attack could be crafted to steal administrator session cookies or execute commands through the WordPress admin interface. The vulnerability was particularly insidious because it exploited the trust relationship between WordPress and its users, allowing attackers to escalate privileges from anonymous commenters to full administrative control. Security researchers identified this issue as a prime example of how improper input validation and CSRF protection flaws can combine to create critical remote code execution vulnerabilities.
This vulnerability aligns with CWE-79 which describes cross-site scripting flaws, and CWE-20 which covers input validation issues. The exploitation patterns match techniques described in the MITRE ATT&CK framework under T1190 for exploit public-facing application and T1059 for command and scripting interpreter. The specific code locations mentioned in wp-admin/includes/ajax-actions.php and wp-includes/comment.php represent critical attack vectors that were patched in WordPress version 5.1.1, emphasizing the importance of keeping content management systems updated. Organizations should implement robust input validation, proper CSRF token implementation, and regular security audits to prevent similar vulnerabilities from compromising their web applications.
The remediation strategy for this vulnerability required immediate patching of WordPress installations to version 5.1.1 or later, which addressed the comment filtering mechanisms and corrected the CSRF protection handling. System administrators should also implement additional security measures including web application firewalls, input sanitization at multiple layers, and regular security scanning to detect similar vulnerabilities. The incident highlighted the critical need for proper security testing of user input handling in web applications and demonstrated how seemingly minor security flaws in core functionality could be exploited to achieve complete system compromise. Organizations running WordPress installations should maintain strict update policies and security monitoring to protect against similar vulnerabilities in the future.