As far as I can tell, there is nothing in the writeup that suggests the file wasn't there from the beginning and present on all Cloud SQL instances. I know this was a standard practice in a few of the products I worked on where we expected the first few layers of security/obfuscation would be peeled off by curious outsiders.
This seems much more likely than OP's exploration being detected in realtime, then someone SSHing into the container manually to put a message.
Disclaimer: I work at Google, not on Cloud SQL, and don't know anything specific about that greetings.txt.
This seems much more likely than OP's exploration being detected in realtime, then someone SSHing into the container manually to put a message.
Disclaimer: I work at Google, not on Cloud SQL, and don't know anything specific about that greetings.txt.