NEW YORK (CNN/Money) - Cell phone networks in major American cities are vulnerable to being shut down by a flood of text messages from malicious hackers, according to a published report.

The New York Times reports on a study by two Pennsylvania State University professors, who said that cellular networks are open to the denial-of-service attacks in which computers send so many messages to a target that other machines are blocked from connecting.

Such attacks occur regularly online, and the professors, Patrick McDaniel and Thomas La Porta, said that a hacker with a single computer with a cable modem could succeed in shutting down networks by pushing 165 text messages a second into a network.

Cellular companies told the newspaper that while such attacks are possible, they have developed systems to prevent such efforts from shutting down their networks.

"If you're not prepared, that could happen," Brian Scott, senior manager for wireless messaging operations at Sprint, told the newspaper. "If you are prepared and you have means in place to identify, detect and mitigate that, it's not as much of a concern."

But other outside experts not associated with the Penn State professors or the cell phone providers agreed the networks are at risk.

"Any time a vulnerability in the physical world exists that can be exploited via computer programs running on the Internet, we have a recipe for disaster," said Aviel D. Rubin, technical director of the Information Security Institute at Johns Hopkins. "It is as though those who wish to harm us have a magic switch that can turn off the cellular network."

The text-messaging system, called S.M.S. for short messaging service, is an increasingly important and popular part of the cellular network, especially with teenage users. But the use of such messages to communicate when cellular voice networks are down, such as following the Sept. 11 terrorist attacks here or the recent Gulf Coast hurricanes, makes them increasingly important.

The most direct solution to such attacks -- simply disconnecting the text-messaging services from the Internet gateways -- is not practical, McDaniel told the newspaper. But technologies to limit the messages being inserted into the network could provide some protection, he added.

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