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This morning, I got a spare machine from of of the labmates. The OS is ubuntu 12.04. I could not log into the system because it is under other person‘s account.
Attack
Just enter into recovery mode when I start the computer.
select root.
passwd [username]
Then there is an error information. authentication token manipulation error.
mount -rw -o remount /
passwd [username]
Countermeasure
[hide recovery mode]
vi /etc/default/grub
update-grub
[set root password]
sudo passwd root
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原文地址：http://www.cnblogs.com/yangzhang/p/5240756.html
