New security scheme whacks text spammers in hours

A new way to track the source of spam text messages can detect culprits within two hours, helping reduce the illicit traffic that can clog cell towers and disconnect legitimate calls, researchers say.

Called Greystar, the scheme invokes the use of phone numbers assigned to laptops, tablets and the like, which don't typically receive any SMS messages, according to the research to be presented at the 22nd USENIX Security conference next week.

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Since these so-called gray numbers aren't likely to receive legitimate SMS messages, the source of texts sent to them yields likely spammer accounts, say the researchers, who are led by Nan Jiang, a Ph.D. student at the University of Minnesota.

The algorithm developed by the researchers has not been made into a commercial product but could be incorporated in software within carrier networks that taps real-time call data to pinpoint where SMS spam comes from. The carriers could then take steps to shut down the offending traffic.

The algorithm parses call data by considering the source and destination of SMS messages and the volumes sent by any given account as it seeks anomalies that indicate misuse of SMS accounts. The algorithm analyzes traffic patterns only, not the content of messages.

Based on their research that used anonymized call records from a North American carrier, the researchers project that Greystar could detect spam attacks within 1.2 hours after they start. Half the time, the Greystar detects spam a full day or more ahead of the method carriers use today -- relying on phone subscribers to report spam, the researchers say.

In its tests, 15% of the spamming numbers the group found went undetected by the unnamed carrier's search method. In culling five months of records, the researchers came up with more than 34,000 spamming numbers, generating only two false positives.

The researchers say their method can cut half the spam traffic from the network and 75% of SMS spam during peak traffic hours. "In this way, Greystar can greatly benet the cellular carriers by alleviating the load from aggressive SMS spam messages on network resources as well as limiting their adverse impact on legitimate mobile users," they say.

SMS messages travel through cell towers nearest senders and then through towers near recipients. In between they cross the SS7 network that is used for signaling in traditional landline phone call setup. By getting rid of the spam, cell towers have less traffic to handle and so have a lower risk of being overwhelmed. Also, enough SMS spam could affect sap SS7 resources and result in dropped calls, the researchers say.

"For example, we have found the SMS trafc volume at cell towers can easily get multiplied by more than 10 times due to the activities of spammers," the researchers say in their paper on the topic. "Therefore, the focus of the SMS spam defense is to control spam numbers as soon as possible before they reach a large number of victims."

The vast majority of spammers -- 95% - generate more than five SMS messages per hour, and more than 70% send more than 50 per hour. In contrast more than 97% of legitimate messages send fewer than 5 messages per hour, the researchers say.

Spamming trafc volume can be 10 times that of normal traffic, and at certain network junctures can account for 80% to 90% of all SMS traffic, they say.

More than 60% of spammers target phone numbers in the area codes of major cities; the rest spread it around, the researchers say.

Tim Greene covers Microsoft and unified communications for Network World and writes the Mostly Microsoft blog. Reach him at and follow him on Twitter @Tim_Greene.

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