Security of what exactly? The protocol & exchange/transfer process is secure, Mt.Gox's website had a security flaw which was known about but not fixed, other exchanges have long since fixed the issue. All in all it's pretty safe, so long as you yourself secure yourself - e.g. make sure your computer with your wallet has antivirus, make sure you protect it with a STRONG password or encrypt the wallet file, make sure it's backed up so if your computer dies you don't lose your wallet/bitcoins etc
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