Dropbox, the file-sharing and synchronization service, has a public folder that is dedicated to sharing content. Dropbox's URL (uniform resource locator) shortening service can be used to create links to content in that public folder.

Spammers have seized on this and are creating shortened links to images stored in the public folder. The images contain a link to online pharmaceutical retailers, Nick Johnston, a senior software engineer at Symantec.

"We saw over 1,200 unique Dropbox URLs being used in spam over a 48-hour period," Johnston wrote. "We have informed Dropbox, providing them with the full list of URLs."

Spammers have often abused URL shortening services. The services are advantageous to spammers since people can't immediately tell where the shortened link will take them.

Twitter introduced its own shortening service, but as a security measure resolves the link to see if it has been reported as malicious. Twitter's URL shortener will also display the target link if users hover the mouse pointer over the shortened link to give users a better idea of where they are going.