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He did this last time when I left my laptop at room. Opened the system recovery mode from advanced Ubuntu options and changed the password using 'passwd usrname ' command.Is there anything I can do for this to not happen in future.

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you can kill him. joking, possible duplicated of… – Geo Aug 15 '14 at 17:17
Heh. This is actually a cool trick. It works for Macs too. I have heard about setting root password, but that causes all sorts of problems in a system. ||| @Geo : Not a dupe at all. That is in regards to not allowing password-setting in Multiuser mode (booted). Root can override that restriction, so it wouldn't help here, nor is it a dupe. – Kaz Wolfe Aug 15 '14 at 17:21
@Mew what problems does it cause? – muru Aug 15 '14 at 17:22
@muru Makes it easier to brute-force your system, adds challenges for you, makes hackers think what you're hiding, and causes a few SSH vulnerabilities. (Had a server). Sudo is much safer/easier, but it does allow this feature. – Kaz Wolfe Aug 15 '14 at 17:25
@Mew What need to brute force if there is no password? SSH by default disallows password authentication for root. Adds only one challenge for the user: remember two passwords. – muru Aug 15 '14 at 17:27

For this user case probably the best "solution" is to re-install Ubuntu with full encryption (LUKS). This would secure the data and prevent changing the root/users password(s).

This could be defeated with a few methods, both hardware (key loggers) and software (custom kernels), but both of those options are unlikely. The OP can assess these risks.

With the above suggestion, I agree with the advice that the ultimate solution is to prevent physical access. In general physical access is root access via any number of techniques.

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This is too much effort to deter pranks. – muru Aug 15 '14 at 17:50

Yes. Set the root password. Open a terminal, and do:

sudo passwd

Enter a password. Then the root shell in recovery mode will always ask for this password.

Note that all this is futile as he can boot into a live USB and change your password.

Ultimately, given physical access, the security battle is lost.

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Block LiveCDs? goes into BIOS | Block BIOS? opens computer and gets HDD | Locked-In HDD? unlocks using lockpicking kit and screwdriver | Encrpyed HDD? cat \dev\null > \dev\sdb. Always a way to do SOMETHING with physical access. My HS tech guy and I would have wars over who could do the worst damage with only physical access. – Kaz Wolfe Aug 15 '14 at 17:26
Also, you're forgetting about Hirens Boot CD. It has a tool to bypass passwords now. – Kaz Wolfe Aug 15 '14 at 17:30
@Mew Doesn't prevent anyone from quickly removing the hard disk, attaching elsewhere and doing whatever they want. And adds yet another password for the user to remember. Let me repeat: physical access, the battle is lost. However, for low-grade fun like this roommate, setting a root password is enough, and simplest. – muru Aug 15 '14 at 17:30
I agree, setting a root password as muru advised is worthless and adds no significant additional security. – bodhi.zazen Aug 15 '14 at 17:42
@bodhi.zazen I disagree, since this is enough to deter the casual prank. Anything more is a sledgehammer to kill an ant. While security is important, the context in which this security is setup is also important. Average users don't care. Otherwise we'd all be using full-disk encryption and BIOS passwords and unique passwords for everything, and encrypt all communication. – muru Aug 15 '14 at 17:46

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