CVE-2021-24503 in Simple Icons Plugin
Summary
by MITRE • 08/02/2021
The Popular Brand Icons – Simple Icons WordPress plugin before 2.7.8 does not sanitise or validate some of its shortcode parameters, such as "color", "size" or "class", allowing users with a role as low as Contributor to set Cross-Site payload in them. A post made by a contributor would still have to be approved by an admin to have the XSS triggered in the frontend, however, higher privilege users, such as editor could exploit this without the need of approval, and even when the blog disallows the unfiltered_html capability.
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Analysis
by VulDB Data Team • 08/06/2021
The vulnerability identified as CVE-2021-24503 affects the Popular Brand Icons - Simple Icons WordPress plugin version 2.7.7 and earlier, representing a critical cross-site scripting weakness that stems from insufficient input sanitization within the plugin's shortcode implementation. This flaw allows attackers with minimal privileges to inject malicious scripts into web pages viewed by other users, creating a significant security risk for WordPress installations that rely on this plugin for brand icon display functionality. The vulnerability specifically targets the plugin's handling of shortcode parameters including color, size, and class attributes, which are processed without proper validation or sanitization measures that would normally prevent malicious code execution.
The technical nature of this vulnerability aligns with CWE-79, which describes Cross-Site Scripting flaws occurring when web applications fail to properly validate or sanitize user-provided input before incorporating it into dynamic web pages. The plugin's shortcode processing mechanism fails to implement adequate sanitization routines for parameters that could contain executable code, allowing malicious payloads to be stored and executed when the shortcode is rendered on the frontend. This weakness exists because the plugin does not properly validate the inputs against a whitelist of acceptable values or apply appropriate encoding techniques that would neutralize potentially harmful script content. The vulnerability's impact is exacerbated by the fact that even users with contributor-level privileges can exploit this flaw, making it particularly dangerous in environments where content moderation is not strictly enforced.
The operational impact of this vulnerability extends beyond simple script injection, as it creates potential pathways for more sophisticated attacks including session hijacking, credential theft, and redirection to malicious websites. While the plugin's design requires posts containing malicious shortcodes to be approved by administrators for the XSS to execute in the frontend, this requirement does not fully mitigate the risk since editors and other higher-privilege users can exploit the vulnerability without approval requirements. This means that even in environments where unfiltered_html capability is disabled, the vulnerability remains exploitable due to the plugin's insufficient input handling. The attack vector becomes particularly dangerous when considering that administrators might inadvertently approve posts containing malicious shortcodes, or when attackers can manipulate the approval process through social engineering or other means.
Security mitigation strategies for this vulnerability should focus on immediate plugin updates to version 2.7.8 or later, which contain the necessary sanitization fixes. Administrators should also implement additional defensive measures including monitoring for suspicious shortcode usage, implementing Content Security Policy headers to limit script execution, and conducting regular security audits of plugin installations. The vulnerability demonstrates the importance of proper input validation and output encoding in web applications, principles that align with the ATT&CK framework's defense evasion techniques and the OWASP Top Ten's emphasis on injection flaws. Organizations should also consider implementing automated scanning tools that can detect similar vulnerabilities in other plugins and themes, as this vulnerability represents a common pattern in WordPress plugin development where insufficient sanitization leads to widespread security risks. Regular security assessments and maintaining updated plugin inventories are essential practices to prevent exploitation of such vulnerabilities in production environments.