Las Vegas ( CNN ) -- 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 . Nohl was scheduled to demonstrate his SIM card hack Wednesday at Black Hat , a computer-security conference in Las Vegas . Instead , he announced that five wireless carriers had rushed to push out updates that patched the problem . Because of the fix , he was only able to demonstrate parts of the hack . Nohl declined to name the carriers involved . Physically replacing millions of compromised SIM cards around the world would have been a huge and costly undertaking for carriers -- and a security concern for phone owners . Instead , the companies came up with a more creative solution : They took advantage of the same Java vulnerability Nohl found and used it to hack into their own cards and rewrite parts of their operating systems . Nohl praised the carriers ' quick action . `` They 're adopting hacking methods to make it more secure , '' he said at a press conference ahead of his talk . `` Abusing the Java vulnerabilities to update the card is the neatest outcome of this . '' With access to a phone 's SIM card , criminals could carry out a range of attacks . They could run up charges on a phone bill , intercept phone calls , remotely control phones , track the location of devices and even access financial information . The bug could be especially damaging in places such as Africa , where many people use their phones not just for communication but as mobile banking systems . A quick fix from a company is the ideal outcome for so-called `` white hat '' hackers such as Nohl , who spend their time finding possible weaknesses in computer systems before criminals can exploit them . Every year at Black Hat , researchers present their latest findings and hacks . The hackers typically receive varying reactions from companies after exposing their vulnerabilities . While some companies do acknowledge issues and respond with a quick fix , others wait until criminals exploit a flaw before taking action . Nohl said Wednesday that companies still have not fixed most of his other computer-bug findings from the past four years .
Expert : Wireless carriers fix a bug that could have allowed criminals to hack into cell phones . A German cryptographer who hacked into SIM cards says he discovered the bug . SIM cards can identify a phone 's owner and store some sensitive personal data . The `` white hat '' hacker presented his findings at a computer-security conference .