Major wireless carriers have fixed a bug that could have allowed criminals to hack into hundreds of millions of cell phones, says a security expert who exposed the flaw. Cryptographer Karsten Nohl of Security Research Labs in Germany says he discovered the bug after spending three years figuring out how to hack SIM cards, those tiny removable plastic cards found in cell phones and other mobile devices. Nohl accessed the SIM cards by exploiting flaws in the encryption keys and sending a hidden SMS text message. SIM cards can identify the phone’s owner and store some sensitive personal data, such as payment information. Nohl’s findings sent ripples through the wireless industry when they were first revealed July 21. While phones are known to be susceptible to a range of security issues and attacks, the old dependable SIM card was considered safe.