This string is a classic indicator of a Path Traversal (or Directory Traversal) attack.

5. Why the encoding format -3A-2F-2F-2F is unusual

Standard URL encoding uses % (e.g., file://file%3A%2F%2F).
The format with hyphens (-3A-2F-2F-2F) suggests:

file:///proc/self/environ: A Linux system file containing the environment variables of the running process.

I notice you're asking about a callback URL that points to a local file path (/proc/self/environ), which contains environment variables of the current process. This pattern raises security concerns, as it resembles:

Log Poisoning Potential: Attackers often target this file because they can sometimes inject malicious code into their own User-Agent string. If the application then includes this file, it can lead to Remote Code Execution (RCE). Recommended Actions