📰 Originally published on SecurityElites — the canonical, fully-updated version of this article.
🎯 BUG BOUNTY COURSE
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Part of the Bug Bounty Course — 60 Days
Day 26 of 60 · 43% complete
⚠️ Legal Disclaimer: Every SSTI technique, payload, and exploitation chain covered here is strictly for authorised bug bounty programs and ethical security research. Testing systems without explicit written permission is illegal under the Computer Fraud and Abuse Act, the Computer Misuse Act, and equivalent legislation worldwide. Always hunt within scope.
I was testing a contact form — the kind that every target has and every hunter ignores. Name field, email field, message box. I dropped {{7*7}} into the name field and hit send. The confirmation email came back: “Hello 49, your message has been received.”
That’s it. That one three-character arithmetic expression just handed me a Critical-severity vulnerability on a program that pays up to $20,000 for RCE. The server wasn’t reflecting my input — it was evaluating it. The template engine was executing my code before the email got rendered. From that confirmation, I had a clear path to full Remote Code Execution on the application server.
SSTI bug bounty vulnerabilities are some of the most underreported Critical findings in the game right now. Most hunters see an injection point, throw an XSS payload, get nothing, and move on. They don’t know to probe for template syntax. They don’t know the engine-specific payload chains. They’re leaving $5,000–$30,000+ reports sitting in fields they already found.
Today I’m changing that. I’ll walk you through exactly how I detect SSTI on live programs, how I fingerprint the template engine from response deltas, and how I build the RCE chain for all five engines you’ll encounter in real bug bounty targets — Jinja2, Twig, Freemarker, Pebble, and Velocity. By the end of this, you’ll be filing Critical SSTI reports that most hunters never even notice.
🎯 What You’ll Master in Day 26
Identify SSTI injection points in live bug bounty targets using a structured payload decision tree
Fingerprint the template engine from response deltas — no guessing, no wrong-engine payloads
Execute confirmed RCE payloads on Jinja2, Twig, Freemarker, Pebble, and Velocity
Write a Critical-severity SSTI bug bounty report that gets triaged and paid fast
Apply WAF bypass techniques when template payloads are filtered
Escalate from arithmetic confirmation to OS command output in a structured, documented chain
⏱️ ~3 hours · 3 exercises ### 📋 Prerequisites - Completed Day 25 — Host Header Injection Bug Bounty — you understand how server-side request manipulation works - Completed Day 24 — CRLF Injection Bug Bounty — familiarity with injection-class vulnerability structure - Burp Suite Community or Pro — Repeater tab open and working. You’ll need it for Exercises 2 and 3. - A free PortSwigger Web Security Academy account — Exercise 2 uses their SSTI labs directly ### SSTI Bug Bounty 2026 — Contents 1. What SSTI Is and Why It Consistently Pays Critical on Bug Bounty Programs 2. SSTI Detection and Engine Fingerprinting — The Payload Decision Tree 3. SSTI Exploitation Chains — RCE Payloads for Jinja2, Twig, Freemarker, Pebble, and Velocity 4. Writing a Critical-Severity SSTI Bug Bounty Report 5. WAF Bypass and Escalation Techniques for SSTI Payloads 6. Frequently Asked Questions You knocked out Day 25 — Host Header Injection and Day 24 — CRLF Injection — two server-side manipulation techniques that operate at the HTTP layer. Today we go deeper. SSTI sits at the top of the injection severity ladder because it doesn’t just manipulate headers or redirect flows — it executes arbitrary code inside the server process. If you want to round out your toolkit before diving in, check out the free SecurityElites tools suite for recon support on your targets. Everything we cover today spans both detection methodology and engine-specific exploitation — because finding SSTI without knowing how to prove RCE leaves money on the table. The full curriculum lives at the Bug Bounty Course hub.
What SSTI Is and Why It Consistently Pays Critical on Bug Bounty Programs
Here’s what’s actually happening when a template engine runs. The application has a template — an HTML file with placeholder expressions like {{username}} or ${order.total}. At runtime, the engine takes the template, resolves those expressions against a data context, and renders the final output. The problem starts when the application constructs that template dynamically using user input — instead of passing your name as a value into a fixed placeholder, it drops your raw input directly into the template string before evaluation.
When that happens, the engine doesn’t see your input as data. It sees it as template code. And it executes it.
Think about the difference between these two approaches. Safe: render("Hello {{name}}", {name: userInput}) — the input goes into the data context, never into the template itself. Vulnerable: render("Hello " + userInput) — the input is concatenated into the template string, so if you send {{7*7}}, the engine evaluates it.
This isn’t an obscure mistake. It shows up constantly in name fields that get rendered into emails, in error messages that echo back what you submitted, in PDF generators that build reports from user-supplied content, in notification systems, in search result pages that include your query in the heading, and in admin dashboards that preview user-submitted data. Anywhere the application uses a template to display your input, and anywhere a developer got lazy with string concatenation instead of proper context separation, SSTI is possible.
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