Overview
File Extension Filter Bypass focuses on an upload feature that attempts to control risk by checking the file extension.
This lab shows why extension-based validation is easy to get wrong. File names are user-controlled metadata, and server behavior can vary depending on parsing rules, storage paths, and web server configuration.
Security Impact
If an extension filter is bypassed, attackers may upload files that the application did not intend to accept. This can lead to stored malicious content, unauthorized file hosting, or server-side execution when uploaded files are interpreted.
Vulnerability Scope
Image uploaders, CMS media libraries, document portals, and older applications often fail here when filename checks become the primary upload security control.
Lab Focus
The lab focuses on extension-based defenses, why file names are not trustworthy, and how canonical filenames, allowlists, and non-executable storage locations reduce risk.
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