Generic Server Side Template Injection (SSTI) Vulnerability Scanner

Web applications frequently employ server-side template engines such as Jinja2, Twig, Freemarker, or Velocity to dynamically generate HTML and other responses. These engines are widely used across industries for developing dashboards, portals, and APIs that render user data on the server before delivering it to clients. They allow developers to create flexible and maintainable user interfaces, but when improperly implemented, they can become a serious attack surface. The scanner is designed to help security teams, developers, and application owners identify these risks by analyzing endpoints and parameters where templates are rendered. Its primary purpose is to detect unsafe template evaluations and confirm potential injection points during both pre-deployment and production testing phases.

Server Side Template Injection (SSTI) occurs when untrusted user input is embedded in a server-side template and interpreted as executable code. When this happens, attackers can inject template expressions or commands that the server processes, potentially exposing sensitive data or executing arbitrary system commands. The vulnerability often arises from insecure rendering functions or unsafe concatenation of user-supplied data within templates. SSTI can lead to information disclosure, unauthorized access, and remote code execution depending on the template engine’s functionality. Because of its potential to compromise the entire host environment, SSTI is considered a high to critical severity vulnerability.

The scanner works by sending crafted payloads to web application parameters, headers, or request bodies to identify if input is evaluated in a template context. It supports both GET and POST methods and can reuse cookies and headers to simulate authenticated sessions. The scanner analyzes the responses for signs of template evaluation, such as arithmetic results, concatenated strings, or specific template error messages. Once it detects these evaluation indicators, it records the affected endpoint, HTTP method, and parameter, helping pinpoint the exact injection vector. This methodology allows accurate detection of SSTI without causing service disruption or unnecessary noise.

If exploited, an SSTI vulnerability can give attackers direct access to internal variables, environment configurations, and file contents stored on the server. In more severe cases, attackers can achieve arbitrary command execution, manipulate application logic, or fully compromise the server. This could result in the theft of sensitive data, defacement, privilege escalation, or lateral movement within the infrastructure. Such attacks not only disrupt service availability but also cause reputational and financial damage, making proactive detection and remediation essential.

REFERENCES

• https://portswigger.net/web-security/server-side-template-injection
• https://owasp.org/www-project-web-security-testing-guide/v41/4-Web_Application_Security_Testing/07-Input_Validation_Testing/18-Testing_for_Server_Side_Template_Injection