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web-file-upload

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Turn a file upload into RCE or stored XSS/SSRF. Load on any upload: avatars, attachments, import CSV/XML, profile images, document processors, "upload your resume". Signals: multipart/form-data, filename in response, an uploads/ path, image thumbnailing, PDF/office parsing.

skills/web/web-file-upload/SKILL.md

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Last checked
Nov 7, 2026

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27
Repository last updated
Oct 14, 2026
License
MIT

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# Malicious file upload

## When it applies
The app accepts a file and later stores, serves, parses, or converts it. Impact depends on
what happens to the file after upload — served from webroot? passed to a parser? rendered?

## Why it works
Validation is usually on the wrong signal (extension or `Content-Type`, both attacker-set)
while the dangerous behaviour is downstream: the web server executes `.php` in the uploads
dir, a parser follows external entities, or the file is served with an HTML content-type.

## Method
> **Payloads & full variation set:** [`cheatsheet.md`](cheatsheet.md) next to this file — work the set, not the first line.
1. **Find the after-upload behaviour**: where is it stored, what URL serves it, what parses it?
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