Server-Side Template Injection (SSTI)

Server-Side Template Injection (SSTI) is a security vulnerability that occurs when user input is incorrectly handled and inserted into server-side templates, allowing attackers to execute malicious code on the server. Templates are used in web applications to dynamically generate HTML, emails, or other formats by embedding variables and logic directly into template files. Common template engines include Jinja2 (Python), Twig (PHP), and FreeMarker (Java).

When an SSTI vulnerability exists, user-supplied data is injected into a template without proper sanitization, allowing attackers to insert malicious payloads. This can lead to the execution of arbitrary code on the server, potentially resulting in severe security breaches.

Common Scenarios Where SSTI Occurs

  1. Rendering User-Generated Content Without Proper Sanitization:

Many web applications allow users to create or modify content, such as profiles, comments, or posts. If this user-generated content is rendered using a server-side template without proper input sanitization, it can lead to SSTI. For instance, if a user’s comment is directly embedded into a template, any malicious code within the comment can be executed by the server.

  1. Using Templates for Email Generation Based on User Inputs:

Templates are often used to generate dynamic emails, such as confirmation emails, newsletters, or password resets. If the content of these emails includes user inputs that are not sanitized, it can result in SSTI. For example, if a user’s email address or name is included in the email template without sanitization, an attacker could input malicious code, leading to the execution of this code on the server when the email is generated.

  1. Dynamic Report Generation Where Users Can Influence the Output:

Some applications generate dynamic reports based on user inputs. These reports might include various data points, filters, or formatting options specified by the user. If these user inputs are integrated into the report template without proper sanitization, it can lead to SSTI. An attacker could manipulate the input to include malicious code, which the server would then execute when generating the report.

How SSTI Works

Mechanism Behind Template Engines

Template engines allow developers to use templates with placeholders for dynamic content. These placeholders are special markers within the template that are replaced with actual data at runtime. The purpose of using template engines is to separate the logic of data processing from the presentation layer, making it easier to manage and maintain code. For example, in a web application, a template might contain placeholders for a user’s name, profile picture, and other personal details. When a user accesses their profile, the template engine replaces these placeholders with the corresponding data from the user’s account.

How Attackers Exploit SSTI

Attackers exploit SSTI by injecting malicious code into the placeholders within templates. This typically happens when user input is integrated into templates without proper sanitization. The process involves the following steps: The attacker first identifies parts of the application where user input is accepted and directly included in the template. This could be in forms, URL parameters, or any input fields. Then, the attacker crafts a payload that includes malicious code designed to exploit the template engine’s capabilities to execute code. Next, the attacker submits the crafted payload through the identified input points. Since the input is not sanitized, the payload is directly inserted into the template. When the template engine processes the template, it replaces the placeholders with the provided data, including the malicious payload. If the template engine is not secured, it executes the injected code. Finally, the injected code is executed on the server, allowing the attacker to perform unauthorized actions. This could range from stealing data to executing commands on the server, potentially leading to a complete system compromise.

Common Template Engines and Their Vulnerabilities

Overview of Popular Template Engines

  1. Jinja2 (Python): 

Jinja2 is a flexible and powerful template engine used in many Python web frameworks. It allows developers to create dynamic web content with ease by embedding Python-like expressions and logic directly within templates.

  1. Twig (PHP): 

Twig is a widely-used template engine in PHP applications. It offers a clean and readable syntax, making it easy for developers to write and maintain templates. Twig also provides features like template inheritance and automatic escaping, which help in building secure and reusable templates.

  1. FreeMarker (Java): 

FreeMarker is a Java-based template engine used in many enterprise applications. It supports complex data models and provides extensive capabilities for generating dynamic content. FreeMarker is often used in large-scale applications where template flexibility and performance are crucial.

Impacts of SSTI

  1. Potential Damage Caused by SSTI Attacks

SSTI vulnerabilities can lead to severe consequences for web applications and their users:

  • Remote Code Execution: Attackers exploit SSTI to execute arbitrary code on the server. This can lead to complete compromise of the server, allowing attackers to take control, install malware, or disrupt services.
  • Data Theft: Sensitive information stored on the server, such as personal data, financial records, and proprietary business information, can be accessed and stolen by attackers leveraging SSTI vulnerabilities.
  • System Compromise: SSTI can compromise the entire server infrastructure, enabling attackers to escalate privileges, gain persistent access, and pivot to other systems within the network.
  1. Case Studies and Real-World Examples

Real-world incidents highlight the severity of SSTI vulnerabilities:

  • Case Study 1: An e-commerce website suffered a security breach due to SSTI, resulting in the theft of customer data, including payment information and personal details.
  • Case Study 2: A financial institution experienced a security incident where SSTI was exploited to gain access to confidential records and sensitive financial data of clients.
  1. Consequences for Businesses and Users

The consequences of SSTI extend beyond technical impacts to affect business operations and user trust:

  • Financial Losses: Organizations face significant financial damages due to data breaches, downtime for remediation, and potential fines or penalties for non-compliance with data protection regulations.
  • Reputation Damage: SSTI breaches can lead to loss of customer trust, negative publicity, and damage to the organization’s reputation in the market.
  • Legal Consequences: Businesses may face legal actions, regulatory investigations, and penalties for failing to protect user data and adhere to data protection laws such as GDPR, CCPA, and others.

Technical Example

  • Vulnerable Code Example:
from flask import Flask, request, render_template_string

app = Flask(__name__)

def greet():
user_name = request.args.get('name')
template = f"Hello, {user_name}!"
return render_template_string(template)

if __name__ == '__main__':

Explanation: In this example, the user_name parameter from the request is directly inserted into the template string without any sanitization. An attacker can exploit this by injecting malicious code.

Exploitation Example: By accessing the URL http://localhost:5000/greet?name={{7*7}}, an attacker can inject {{7*7}} into the template, resulting in the output Hello, 49!. This demonstrates code execution within the template.

  • Mitigated Code Example:
from flask import Flask, request, render_template_string
import jinja2

app = Flask(__name__)

def greet():
user_name = request.args.get('name', 'Guest')
template = "Hello, {{ user_name }}!"
return render_template_string(template, user_name=jinja2.escape(user_name))

if __name__ == '__main__':

Explanation: In the mitigated example, jinja2.escape is used to sanitize the user input, ensuring that any injected code is treated as plain text rather than executable code


Server-Side Template Injection (SSTI) represents a critical security risk for web applications, enabling attackers to execute malicious code by exploiting input handling within server-side templates. Understanding the mechanics of SSTI, such as how template engines process user-provided data, is fundamental for developers and security teams.

Mitigating SSTI vulnerabilities requires adopting secure coding practices, such as thorough input validation and proper sanitization of user inputs before they are integrated into templates. For example, utilizing functions like jinja2.escape in Python’s Jinja2 engine prevents user inputs from being interpreted as executable code.

Real-world incidents underscore the severe consequences of SSTI, including data breaches, operational disruptions, and reputational damage. These incidents highlight the necessity for organizations to prioritize security measures and stay vigilant against emerging threats.

In conclusion, by implementing proactive security measures and maintaining awareness of SSTI risks, organizations can effectively protect their web applications from exploitation and uphold the integrity of their systems and data.

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