Thieves who target backpacks usually use one of four methods to gain access to the pack. They stand close and unzip the bag while the owner is distracted or unaware of their presence, or if the owner has set the bag down, they will swipe it, often using an accomplice to distract the owner. If the owner is wearing the backpack, thieves will use a knife or razor to cut the pack and expose the contents or will cut the strap and run off with the bag.
A new generation of backpacks has come to market, designed to defeat or at least discourage thieves and pickpockets from robbing you of your possessions. They use locking pockets and zippers, fabrics that are difficult or impossible cut, and features such as steel cables within the shoulder straps, to make sure any attempt to cut the straps is crushed and your possessions remain yours.
These bags are available from many retailers and you can expect to pay $40 or more, depending on the features and quality. These prices are about half of what they were a year ago as these bags are now flooding the market. But remember, you get what to pay for, so a bag in the $100-$150 range should be a good starting point when shopping for one.