Twitter accounts belonging to dozens of high-profile celebrities - including Barack Obama and Britney Spears - have been compromised and defaced with offensive messages.
The company believes the attacks to be the result of a single hacker - possibly a disgruntled former employee.
The hacker targeted the accounts of at least 33 high-profile users in the latest attack on the social networking site.
Twitter says it has identified and blocked the cause of the hacks, and is working to restore the compromised accounts. It has warned all users to change their passwords.
Security consultancy Sophos says the hacker used internal tools used by Twitter's technical support team to reset locked-out member accounts.
According to Sophos senior technology consultant Graham Cluley, the attacks are more serious than if the compromised account-holders had fallen for a simple phishing attack.
"It appears that Twitter's systems were potentially exposing everybody's account to the danger of being taken over by hackers - this breach could actually have been much more serious and affected many more of Twitter's users,' he said.
Twitter responded by "immediately [taking] the support tools offline," Twitter co-founder Biz Stone has announced in a statement. "We'll put them back only when they're safe and secure," he added.
Twitter has also recently been the target of a sustained phishing attack. Targeted users have received direct messages - or "tweet" - purporting to link to a website or blog mentioning the user. In reality, the link links to a duplicate of the Twitter login page designed to steal usernames and passwords. The phishers have also launched a similar attack targeting Facebook users.