CVE-2023-0147 in Flexible Captcha Plugin
Summary
by MITRE • 02/06/2023
The Flexible Captcha WordPress plugin through 4.1 does not validate and escape some of its shortcode attributes before outputting them back in a page/post where the shortcode is embed, which could allow users with the contributor role and above to perform Stored Cross-Site Scripting attacks
Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.
Analysis
by VulDB Data Team • 03/05/2023
The Flexible Captcha WordPress plugin version 4.1 and earlier contains a critical stored cross-site scripting vulnerability that affects WordPress installations using this plugin. This vulnerability stems from inadequate input validation and output escaping mechanisms within the plugin's shortcode processing functionality. The flaw specifically impacts users with the contributor role and above, making it particularly concerning for WordPress environments where multiple user roles exist with varying permission levels.
The technical implementation of this vulnerability occurs within the plugin's shortcode handling code where certain attributes passed to the captcha shortcode are not properly sanitized before being rendered back to the page. When a contributor or higher-privileged user embeds a malicious shortcode containing crafted script payloads, these scripts become permanently stored within the WordPress database. The vulnerability manifests when the page containing the shortcode is subsequently rendered, causing the malicious JavaScript to execute in the context of other users who view the affected content.
This stored XSS vulnerability operates under the CWE-79 classification as a classic cross-site scripting flaw where untrusted data flows from user input into web output without proper sanitization. The attack vector specifically targets the shortcode attribute processing system, which falls under the ATT&CK technique T1566.001 for initial access through malicious content injection. The vulnerability's impact is amplified by the contributor role requirement, as this level of access typically allows users to create and modify posts and pages, making the attack surface more accessible in typical WordPress environments.
The operational consequences of this vulnerability extend beyond simple script execution, as attackers could potentially steal session cookies, redirect users to malicious sites, or perform actions on behalf of other users within the WordPress environment. The stored nature of the vulnerability means that the malicious payloads persist even after the initial injection, creating a continuous threat vector that affects all users who encounter the compromised content. This makes the vulnerability particularly dangerous in multi-user environments where contributors regularly create content that gets displayed to other users.
Mitigation strategies for this vulnerability include immediate patching to version 4.2 or later where the sanitization issues have been addressed. Administrators should also implement additional security measures such as role-based content review processes, regular security audits of user-generated content, and monitoring for suspicious shortcode usage patterns. The WordPress security team recommends that all users upgrade to the latest plugin version and consider implementing content security policies to further reduce the impact of potential XSS attacks. Additionally, administrators should review and restrict contributor permissions where possible, ensuring that users with elevated roles undergo additional scrutiny for content they create and publish.