CVE-2021-39201 in WordPress
Summary
by MITRE • 09/10/2021
WordPress is a free and open-source content management system written in PHP and paired with a MySQL or MariaDB database. ### Impact The issue allows an authenticated but low-privileged user (like contributor/author) to execute XSS in the editor. This bypasses the restrictions imposed on users who do not have the permission to post `unfiltered_html`. ### Patches This has been patched in WordPress 5.8, and will be pushed to older versions via minor releases (automatic updates). It's strongly recommended that you keep auto-updates enabled to receive the fix. ### References https://wordpress.org/news/category/releases/ https://hackerone.com/reports/1142140 ### For more information If you have any questions or comments about this advisory: * Open an issue in [HackerOne](https://hackerone.com/wordpress)
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Analysis
by VulDB Data Team • 09/12/2021
The vulnerability identified as CVE-2021-39201 represents a cross-site scripting flaw within the WordPress content management system that specifically affects users with contributor or author roles. This security issue arises from insufficient input validation and sanitization mechanisms within the WordPress editor interface, creating a pathway for authenticated users to inject malicious scripts that can execute in the context of other users' browsers. The flaw is particularly concerning because it undermines the permission model that WordPress implements to restrict certain HTML content based on user roles, allowing lower-privileged users to bypass these security controls through a carefully crafted payload.
The technical exploitation of this vulnerability occurs through the WordPress editor's handling of user input, where the system fails to properly sanitize or escape content that is processed through the visual editor interface. This weakness enables an attacker with contributor or author privileges to insert malicious JavaScript code that gets executed when other users view the affected content, potentially leading to session hijacking, data theft, or further escalation of privileges. The vulnerability specifically targets the editor environment where users without the unfiltered_html capability are normally restricted from posting raw HTML, yet the flaw allows bypassing these protections through the XSS vector.
From an operational perspective, this vulnerability presents a significant risk to WordPress installations where contributors or authors have access to the system, as these users typically have limited capabilities but can now potentially execute malicious scripts against other users. The impact extends beyond simple script execution to potentially enable more sophisticated attacks such as credential theft, privilege escalation, or redirection to malicious sites. The vulnerability's exploitation requires minimal user interaction since it operates within the editor context where users are already authenticated, making it particularly dangerous in environments where multiple users contribute content.
The mitigation strategy for CVE-2021-39201 involves upgrading to WordPress version 5.8 or later, which includes the necessary patches to address the XSS vulnerability in the editor interface. WordPress automatically pushes these security updates to older versions through minor releases, making it crucial for administrators to maintain automatic updates enabled to receive the fix. The patch addresses the root cause by implementing proper input sanitization and output escaping mechanisms within the editor's content processing pipeline, ensuring that user-supplied content is properly validated before being rendered in the browser context. Security practitioners should also consider implementing additional monitoring for suspicious content submissions and user behavior patterns that might indicate exploitation attempts.
This vulnerability aligns with CWE-79, which describes Cross-Site Scripting flaws in software systems, and represents a specific implementation weakness in WordPress's content handling architecture. The attack pattern follows ATT&CK technique T1059.007 for command and control through web shell execution, though the immediate impact is limited to the XSS vector within the editor environment. Organizations should also consider the broader implications of this vulnerability within their WordPress security posture, particularly in multi-user environments where contributor and author roles are widely distributed, as it demonstrates how privilege escalation can occur through seemingly minor security gaps in content management systems. The vulnerability serves as a reminder of the critical importance of maintaining up-to-date software versions and implementing proper input validation across all user-facing interfaces in web applications.