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Mobile phone networks could be swamped by text messages to phones in a denial of service attack by hackers, academics warn. A paper by Enck, Traynor, McDaniel, and La Porta of Pennsylvania State University explains that if too many text messages are sent to phones in the the same cell of a mobile network at the same time the cell's control channel might be monopolised, preventing new calls from being initiated.

The academics suggest it might be possible to deny voice service to cities the size of Washington with "little more than a cable modem" by sending hundreds of SMS messages a second from a broadband connected PC.

"Moreover, attacks targeting the entire United States are feasible with resources available to medium-sized zombie networks," the Exploiting Open Functionality in SMS-Capable Cellular Networks paper (PDF) states.

The Penn researchers suggest that reconnaissance techniques can be used to draw up a "hit list" of numbers needed to target attacks. They suggest a wealth of data on cell phone numbers and geographical location is available for harvesting on the web, though help from an insider would seem to be an easier approach.

The closest thing we have to a DDoS event on mobile phone in the UK comes on New Year's Eve. Last year 111m texts were sent between midnight on 31 December and midnight on 1 January 2004 - nearly twice the daily average for 2003 and eight per cent up on the year before. Texts took longer to get through at such times but there's only limited anecdotal evidence (based on limited tests conducted on New Year's Eve 2002/2003) that any texts are lost. US tests by Web performance monitoring outfit Keynote Systems in 2003 suggest that one in 12 (7.5 per cent) text messages, originated by email, are either tardy or lost on their way to US mobile subscribers.

Network capacity has increased since then and has moved on by leaps and bounds since the millennium when sheer weight of numbers made it virtually impossible to make calls in central London when 2000 dawned. That took 10,000s of attempted connections from an area around the river of around a square mile so the Penn researchers claim the a mere hundreds of messages a second are enough to degrade performance within a small cell are open to question.

Security experts are also doubtful about the feasibility of the attack. According to Mikko Hyppönen, chief research officer at F-Secure, hackers would find it difficult to get a list of mobile phones in a particular cell necessary to conduct an attack. But Graham Cluley, senior technology consultant at Sophos, said the attack is consuming the resources it needs to succeed. Beyond a certain point, SMS messages would not go through and congestion would not get any worse, he said.

Speaking informally at the Virus Bulletin conference in Dublin last week, other security researchers said it was likely that service providers had protective measures in place to thwart this type of attack. ®

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