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I went to get breakfast in the cafeteria and when I came back I noticed my lockscreen hadn't faded, as if it was accessed in the past 5 minutes.

There is the log file snippet for the keyword 'guest':

Ma│May 21 10:24:44 machine-12 groupadd[3518]: group added to /etc/group: name=guest-cA5vc1, GID=126                                                                             │nd
ic│May 21 10:24:44 machine-12 groupadd[3518]: group added to /etc/gshadow: name=guest-cA5vc1                                                                                    │co
ns│May 21 10:24:44 machine-12 groupadd[3518]: new group: name=guest-cA5vc1, GID=126                                                                                             │
Ma│May 21 10:24:44 machine-12 useradd[3522]: new user: name=guest-cA5vc1, UID=115, GID=126, home=/, shell=/bin/bash                                                             │
Ma│May 21 10:24:44 machine-12 usermod[3527]: change user 'guest-cA5vc1' password                                                                                                │
Ma│May 21 10:24:44 machine-12 chage[3532]: changed password expiry for guest-cA5vc1                                                                                             │
Ma│May 21 10:24:44 machine-12 chfn[3535]: changed user 'guest-cA5vc1' information                                                                                               │
Ma│May 21 10:24:44 machine-12 usermod[3543]: change user 'guest-cA5vc1' home from '/' to '/tmp/guest-cA5vc1'                                                                    │
Ma│May 21 10:24:44 machine-12 su[3548]: Successful su for guest-cA5vc1 by root                                                                                                  │
Ma│May 21 10:24:44 machine-12 su[3548]: + ??? root:guest-cA5vc1                                                                                                                 │
Ma│May 21 10:24:44 machine-12 su[3548]: pam_unix(su:session): session opened for user guest-cA5vc1 by (uid=0)                                                                   │
Ma│May 21 10:24:44 machine-12 su[3548]: pam_unix(su:session): session closed for user guest-cA5vc1                                                                              │
Ma│May 21 10:24:44 machine-12 lightdm: pam_unix(lightdm-autologin:session): session opened for user guest-cA5vc1 by (uid=0)

I had never used a guest account before so it appears that the guest account had been created when someone logged into it. Is this the case?

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2 Answers

The log file indicates that someone logged on as guest from the login screen.

The guest account in Ubuntu is created dynamically when someone selects it from the login screen. By default Ubuntu is (or should be) configured so that you cannot access other users' documents or perform any administrative tasks while using the guest account. The log file shows what you always see when you log in as a guest--the account is created dynamically and the login screen runs su to change the user's identity from root (which the login screen runs as) to the limited guest account.

If you want to prevent other people from using the guest account, you should disable guest sessions.

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Yes someone has accessed your computer using a guest account at May 21 10:24:44.

But the question is how did he login back to your account ?

Does your computer auto login without a password ?

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