For all the convenience of short URLs, some Internet security experts worry that they could be used to camouflage spam and phishing attacks and redirect people to malicious Web sites.

“People have no way to know where they’re going,” said Patrik Runald, chief security advisor at F-Secure Security Labs, a maker of security software. “These services are great and they serve a purpose, but at the same time, there is a darker side.”

And if a shortening site shuts down, any links funneled through it would be lost forever, Mr. Runald said.