Sarah Brown, wife of former PM Gordon Brown, was one of those affected Twitter has patched a flaw in its website that was being exploited to pump out pop-up messages and links to porn sites.
Initially, users only had to move their mouse over a message containing a link – not click it – to open it in the browser.
The code was spread by worms, self-replicating, malicious pieces of code.
Thousands of users were caught out by the flaw, including Sarah Brown, the wife of the UK’s former Prime Minister.
“The exploit is fully patched,” Twitter said on its status blog.
People using third-party Twitter software – such as Tweetdeck – were unaffected by the problem.
‘No regrets’
The code exploited what is known as a cross-site scripting (XSS) vulnerability, a flaw in a website that can be exploited by relatively simple code.
In the case of the most recent flaw, the command – written in a programming language called Javascript – automatically directed users to another website, some of which contained pornography.
The malicious links looked like a random URL and contained the code “onmouseover”, which triggered when the cursor hovered over the link.
“There is no legitimate reason to tweet Javascript,” Graham Cluley, a researcher at security firm Sophos, told BBC News.
The first message to contain the code seemed to have been sent by a developer called Magnus Holm.
“I wrote the first worm that has been spreading,” he told BBC News.
“It was only a matter of time before more serious worms started”
Magnus Holm
“I simply wanted to exploit the hole without doing any ‘real’ harm,” he said. “It started off as ‘ha, no way this is going to work’.”
He said the flaw had been identified by others and had already been used for other means.
“There were several other tiny hacks using the exploit – I only created the worm,” he said.
So far, said Mr Holm, he had seen his worm passed around in at least 200,000 messages.
But he warned there were now several other variants spreading that used “other nasty or smart tricks”.
“It was only a matter of time before more serious worms started.”
However, he said he had no regrets and was “not sure” whether he would receive a call from Twitter.
It is not the first time the service has suffered an attack.
In April 2009, another worm spread links to a rival site, again showing unwanted messages on infected user accounts.
Mr Cluley said that Twitter needs “much tighter control” over what users can contain in a tweet to prevent similar problems in the future.
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