CVE-2021-24516 in PlanSo Forms Plugin
Summary
by MITRE • 10/18/2021
The PlanSo Forms WordPress plugin through 2.6.3 does not escape the title of its Form before outputting it in attributes, allowing high privilege users such as admin to set XSS payload in it, even when the unfiltered_html is disallowed, leading to an Authenticated Stored Cross-Site Scripting issue.
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Analysis
by VulDB Data Team • 10/22/2021
The vulnerability identified as CVE-2021-24516 affects the PlanSo Forms WordPress plugin version 2.6.3 and earlier, representing a critical authenticated stored cross-site scripting flaw that exploits improper output escaping mechanisms. This vulnerability specifically targets the plugin's handling of form titles within HTML attributes, creating a persistent XSS vector that can be exploited by users with administrative privileges or higher access levels. The flaw occurs when the plugin fails to properly sanitize or escape the title field before rendering it in HTML attributes, allowing malicious payloads to be stored and subsequently executed in the context of other users' browsers.
The technical implementation of this vulnerability stems from a failure in input validation and output escaping practices within the WordPress plugin architecture. When administrators or privileged users create or edit forms through the plugin interface, they can inject malicious JavaScript code into the form title field. This payload persists in the database and gets rendered back to users' browsers whenever the form data is displayed, particularly in HTML attributes where the unfiltered_html capability is restricted. The vulnerability demonstrates a classic case of improper output escaping, which is categorized under CWE-79 as "Cross-site Scripting" and specifically aligns with CWE-116 as "Improper Encoding or Escaping of Output" in the context of HTML attribute contexts.
The operational impact of this vulnerability extends beyond simple XSS execution, as it can be leveraged for more sophisticated attacks within the WordPress environment. An authenticated attacker with administrative privileges can craft malicious payloads that execute in the context of other users who view affected forms, potentially leading to session hijacking, privilege escalation, or data exfiltration. The vulnerability is particularly concerning because it operates even when the WordPress environment has strict HTML filtering enabled, making it difficult to detect and prevent through standard security measures. This flaw can be exploited to bypass security controls that typically protect against XSS attacks by virtue of its persistence and the specific context in which it occurs.
Mitigation strategies for this vulnerability should focus on both immediate remediation and long-term security hardening. The primary solution involves updating to the patched version of the PlanSo Forms plugin, which addresses the improper output escaping issue by implementing proper HTML attribute escaping mechanisms. Organizations should also implement additional security measures such as restricting administrative privileges to only necessary personnel, implementing web application firewalls with XSS detection capabilities, and conducting regular security audits of WordPress plugins. From an ATT&CK framework perspective, this vulnerability maps to T1548.001 as "Abuse Elevation Control Mechanism" and T1059.007 as "Command and Scripting Interpreter: JavaScript," highlighting the potential for privilege escalation and malicious code execution through the XSS vector. Regular monitoring of plugin updates and maintaining a comprehensive inventory of installed WordPress plugins can help prevent similar vulnerabilities from being exploited in the environment.