OpenTSDB 2.4.0 unauthenticated command injection

This module exploits an unauthenticated command injection
vulnerability in the yrange parameter in OpenTSDB through
2.4.0 (CVE-2020-35476) in order to achieve unauthenticated
remote code execution as the root user.

The module first attempts to obtain the OpenTSDB version via
the api. If the version is 2.4.0 or lower, the module
performs additional checks to obtain the configured metrics
and aggregators. It then randomly selects one metric and one
aggregator and uses those to instruct the target server to
plot a graph. As part of this request, the yrange parameter is
set to the payload, which will then be executed by the target
if the latter is vulnerable.

This module has been successfully tested against OpenTSDB
version 2.3.0.