📰 Originally published on SecurityElites — the canonical, fully-updated version of this article.
🐛 BUG BOUNTY COURSE
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Part of the Bug Bounty Hunter Course
Day 27 of 60 · 45% complete
⚠️ Legal Disclaimer: All path traversal, LFI, log poisoning, and /proc enumeration techniques covered here are strictly for authorised security testing and educational purposes. Never test any system without explicit written permission from the owner. Unauthorised access is illegal.
The target was a SaaS invoice platform — mid-sized company, active bug bounty program, $5,000 Critical cap. I found a file= parameter buried in a PDF export endpoint. Six characters: ../../../. Thirty seconds later I had /etc/passwd in my Burp Repeater response. That alone was a High. Then I pivoted to /proc/self/environ and pulled the application’s SECRET_KEY, DATABASE_URL, and an AWS access token — all sitting in the process environment, handed to me by the Linux kernel. That’s the Critical. Path traversal LFI bug bounty chains don’t require sophisticated tooling or days of research. They require knowing exactly which files to ask for, which filters to break, and how to document the impact so the triage team has no choice but to escalate. That’s what today covers — the full chain from parameter discovery to log poisoning proof-of-concept, exactly the way I run it on real engagements.
🎯 What You’ll Master in Day 27
Detect path traversal candidates in URLs, POST bodies, and JS-mined endpoints using practitioner recon methodology
Break WAF and developer filters with encoding bypasses, null bytes, and nested traversal sequences
Extract high-value data from the Linux /proc filesystem — environment variables, secret keys, open file descriptors
Chain LFI to log poisoning for remote code execution proof-of-concept in your bug bounty report
Write a Critical-severity bug bounty report that earns maximum payout — impact framing, reproduction steps, remediation
⏱️ 3 hours total · 3 exercises · Beginner-friendly with Burp Suite ### Before You Start Day 27 - Completed Day 26 SSTI Bug Bounty — server-side template injection builds the same parameter-exploitation mindset you’ll use here - Burp Suite Community or Professional installed and intercepting your browser traffic — Proxy and Repeater tabs both working - Basic Linux filesystem knowledge: you know what /etc/passwd, /var/log/, and /proc/ are - Active PortSwigger Web Security Academy account — free at portswigger.net — needed for Exercise 2 - Revisit the full Bug Bounty Hunter Course hub if you need to catch up on earlier days before continuing ### Day 27 — Path Traversal & LFI Bug Bounty 2026 1. What Path Traversal and LFI Actually Are (And Why They’re Still Paying Critical in 2026) 2. Recon: How I Find Traversal Parameters Before I Ever Send a Payload 3. 🛠️ Exercise 1 — Browser Recon for Traversal Parameters 4. Filter Bypass: Every WAF and Developer Defence I’ve Broken on Path Traversal 5. /proc Leaks: What the Linux Process Filesystem Hands You on a Plate 6. 🌐 Exercise 2 — PortSwigger Path Traversal Labs 7. Log Poisoning: Chaining LFI to Remote Code Execution 8. ⚡ Exercise 3 — Full Chain in Burp Suite 9. Writing the Critical Report — Impact Framing That Earns Maximum Payout 10. 📋 Commands Reference Card — Day 27 11. FAQs — Path Traversal & LFI Bug Bounty Yesterday in Day 26 you learned how server-side template injection turns a rendering engine into a code execution primitive. Path traversal and LFI sit one level below that — instead of hijacking the template engine, you’re hijacking the file system itself. These two bugs appear together in almost every PHP monolith, Node.js file-serving endpoint, and legacy Java web app I’ve audited. They’re also some of the most underreported vulnerabilities in bug bounty because hunters either stop at /etc/passwd (which is a High, not a Critical) or they never find the parameter at all. By the end of today you’ll know how to find it, break its filters, and chain it all the way to RCE. Check the full Bug Bounty Hunter Course hub for where this fits in the 60-day roadmap.
What Path Traversal and LFI Actually Are (And Why They’re Still Paying Critical in 2026)
Here’s what’s happening at the server level. The application takes a filename from user input — a URL parameter, a POST field, a cookie — and passes it directly to a file-reading or file-including function without validating that the resolved path stays inside the intended directory. That’s it. The entire vulnerability class in one sentence.
Path traversal and local file inclusion are two expressions of the same root cause, but they behave differently in exploitation:
Path traversal means the application reads and returns the file contents in the HTTP response — think readFile() in Node or file_get_contents() in PHP. You get the raw content of whatever file you point it at. The payload is the classic ../../../etc/passwd sequence — each ../ climbs one directory level toward the filesystem root.
LFI (local file inclusion) means the application uses the input to include a PHP file and execute it — include(), require(), include_once(). This is more powerful because it doesn’t just read files — it runs them. That’s the primitive that enables log poisoning to RCE, which we’ll cover in full later today.
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