Maltrail Unauthenticated Command Injection

Maltrail is a malicious traffic detection system, utilizing publicly
available blacklists containing malicious and/or generally suspicious trails.
The Maltrail versions The `subprocess.check_output` function in `mailtrail/core/httpd.py` contains
a command injection vulnerability in the `params.get("username")` parameter.
An attacker can exploit this vulnerability by injecting arbitrary OS commands
into the username parameter. The injected commands will be executed with the
privileges of the running process. This vulnerability can be exploited remotely
without authentication.

Successfully tested against Maltrail versions 0.52 and 0.53.