Server-Side Template Injection (SSTI) is a critical web vulnerability that arises when an attacker can inject malicious code into a server-side template, leading to unauthorized code execution on the server. This occurs when user inputs are improperly sanitized and directly embedded into templates. Understanding SSTI is essential for developers and security professionals to safeguard web applications.
Understanding SSTI
In web development, templates are used to dynamically generate HTML pages by combining static content with dynamic data. Template engines like Jinja2 (Python), FreeMarker (Java), and Thymeleaf (Java) facilitate this process. However, if user inputs are directly inserted into templates without proper validation, attackers can exploit this by injecting malicious expressions. For instance, in a Jinja2 template, if user input is handled as follows:
output = template.render(name=request.args.get('name'))
An attacker could manipulate the name parameter:
http://vulnerable-website.com/?name={{7*7}}
If the application is vulnerable, the server processes {{7*7}}, and the output becomes 49 or 7777777. This indicates that the template engine is evaluating the injected expression, confirming the presence of SSTI.
Detecting SSTI
To identify SSTI vulnerabilities, testers can inject specific payloads and observe the server's response. Common payloads include:
{{7*7}}${7*7}<%= 7*7 %>
If the server returns 49, it suggests that the template engine is evaluating the expression, indicating a potential SSTI vulnerability. Additionally, analyzing error messages can help identify the specific template engine in use, aiding in crafting more targeted payloads.
Exploiting SSTI
Once an SSTI vulnerability is confirmed, attackers may attempt to execute arbitrary code on the server. For example, an attacker might use a polygot:
${{<%[%'"}}%\
This payload attempts to access the application's configuration items, potentially revealing sensitive information. The specific payloads and their effects depend on the template engine and the application's context. A poligot is payload that is universal and it’s easy to generate a POC.
To prevent SSTI vulnerabilities, developers should:
Avoid Directly Embedding User Inputs: Refrain from inserting user inputs directly into templates.
Implement Input Validation and Sanitization: Ensure that all user inputs are properly validated and sanitized before processing.
Use Context-Aware Escaping: Apply appropriate escaping mechanisms based on the context in which the data is used.
Employ Security Features of Template Engines: Utilize built-in security features provided by template engines to mitigate risks.
By adhering to these practices, developers can significantly reduce the risk of SSTI vulnerabilities in their applications. Understanding its mechanics, detection methods, and preventive measures is crucial for maintaining the security of web applications.
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