I recently signed up on blockchain.info and on the manage addresses section I can see my wallet Extended Public Key (xPub). There is a warning message saying:
You should only give this Extended Public Key (xPub) to those you trust. With this information, they may be able to keep track of your payments, and may be able to disrupt your access to your wallet.
I'd like to know how this works. How could someone keep track of payments that a wallet receives and sends if he knows it's Extended Public Key?