CVE-2021-28121 in Virtual Robots.txt
Summary
by MITRE • 08/13/2021
Virtual Robots.txt before 1.10 does not block HTML tags in the robots.txt field.
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Analysis
by VulDB Data Team • 08/18/2021
The vulnerability identified as CVE-2021-28121 affects the Virtual Robots.txt plugin version 1.10 and earlier, representing a significant security flaw in web application configuration management. This issue stems from inadequate input validation within the robots.txt field processing functionality, where the system fails to properly sanitize or filter HTML tags that may be present in user-supplied content. The vulnerability resides in the plugin's inability to distinguish between legitimate configuration directives and potentially malicious HTML content that could be injected into the robots.txt file.
The technical implementation flaw manifests when the plugin processes user input for the robots.txt field without proper sanitization measures. This creates an environment where HTML tags can be inadvertently or maliciously included in the configuration file, potentially allowing attackers to manipulate how web crawlers and bots interpret the site's access rules. The vulnerability specifically affects the parsing and handling of HTML elements within the robots.txt field, which should normally contain only standard robots.txt directives such as User-agent, Disallow, and Allow statements.
From an operational impact perspective, this vulnerability could enable attackers to bypass intended access restrictions by injecting HTML content that manipulates crawler behavior or creates unexpected parsing scenarios. The flaw essentially allows for a form of input injection that could potentially lead to information disclosure, denial of service, or other malicious activities depending on how the affected system processes the corrupted robots.txt content. Security researchers have classified this as a configuration management issue that could undermine the effectiveness of web application security controls.
The vulnerability aligns with CWE-79, which addresses Cross-Site Scripting (XSS) conditions where web applications fail to properly validate or sanitize user-provided data before incorporating it into dynamic content. Additionally, this issue relates to ATT&CK technique T1071.004, which involves application layer protocol manipulation through improper input handling. The affected plugin's failure to implement proper input validation and sanitization creates a pathway for attackers to inject malicious content that could be interpreted by web crawlers or other automated tools.
Organizations should immediately upgrade to Virtual Robots.txt version 1.10 or later, which includes proper input sanitization and validation mechanisms. The recommended mitigation strategy involves implementing comprehensive input filtering that removes or encodes HTML tags from user-supplied content before processing. Security teams should also conduct thorough audits of all plugins and applications that handle user-provided configuration data, ensuring proper sanitization measures are in place. Network monitoring should be enhanced to detect unusual patterns in robots.txt file access or modifications that could indicate exploitation attempts. Regular security assessments of web application configurations are essential to prevent similar vulnerabilities from emerging in other components of the system infrastructure.