Can a locked gnu screen session terminal be accessed?

Solution 1:

Nope, you're safe.

If I had to guess about what you found confusing about the man-page, I'd say it was the warning-part. What they are trying to say is that it isn't the screen-session itself that is locked, but that instance on that specific terminal. If you have left shells where the attacker can start screen as your user, they can get access to what you have running in the locked screen.

I'm not sure why this is a scenario though. If it's a server, why don't you just login using SSH? If it's a desktop machine - why don't you run xlock instead?

Solution 2:

To my knowledge they cannot use your creds to get into the system.

They could use ALT-F2, ALT-F3 and so on to get command shells but would have to authenticate...

If they don't reboot the machine then I don't know anyway to compromise the system though the GNU or Lock screen....

If they reboot they can put the machine into single user mode and compromise the system..but AFAIK based on your senerio above you should be ok.