Server Side Includes (SSI) is a technique that allows HTML documents to be dynamically generated on the server side. SSI uses special commands embedded within HTML comments, which are interpreted and executed by the web server before the page is sent to the user's browser.

Functions and Applications of SSI:

1. Including Content: SSI allows content from other files or dynamic sources to be inserted into an HTML page. For example, you can reuse a header or footer across multiple pages by placing it in a separate file and including that file with SSI.

Server Side Includes (SSI) Injection is a security vulnerability that occurs in web applications that use Server Side Includes (SSI). SSI is a technique allowing HTML files to be dynamically generated on the server by embedding special commands within HTML comments. These commands are interpreted and executed by the web server before the page is delivered to the client.

How does SSI Injection work?

In an SSI Injection attack, an attacker injects malicious SSI commands into input fields, URLs, or other mechanisms through which the application accepts user data. If the application does not properly validate and filter these inputs, the injected commands can be executed on the server.

Example of an SSI command:

This command would list the contents of the current directory on a vulnerable server.

Potential impacts of SSI Injection:

• File System Manipulation: Attackers can read, modify, or delete files.
• Remote Code Execution: Execution of arbitrary commands on the server, potentially leading to full system compromise.
• Information Theft: Access to sensitive information, such as configuration files or database contents.
• Denial of Service: Executing commands that crash or overload the server.

Mitigation measures against SSI Injection:

1. Validate and Sanitize Inputs: All user inputs should be thoroughly validated and restricted to acceptable values.
2. Use of Prepared Statements: Where possible, use prepared statements and parameterized queries to minimize the risk of injections.
3. Limit SSI Usage: Avoid using SSI if it is not necessary, to reduce exposure to such vulnerabilities.
4. Leverage Server Security Features: Configure the web server to accept only trusted SSI commands and avoid executing dangerous shell commands.

By implementing these measures, the risk of SSI Injection can be significantly reduced.