The iPhone 5S, which went on sale Friday, includes a fingerprint sensor that lets users tap the phone's home button to unlock their phone, rather than enter a four-digit passcode.
But Franken said that the fingerprint system could be potentially disastrous for users if someone does eventually hack it. While a password can be kept a secret and changed if it's hacked, he said, fingerprints are permanent and are left on everything a person touches, making them far from a secret.
"Let me put it this way: if hackers get a hold of your thumbprint, they could use it to identify and impersonate you for the rest of your life," the Minnesota Democrat said in a letter to Apple CEO Tim Cook.
Apple Inc. officials didn't immediately return an email seeking comment on Franken's letter.
But the Cupertino, Calif., company has said that this kind of technology significantly boosts security for users.
According to Apple, the fingerprint data is stored on the phone in a place that's inaccessible to other apps and to Apple's remote servers. In addition, Apple says it's not possible to convert a fingerprint from a police file into something the phone will recognize, as the sensor reads a sub-epidermal layer of the finger.
Meanwhile, anyone worried about fingerprint scan has the option of disabling the feature and sticking with the passcode.
Follow Bree Fowler on Twitter at https://twitter.com/APBreeFowler
Copyright 2013 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.