📰 Originally published on SecurityElites — the canonical, fully-updated version of this article.
60-DAY BUG BOUNTY COURSE
DAY 12
Day 13: XXE →
Course Progress
Day 12 of 60 — 20% Complete
Every web application that lets users upload files is asking one question: do I trust what just came in? Most applications try to answer that question with a file extension check, a MIME type header check, or a signature scan. Every one of those checks can be bypassed. File upload vulnerabilities are one of the most consistently rewarding bug classes in bug bounty — they can escalate from a minor misconfiguration to full Remote Code Execution on the server, earning payouts in the tens of thousands of dollars. Today you are going to learn every bypass technique from first principles, practise them in hands-on labs, and walk away knowing exactly how to find, confirm, and report file upload bugs professionally. Lets start our session on File Upload Vulnerabilities Bug Bounty.
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After completing Day 12 you will be able to:
Explain every category of file upload vulnerability and their relative severity · Bypass extension, MIME type, magic bytes, and null byte upload filters · Upload a web shell to DVWA and execute remote commands · Find and exploit SVG-based stored XSS via file upload · Chain a file upload bug with path traversal for maximum impact · Write a complete Critical severity file upload report that gets paid
~24
min read
📊 DIFFICULTY CHECK
How much do you know about file upload vulnerabilities going in?
🟡 Complete beginner — I know file uploads exist but don’t know how they’re attacked
🟠 Getting there — I’ve seen the concept but never actually tested for it
🟢 Confident — I know the basics and want advanced bypass techniques
🔵 Already knew this — here for the chaining and report writing sections
✅ Recorded — this lesson covers all levels. The article flows from fundamentals through advanced bypasses to chaining and reporting.
📋 Day 12 Contents
- Why File Upload Bugs Are Among the Most Valuable in Bug Bounty
- How Upload Defences Work — and Why Each One Fails
- The Complete Bypass Playbook — 6 Techniques with Payloads
- Web Shell Upload — From File to Remote Code Execution
- SVG XSS — The File Upload Bug That Bypasses PHP Filters
- Chaining — How to Turn a Low-Severity Upload Bug into a Critical
- Hands-On Lab — File Upload in DVWA (Full Walkthrough)
- Writing the Report — Complete File Upload Bug Report Template
Why File Upload Bugs Are Among the Most Valuable in Bug Bounty
File upload functionality exists on almost every modern web application. Profile pictures, document attachments, import tools, media galleries, resume submissions, support ticket attachments — each one is an opportunity for a user to put data onto the server. And each one is an opportunity for an attacker to put code onto the server, disguised as data.
What makes file upload bugs particularly valuable in bug bounty is the potential impact ceiling. Most web vulnerabilities give you data access — you can read things you shouldn’t. A successful file upload exploit can give you code execution — you can run things on the server. Remote Code Execution (RCE) is the highest-severity finding in web application security. It earns Critical severity ratings and the largest payouts on every major programme.
Even when RCE is not achievable, file upload bugs can deliver stored XSS via SVG uploads, path traversal via filename manipulation, denial of service via oversized uploads, and information disclosure via uploaded files that the server processes incorrectly. Each of these has a place on a payout table. Understanding the full impact spectrum — and how to escalate a low-severity finding toward a higher one — is the skill that separates methodical hunters from lucky ones.
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FILE UPLOAD VULNERABILITY IMPACT SPECTRUM — FROM LOW TO CRITICAL
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Unrestricted File Upload → Web Shell → RCE
Upload PHP/ASP/JSP shell. Access via URL. Execute OS commands. Full server compromise. Lateral movement into internal network.
CRITICAL
$3K–$30K+
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Path Traversal via Filename → Arbitrary File Write
Filename like ../../webroot/shell.php stores the file outside the upload directory in a web-accessible, executable location.
HIGH
$1K–$10K
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SVG Upload → Stored XSS
Upload SVG with embedded JavaScript. When other users view the image, the script runs in their browser session. Cookie theft, session hijacking possible.
MEDIUM–HIGH
$500–$3K
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Stored File Accessible Without Auth → Info Disclosure
Files uploaded by one user are accessible to all users or unauthenticated visitors via predictable URLs. Exposure of private documents, invoices, sensitive attachments.
LOW–MEDIUM
$100–$500
File Upload Vulnerability Impact Spectrum — from unrestricted upload enabling RCE at the top to information disclosure at the bottom. The same upload endpoint can yield different severity findings depending on what the server does with the uploaded file. Your job as a hunter is to find the upload, test all paths, and escalate to the highest achievable impact before reporting.
How Upload Defences Work — and Why Each One Fails
📖 Read the complete guide on SecurityElites
This article continues with deeper technical detail, screenshots, code samples, and an interactive lab walk-through. Read the full article on SecurityElites →
This article was originally written and published by the SecurityElites team. For more cybersecurity tutorials, ethical hacking guides, and CTF walk-throughs, visit SecurityElites.
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