Day 27 of 30 Day — 30 Vulnerabilities | Server-Side Template Injection (SSTI)

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Day 27: Mastering Server-Side Template Injection (SSTI) — Essential Tricks & Techniques Based on Personal Experience and Valuable POCs

In collaboration with Sunil Kumawat (LinkedIn | Twitter)

Hey security enthusiasts, Abhijeet (Twitter) here, bringing you another deep dive into the world of web security. Today’s focus is on Server-Side Template Injection (SSTI) — a powerful and often overlooked vulnerability that can lead to severe security breaches. Let’s break down the essentials.

What is Server-Side Template Injection (SSTI)?

Server-Side Template Injection (SSTI) is a vulnerability that occurs when user input is unsafely incorporated into server-side templates. By exploiting this flaw, an attacker can execute arbitrary code on the server, leading to unauthorized access, data theft, or full system compromise.

How Does It Work?

SSTI typically arises when an application uses templating engines (like Jinja2, Thymeleaf, or Velocity) that render templates on the server-side. If user input is inserted into templates without proper validation or sanitization, an attacker can inject malicious code or expressions that the template engine will execute.

User-Supplied Data in Templates:

Identify parts of the application where user-supplied data is embedded within templates. This includes email templates, web pages, and report generation.

Templating Engines:

Review the application’s use of templating engines. Common ones like Jinja2, Freemarker, or Thymeleaf are often vulnerable if not handled securely.

Dynamic Content Rendering:

Check for any functionality that dynamically renders content based on user input, especially in template-based rendering systems.

Manual Testing:

Inject template expressions (e.g., {{7*7}} in Jinja2 or ${7*7} in Velocity) into inputs that appear to be used in template rendering. Observe the output to see if the expression is executed.

Automated Tools:

Use tools like Burp Suite with SSTI-specific payloads or custom scripts to automate the discovery process. These tools can inject common SSTI payloads and detect if the response indicates template execution.

Error Messages:

Look for error messages that indicate the template engine’s presence, such as syntax errors in the response. These can provide clues about the type of template engine in use.

Basic Payloads:

Mathematical Operations: Inject basic operations like {{7*7}} to check if the engine evaluates the expression and returns 49.String Manipulation: Inject expressions that manipulate strings, like {{'test'.upper()}}, to see if the engine processes it.

Advanced Payloads:

Arbitrary Code Execution: Exploit the template engine to execute code on the server, such as {{''.class.mro()[1].__subclasses__()[40]('/bin/sh',shell=True,stdout=-1).communicate()}} in Jinja2.File System Access: Use SSTI to read or write files on the server, potentially leading to information disclosure or server compromise.

Filter Evasion:

Bypass basic input filters by using alternative syntax or encoding techniques that the application’s input validation doesn’t catch.

Contextual Injection:

Inject payloads within different contexts, such as within nested templates or within loops, to bypass simple sanitization techniques.

Logic Injection:

Exploit logical expressions within templates to manipulate the flow of execution, such as injecting conditions or loops to perform complex attacks.

POC 1: Simple SSTI Execution

A. Description:

Demonstrate a basic SSTI by injecting a simple mathematical operation and observing the server’s response.

B. Steps to Reproduce:

Identify an input field or endpoint where user input is used within a template.Inject {{7*7}} or equivalent syntax based on the templating engine.Observe if the response includes 49, indicating successful SSTI.

C. Impact:

Demonstrates the ability to inject and execute arbitrary expressions within the server-side template.

POC 2: Arbitrary Code Execution via SSTI

A. Description:

Exploit SSTI to achieve arbitrary code execution on the server, demonstrating the severity of the vulnerability.

B. Steps to Reproduce:

Inject a payload designed to execute server-side code, such as spawning a shell or reading a file.Example for Jinja2: {{''.class.mro()[1].__subclasses__()[40]('/bin/sh',shell=True,stdout=-1).communicate()}}.Verify the impact by observing the server’s response or the effect of the code execution.

C. Impact:

Full compromise of the server, leading to unauthorized access, data exfiltration, or system control.

POC 3: File Access via SSTI

A. Description:

Use SSTI to read sensitive files on the server, potentially leading to information disclosure.

B. Steps to Reproduce:

Inject a payload designed to read a file, such as {{config.from_pyfile('/etc/passwd')}}.Observe the response to see if the file’s contents are returned.

C. Impact:

Exposure of sensitive information, which could lead to further exploitation or targeted attacks.

Mitigation Strategies:

Input Validation: Strictly validate and sanitize all user inputs before embedding them into templates, ensuring that no malicious content can be injected.Use Safe Template Engines: Prefer template engines that offer safe rendering options, such as escaping user input by default.Content Security Policies: Implement strong Content Security Policies (CSP) to reduce the impact of successful SSTI exploitation.Regular Audits: Conduct regular security audits and code reviews focusing on template rendering logic to identify and mitigate potential SSTI vulnerabilities.

Final Thoughts: Server-Side Template Injection is a powerful attack vector that can lead to significant security breaches if not properly managed. Understanding how these vulnerabilities work and implementing strong defenses is critical for securing your web applications (OWASP, PortSwigger).