The hacker, who uses the cyber name "Masato Kinugawa," says he warned Twitter about the loophole back in August, but the company did nothing to resolve the issue.
Through his Twitter account and personal blog, Kinugawa regularly tracks down computer security loopholes and notifies companies of their existence. Earlier this year, he pointed out several scripting problems to Japanese Internet company Livedoor, which thanked him with a $177 gift certificate.
San Francisco-based Twitter said it does not believe that any user information was compromised and that the "vast majority" of the breaches were pranks or promotions. Twitter says it is safe to "Tweet" again.
It is estimated that the Twitter take-down affected 100,000 people worldwide.
The Associated Press contributed to this report.