Missing ownership check on the shared memory object named by the kitty askpass escape code

Summary

Missing Authorization in the askpass escape code handler in kitty from 0.25.0 before 0.49.0 allows a local user other than the one running the terminal to obtain the text typed into a prompt that kitty itself displays, because handle_remote_askpass() in kitty/window.py opens the POSIX shared memory object named in the escape code, parses a prompt definition out of it, and writes the user's answer back into an object of that same name, without at any point checking that the object is owned by the user running kitty or that its permissions exclude other users. The equivalent consumer of the same SharedMemory class in the ssh kitten performs exactly that check; the askpass path did not. The handler is reached through a device control string processed from the byte stream of the window, so the attacker must also cause bytes of their choosing to be displayed by the victim's terminal. Where the POSIX shared memory namespace is shared between the two users, a second local user can create an object with permissions that allow the victim to read and write it, cause the victim's kitty to render a prompt of the attacker's choosing, including a masked password prompt, and read the typed secret back out of the object afterwards. The prompt text is additionally passed to the display without control character sanitisation, so it can overwrite the warning line kitty prints above it. The answer is written by reopening an object of that name when the user answers, rather than through the handle already held. This results in disclosure of a secret typed by the victim to a second local user, and does not require any privilege on the victim's account.