Improper Neutralization of Server-Side Includes (SSI) Within a Web Page

Server-Side Includes (SSI) are directives placed in HTML pages that are executed on the server before the page is sent to the user's browser. They are useful for including reusable content like headers or footers. However, if an application inserts user input directly into a page that gets parsed for SSI, an attacker can inject directives like `<!--#exec cmd="rm -rf /"-->` or `<!--#include virtual="/etc/passwd"-->` to execute commands, read sensitive files, or otherwise compromise the server. To prevent this, developers must treat all user input as untrusted and implement strict output encoding or validation specifically for SSI contexts. This involves either disabling SSI parsing on pages that handle user input, using a safe templating system that automatically escapes special characters, or employing a positive allowlist of permitted characters that explicitly excludes SSI directive syntax (like `<!--#` and `-->`). Regular input validation for SQL or XSS is not sufficient, as the attack targets a different parsing layer on the server.