Server-Side Template Injection (SSTI) Vulnerability and Protection

Server-Side Template Injection (SSTI) Vulnerability and Protection

Description
Server-Side Template Injection (SSTI) is a security vulnerability that occurs on the server side of web applications. It arises when an application, while rendering user input, fails to strictly filter it and directly concatenates user input containing malicious template directives into the template for execution. Attackers can exploit this vulnerability to execute arbitrary code on the server, read sensitive files, or even gain complete control of the server. Common template engines include Jinja2 (Python), Freemarker/Thymeleaf (Java), Twig (PHP), Smarty (PHP), etc.

Problem-Solving Process

  1. Understanding How Template Engines Work

    • Goal: Template engines are designed to embed dynamic data (such as usernames, article content) into static page structures (HTML templates). They use special syntax (e.g., {{ variable }}, {% logic control %}) to mark the location of dynamic content and control the logic flow.
    • Normal Workflow:
      1. The developer writes a template file (e.g., welcome.html) containing template syntax: <h1>Welcome, {{ username }}!</h1>.
      2. Backend code (e.g., a Python Flask view function) receives a user request and retrieves data (e.g., reads username = "Alice" from a database).
      3. The backend code calls the template engine's rendering function, passing in the template file and data (context): render_template("welcome.html", username=username).
      4. The template engine performs the rendering, replacing {{ username }} with the concrete value "Alice", generating the final HTML: <h1>Welcome, Alice!</h1>.
      5. The generated HTML is sent to the user's browser.
    • Key Point: The template engine needs to "parse" and "execute" the special syntax within the template. If a user can control the content that gets "executed," a vulnerability is created.

  2. Identifying SSTI Vulnerability Points

    • Goal: Find places in the application where user input is directly used for template rendering.
    • Cause of Vulnerability: When developers mistakenly concatenate user input directly into the template string, instead of passing it as data to the template.
      • Incorrect Example (Python Flask/Jinja2): 
        # Dangerous! Directly uses user input as part of the template for rendering
from flask import Flask, request
app = Flask(__name__)

@app.route('/greet')
def greet():
    name = request.args.get('name', 'Guest')
    # Error: Using string formatting or concatenation to "construct" template content
    template = f"<h1>Hello, {name}!</h1>"  # Or "<h1>Hello, " + name + "!</h1>"
    return render_template_string(template)  # Directly renders the string template
      • Correct Example: 
        # Safe: Passes user input as data to a predefined template
@app.route('/greet_safe')
def greet_safe():
    name = request.args.get('name', 'Guest')
    # Correct: Uses a separate template file; user input is a variable in the context
    return render_template("greet.html", username=name)
# In greet.html: <h1>Hello, {{ username }}!</h1>
    • Detection Methods: In places where user input might be rendered (e.g., search boxes, profile pages), try entering simple template expressions.
      • Input: {{ 7*7 }}
      • Observation: If the returned page content includes 49 instead of the original {{ 7*7 }}, there is a high likelihood of an SSTI vulnerability because the template engine executed the multiplication.

  3. Exploiting the SSTI Vulnerability

    • Goal: After confirming SSTI, construct malicious payloads to execute system commands or read files, based on the type of template engine.
    • Steps:
      1. Identify the Template Engine Type: Different engines have different syntax and built-in objects/functions. Use probing payloads to observe error messages or behavioral differences.
        • Input {{ 7*'7' }}: Jinja2 returns 7777777, Twig returns 49.
        • Input ${7*7}: Might be Freemarker/Velocity.
        • Input <%= 7*7 %>: Might be ERB (Ruby)/JSP.
      2. Find Available Classes and Methods: Template engines typically provide a context environment containing built-in objects (e.g., __builtins__, os module in Python, Runtime class in Java). The attacker's goal is to call methods of these objects.
      3. Construct Malicious Payloads (using Jinja2 as an example):
        • Read Sensitive Files: Attempt to access the file system.
          • {{ ''.__class__.__mro__[1].__subclasses__() }}: The purpose of this payload is to list all Python built-in classes. '' is a string object; its __class__ is the str class; __mro__ (method resolution order) shows its inheritance chain (e.g., str, object); [1] is usually the base class object; __subclasses__() returns all classes inheriting from object. From the large list of output classes, one needs to find a class that can be used to execute commands or read files (e.g., <class 'os._wrap_close'>, <class 'subprocess.Popen'>) and note its index in the list.
        • Execute System Commands:
          • Assume <class 'subprocess.Popen'> is found at index 400 from the list above.
          • Payload: {{ ''.__class__.__mro__[1].__subclasses__()[400]('whoami', stdout=-1).communicate() }}
          • Explanation: Retrieve the Popen class via index 400, instantiate it, pass the command whoami, and finally call the communicate() method to execute the command and retrieve the output.

  4. Protecting Against SSTI Vulnerabilities

    • Goal: Fundamentally eliminate the possibility of user input being executed as template directives.
    • Core Principle: Strictly separate code (template syntax) and data (user input).
    • Specific Measures:
      1. Avoid Dynamic Template Rendering: Never use functions like render_template_string to directly render strings containing user input. Always insist on using predefined, static template files (e.g., .html, .j2 files) and pass user input as data (variables) to the template engine.
      2. Strictly Filter and Escape User Input: If a business scenario genuinely requires some dynamism (should be avoided if possible), user input must be whitelist-filtered, allowing only safe characters. For all data output to the page, ensure the template engine's auto-escaping feature is enabled (usually default in modern engines), so characters like <, > are escaped to &lt;, &gt;, preventing them from being executed as HTML or scripts. Note, however, that auto-escaping is for the HTML context; for template injection itself, mere escaping is insufficient because the attack payload (e.g., {{...}}) is legitimate template syntax.
      3. Run the Template Engine in a Sandbox Environment: Some template engines support a sandbox mode, which restricts accessible classes and functions, thereby reducing the success rate of exploits. However, this is not absolutely secure, as experienced attackers might find sandbox escape methods.
      4. Code Auditing and Security Testing: During development, conduct regular code reviews, especially focusing on all template rendering-related code. During the testing phase, use SAST/DAST tools and manually perform SSTI vulnerability testing.