Proxy-url-file-3a-2f-2f-2f Guide

If proxy-url-file:/// is mishandled, an attacker might read local files:
proxy-url-file:///etc/passwd
proxy-url-file:///C:/Windows/win.ini

In Windows or macOS, system proxy configurations may use PAC (Proxy Auto-Configuration) files or WPAD. A malformed PAC file might return a string like proxy-url-file:///... if a developer incorrectly concatenated variables.

To understand the fragment, we must first decode it. The string contains 3A and 2F, which are hexadecimal byte values in Percent-Encoding (also known as URL encoding). proxy-url-file-3A-2F-2F-2F

In standard URL encoding:

Look closely at your string: proxy-url-file-3A-2F-2F-2F If proxy-url-file:/// is mishandled, an attacker might read

Notice there are no percent signs (%) before the 3A and 2F. This is the first sign of corruption. The original data likely looked like this:

Original Intact String: proxy-url-file%3A%2F%2F%2F that becomes: proxy-url-file:///

When decoded, that becomes: proxy-url-file:///

| Scenario | Description | |----------|-------------| | Proxy auto-config (PAC) files | Some tools let you load a PAC file from disk using a custom URI scheme. | | Browser/OS proxy settings | Advanced proxy extensions or debugging tools may log internal URIs like this. | | Network debugging | Tools like Fiddler, Charles Proxy, or mitmproxy might use such a scheme internally. | | Misconfigured software | A broken proxy setting might display this instead of a valid file:// path. |