Server Side Template Injection (SSTI) is a web vulnerability that occurs when a web application allows user input to be inserted into a server-side template engine without proper validation. Template engines are commonly used to generate dynamic HTML pages by combining data with predefined templates. When SSTI happens, attackers may inject malicious template code that gets executed by the server. This vulnerability can lead to serious consequences such as data leakage or remote code execution. SSTI is often found in applications using template engines like Jinja2, Twig, Velocity, or Freemarker. Because it affects server-side logic, SSTI is considered a high-impact security risk.
SSTI works by abusing the syntax of template engines to execute unexpected commands. Instead of treating input as plain text, the server processes it as template expressions. For example, an attacker may inject special characters like {{ }} or ${ } depending on the template engine. If the server renders this input directly, the attacker can access internal variables or system functions. In advanced cases, SSTI can allow attackers to run OS-level commands on the server. This makes SSTI one of the most dangerous vulnerabilities in web application security.
Using SSTI in security testing involves checking whether user-controlled input is being rendered by the template engine. Testers usually insert harmless payloads such as {{7*7}} to see if the output becomes 49. If the output changes, it confirms that the input is being executed as template code. After confirmation, testers identify which template engine is being used by testing different payload styles. Tools like Burp Suite can help intercept requests and quickly modify parameters for testing. Proper testing should always be done only in authorized environments to avoid illegal exploitation.
The main function of SSTI knowledge in cybersecurity is to help security professionals detect and prevent template-based vulnerabilities. SSTI testing is often part of penetration testing, bug bounty research, and web application audits. It helps reveal weaknesses in how applications handle dynamic content generation. Developers can prevent SSTI by avoiding direct template rendering of user input. Input validation, escaping, and using safe template functions reduce the risk significantly. Security teams also implement WAF rules and secure coding practices to block suspicious template payloads.
Overall, SSTI is a critical vulnerability because it can bypass normal security controls and attack the server directly. Unlike client-side attacks, SSTI happens within the backend environment where sensitive data is stored. This makes it highly valuable for attackers seeking database credentials, system secrets, or server access. Security teams treat SSTI findings as high priority due to the possibility of full server compromise. Regular code reviews and penetration testing help detect SSTI early. Understanding SSTI is essential for building secure modern web applications.
Common Payloads Below
- {{77}}
- ${77}
- <%= 77 %>
- ${{77}}
- {7*7}
