One of the most dangerous misconceptions in application security is the belief that file:// is safe because it only accesses local files. This is dangerously incomplete reasoning.

So, 3A-2F-2F-2F decodes to :/:/// .

This specific syntax represents the instruction to use the command-line tool to request or read a resource using the local file system schema ( file:/// ) rather than a remote network protocol like HTTP or FTP.

# View the contents of the local hosts file curl file:///etc/hosts Use code with caution. 2. Windows Systems

While curl is primarily known for network transfers (HTTP, FTP, etc.), its support for the FILE protocol is a powerful, though often overlooked, feature that carries significant security implications. Understanding the file:/// Protocol in curl

# NEVER do this without sanitization curl "file:///$USER_SUPPLIED_PATH"

The file:/// scheme is used to access local files on the system. When you use curl with the file:/// scheme, you're essentially telling curl to read from a local file instead of making a network request.

file:///path/to/your/file

So: curl-url-file-3A-2F-2F-2F → curl-url-file-:///

The string contains URL encoding (also known as Percent-encoding). Web servers and browsers use this format to transmit special characters that might otherwise be interpreted as command syntax.

To download a file using curl , you can use the following command:

: If a URL is improperly formatted or contains illegal characters, typically returns Exit Code 3 ("URL using bad/illegal format"). Stack Overflow Example Scenarios Fetching a local file curl file:///etc/passwd

When you see curl file-3A-2F-2F-2F/path/to/file , it is functionally identical to running curl file:///path/to/file . This syntax is often used when constructing URLs dynamically in scripts where slashes or colons might be misinterpreted by the shell or the application handling the URL string. How to Use cURL for Local Files

: If using libcurl bindings in languages like PHP or Python, explicitly disable local file loading options within your network request configuration. Share public link

The issue has been reported through official security channels. One disclosure noted that when processing URLs containing ../ sequences through the file:// protocol handler, cURL fails to validate the path before opening the file, leading to unauthorized access to arbitrary files on the system.