Twitter did not return messages seeking comment.

It is not clear whether Twitter is filtering the malicious domains itself or using some other services such as Google's Safe Browsing API (application programming interface). "This is a common problem with this sort of filtering service," said Chris Boyd, director of malware research with FaceTime Security Labs.

Boyd said it will probably take Twitter "a while" to get its Web filtering up and running properly, "but even some protection is better than none."