exploiting-file-upload-vulnerabilities

Extension filter bypasses include double extensions like shell.php.jpg, case toggling such as .pHp, alternative PHP extensions like .phtml or .phar, null byte injection, and trailing dots or spaces that Windows strips on save. Test each variant and observe which the server accepts and executes.

Content-Type validation is bypassed by declaring an image MIME type such as image/jpeg while sending executable content, since the header is client-controlled. Combine this with magic-byte prefixing, for example GIF89a or JPEG FFD8FF bytes before PHP code, to also pass server-side content sniffing.

A polyglot file is valid in multiple formats simultaneously, such as a JPEG that is also executable PHP. It passes magic-byte validation as an image while the server-side handler still parses and executes the embedded code.

Yes, SVG files can carry stored XSS via onload handlers that execute in viewers' browsers, and XXE entities that read local files or trigger SSRF. Applications serving uploaded SVGs without sanitization or Content-Disposition: attachment are exposed.

Webshells fail to execute when uploads are stored outside the web root, served with Content-Disposition: attachment, renamed to non-executable extensions, or when the server lacks a handler mapping for that extension. Check the served Content-Type and try .htaccess or web.config uploads to enable execution.

Upload testing requires written authorization explicitly covering upload functionality in the penetration testing agreement. It should not be performed on production systems without scope approval, since webshells and traversal payloads can alter server state.