The shows the need for legislation to address how carriers use location information, said Senator Al Franken, a Minnesota Democrat who asked for the study. Franken's would require mobile carriers and app makers to get customer permission before collecting location information and before sharing it with other companies.

"I believe Americans have a fundamental right to privacy: to know what information is being collected about them and to be able to control whether or not that information is shared with third parties," Franken said in a statement. "And this report clearly shows that mobile industry companies often fail to respect that right, giving out consumers' location data without their knowledge or explicit consent."

Mobile customers can benefit from location-based services on smartphones, and mobile companies can use the data to increase advertising revenue and improve service, the GAO report said. And many mobile carriers and app providers have developed privacy policies. But they have "not consistently or clearly disclosed to consumers what the companies are doing with these data or which third parties they may share them with," the report added.

Privacy and industry groups have recommended location privacy practices, but those recommendations aren't consistently followed, the report said.

"We found that there aren't very many rules," Mark Goldstein, the GAO's director of physical infrastructure, said in a . "In many ways, this is still the Wild West of the electronic era. There's a lot of work that still needs to be done."