According to the explanation of the problem, the authentication of the flag06 account came from a former Unix system.
If you open the /etc/passwd file like the above image, only the flag06 account's password is stored encrypted.
This is characteristic of older Unix systems. Current systems manage passwords at /etc/shadow.
Since the encryption level seems low, install the password cracker tool "John the Ripper"
→ wget http://www.openwall.com/john/j/john-1.8.0.tar.gz
The downloaded file is in home directory. Unpack it and install it in src directory with make command.
→ make clean generic
Execute the john in run directory.
→ ./john /etc/passwd
You can find the password hello.
Log in the account flag06 with the password hello and clear the level with getflag command.
→ su flag06
→ getflag