# Server-Side Template Injection (SSTI)

> Detects user-controlled input passed directly into template rendering engines, enabling code execution.

- Skill: `zakirkun/server-side-template-injection-ssti` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds@latest add zakirkun/server-side-template-injection-ssti`
- Raw SKILL.md: https://api.skillmd.com/api/skills/zakirkun/server-side-template-injection-ssti/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: zakirkun (https://skillmd.com/u/zakirkun)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/zakirkun/server-side-template-injection-ssti

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# Server-Side Template Injection (SSTI)

## Overview
SSTI occurs when user input is embedded directly into a template string before rendering. Template engines like Jinja2, Twig, Pebble, Freemarker, and Mustache can interpret injected expressions as code, leading to Remote Code Execution (RCE).

Classic payload: `{{7*7}}` → if output is `49`, the template is evaluating user input.

## Detection Strategy
Look for template `render()` calls that concatenate user input directly into the template string rather than passing it as a context variable.

Key patterns:
- `render_template_string(user_input)` in Flask/Jinja2
- `new Template(user_input).render()` in various engines
- String format operations feeding user data into template strings

## Remediation
Never put user input in the template string. Always pass user data as template variables.

**Vulnerable (Python/Jinja2):**
```python
from flask import render_template_string, request
@app.route('/greet')
def greet():
    name = request.args.get('name')
    return render_template_string(f"Hello {name}!")  # SSTI!
```

**Safe (Python/Jinja2):**
```python
@app.route('/greet')
def greet():
    name = request.args.get('name')
    return render_template_string("Hello {{ name }}!", name=name)
```