Template Injection

Server-Side Template Injection (SSTI) is a critical web application vulnerability that occurs when user input is embedded directly into server-side template engines without proper sanitization. Unlike client-side injection attacks, SSTI allows attackers to execute arbitrary code on the web server itself, making it one of the most dangerous vulnerability classes in modern web applications.

How Server-Side Template Injection Works

Web applications commonly use template engines like Jinja2, Twig, Freemarker, and Velocity to dynamically generate HTML pages. These engines process template syntax - special expressions enclosed in delimiters like {{ }} or <% %> - and replace them with computed values before sending the page to the user. When developers insert user-controlled input directly into a template string rather than passing it as a data parameter, the template engine interprets that input as executable code.

For example, if a support ticket system passes a customer's description through a Jinja2 template without sanitization, an attacker could submit template expressions like {{7*7}} to test for SSTI. If the application returns 49 instead of the literal string, the template engine is processing the input as code. From there, attackers can escalate to reading files, accessing environment variables, and achieving full remote code execution on the server.

Real-World Impact of SSTI

Server-Side Template Injection vulnerabilities have been discovered in major platforms and frameworks. Notable real-world cases include vulnerabilities in Uber's Jinja2-based systems, Shopify's Liquid templates, and numerous enterprise applications. The impact is severe because SSTI often leads to complete server compromise - attackers can read sensitive configuration files, access databases, pivot to internal networks, and establish persistent backdoors.

Prevention and Mitigation

The most effective defense against SSTI is to never concatenate user input into template strings. Instead, pass user data as context variables to the template engine's render function. Additional protections include using sandboxed template environments, implementing strict input validation, and deploying Web Application Firewalls (WAFs) with SSTI detection rules. Regular security assessments that specifically test for template injection are essential for organizations using template-based rendering.