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I am running Ubuntu 13.04 with KDE. Today, Software update notification indicated security updates containing updates to libfs6, libx11-6, libx11-data, etc.

When proceed with the updates, it asks for my password and then gives a message:

The following pieces of software cannot be verified. WARNING: Installing unverified software represents a security risk, as the presence of unverifiable software can be a sign of tampering. Do you wish to continue/ cancel

I have, of course, canceled updates. What could be the issue/ how do I set it right? More importantly, is my system hacked/ compromised?

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up vote 3 down vote accepted

I think this means that you have an old copy of Ubuntu's APT keys (used to verify that the packages you're installing are legit); you probably have not been hacked. Does it continue to happen after a sudo apt-get update to get the latest keys?

Here's a basic intro:

Another problem you may encounter if using testing or unstable is that if you have not run apt-get update lately and apt-get install a package, apt might complain that it cannot be authenticated (why does it do this?). apt-get update will fix this.

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Perfect. sudo apt-get update did the trick. – deshmukh Jun 10 '13 at 15:12

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