Template injection is a vulnerability found in web applications that use templates to generate dynamic content. This vulnerability arises when user-supplied input is improperly validated and injected into a template, allowing attackers to insert malicious code. The injected code is then executed by the server-side rendering engine, potentially compromising the application.

Templates are commonly used in web development to separate presentation logic from business logic, making it easier to manage and update web content. However, if the input is not properly sanitized, it can lead to template injection vulnerabilities. These vulnerabilities can affect various template engines or frameworks, with specific exploitation techniques varying by the targeted system.

How does Template Injection Work?

Template injection works by exploiting the way template engines process user input. When a web application uses a template engine to render dynamic content, it often incorporates user-supplied data into the template. If this input is not properly sanitized, an attacker can inject malicious code into the template.

The injected code is then interpreted and executed by the server-side rendering engine. This occurs because the template engine mistakenly treats the malicious input as part of the template syntax. As a result, the attacker can manipulate the template engine to execute arbitrary commands or access sensitive information.

In essence, the vulnerability arises from the template engine's ability to execute code embedded within the template. By carefully crafting the input, attackers can exploit this feature to gain control over the server or extract confidential data, depending on the capabilities of the template engine in use.

What are Examples of Template Injection?

Examples of template injection can be found in various web applications and frameworks. For instance, in a Vue.js application, a client-side template injection (CSTI) vulnerability might occur if user input is directly concatenated into the template without proper validation. An example payload like {{8*8}} could be used to test the vulnerability, rendering as Hello 64 if the application is susceptible.

Server-side template injection (SSTI) is another common scenario, often seen in applications using template engines like Jinja2 in Flask. A typical example involves injecting a payload such as {{8*8}} into a vulnerable template, which would also render as Hello 64. More advanced payloads can exploit the vulnerability to execute arbitrary code, such as listing directory contents or even taking control of the server.

What are the Potential Risks of Template Injection?

Template injection vulnerabilities pose significant risks to web applications. Here are some of the potential dangers:

• Remote Code Execution: Attackers can execute arbitrary code on the server, potentially gaining full control over the application and underlying infrastructure.

• Unauthorized Data Access: Sensitive information, such as database credentials and user data, can be exposed, leading to data breaches.

• System Compromise: The entire system can be compromised, allowing attackers to manipulate or corrupt data and files.

• Service Disruption: Exploiting these vulnerabilities can lead to denial of service, disrupting the availability of the application.

• Reputation Damage: Successful attacks can harm the organization's reputation, leading to loss of customer trust and potential financial losses.

How can you Protect Against Template Injection?.

Protecting against template injection requires a multi-faceted approach. Here are some key strategies:

• Validate User Input: Always validate and sanitize user inputs before incorporating them into templates to ensure they do not contain malicious code.

• Use Logic-less Template Engines: Opt for template engines that do not allow code execution, such as Mustache, to minimize the risk of injection.

• Implement Access Controls: Configure strict access controls to ensure only authorized users can interact with sensitive parts of the application.

• Regular Security Audits: Conduct frequent security audits and code reviews to identify and mitigate potential vulnerabilities in your templates.

• Employ Security Libraries: Utilize security libraries and frameworks that offer built-in protections against template injection attacks.