Preventing Server-Side Template Injection (SSTI)

Server-Side Template Injection (SSTI) occurs when untrusted user input is directly embedded into a template and executed on the server, often leading to remote code execution. Securing web applications against this vulnerability requires a combination of secure coding practices, architecture-level defenses, and engine-specific controls. This guide outlines the most effective mechanisms developers use to safeguard applications against SSTI attacks.

Use Logic-Less Template Engines

The most resilient defense against SSTI is adopting a logic-less template engine, such as Mustache or Handlebars (in restricted modes). These engines strictly separate presentation from program logic, preventing templates from invoking system functions, importing libraries, or executing arbitrary executable code. When business logic cannot be embedded in templates, attackers cannot exploit them to execute shell commands or access internal APIs.

Strictly Separate Data and Template Definitions

Developers often introduce SSTI vulnerabilities by concatenating user-supplied strings directly into the template body before compilation. To prevent this:

  • Pass user input exclusively as dynamic context (view-model data), never as part of the template markup itself.
  • Load templates from static, version-controlled files stored on the server rather than compiling ad-hoc templates generated from user requests.

Implement Sandboxed Template Environments

If user-customizable templates are a required business feature, run the template engine within an isolated, sandboxed environment. Many modern template engines offer built-in sandboxing capabilities:

  • Twig (PHP): Provides a sandboxed environment that restricts access to specific tags, filters, and object methods through an explicit allowlist.
  • Jinja2 (Python): Includes jinja2.sandbox.SandboxedEnvironment, which intercepts and blocks attempts to access private attributes (such as __class__ or __subclasses__) commonly used in exploitation chains.

Sandboxing policies must follow a strict default-deny approach, only permitting methods and variables that are strictly necessary for rendering.

Restrict Access to Internal Objects and Methods

Template engines often grant access to underlying system classes and global variables by default. To neutralize SSTI:

  • Remove standard libraries, execution utilities (such as os, exec, or eval), and reflection tools from the template context.
  • Disable template execution extensions that interface directly with the file system or run external processes.
  • Limit context variables strictly to the primitive types or data transfer objects (DTOs) required by the view.

Enforce Strict Input Validation and Sanitization

While input validation should not serve as the primary line of defense against injection, filtering user data adds an essential layer of depth:

  • Apply strict allowlists for user inputs using regular expressions that only allow expected alphanumeric characters.
  • Detect and strip common template engine syntax delimiters, such as {{, }}, ${, }, <%, and %>, if user input must be processed as raw text.

Apply the Principle of Least Privilege

If an SSTI vulnerability is discovered and bypassed, defense-in-depth measures minimize the damage:

  • Run the application under an unprivileged system user account with restricted filesystem access.
  • Deploy applications inside read-only containers or use system sandboxing (like SELinux or AppArmor) to prevent attackers from executing local binaries or persisting access.