Server-Side Template Injection
Description
Server-Side Template Injection (SSTI) is a vulnerability that occurs when user-supplied input is embedded directly into server-side templates without proper sanitization. Template engines are designed to combine static templates with dynamic data, but when attackers can inject template directives or expressions, they can manipulate the template rendering process. This vulnerability allows attackers to inject malicious template syntax that gets executed on the server, potentially leading to remote code execution, data exposure, and complete server compromise.
Remediation
To remediate Server-Side Template Injection vulnerabilities, implement the following measures:
1. Never construct templates from user input
Do not concatenate or embed user-controlled data directly into template strings. Instead, pass user data as parameters to pre-defined templates.
Vulnerable code example:
// VULNERABLE - Don't do this template = "Hello " + user_input render(template)
Secure code example:
// SECURE - Use parameterized templates
template = "Hello {{name}}"
render(template, {name: user_input})2. Use a logic-less template engine
Choose template engines that separate logic from presentation (e.g., Mustache) and avoid engines that allow arbitrary code execution within templates.
3. Implement strict input validation
If template selection must be dynamic, use a whitelist approach to validate template names against a predefined set of allowed values.
4. Apply sandboxing
If dynamic template rendering is absolutely necessary, use a sandboxed environment with restricted access to sensitive objects, functions, and system resources.
5. Apply the principle of least privilege
Run the application with minimal permissions to limit the impact of successful exploitation.