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Yahoo! has blamed cross-site scripting security bugs, which it claims to have squashed, for a recent upsurge in webmail account takeovers.

Over the last few days several Reg readers have been in touch to complain that their Yahoo! webmail accounts have been hijacked or to point us towards complaints on various support forums about the issue.

One tipster told us: "Lots of Yahoo! Mail accounts were broken into last week by computers all over the world. It seems a botnet was used to do it. The hackers might have accessed some of the accounts through Apple iPhone's Yahoo! Mail app, as account security logs show that as one of the hack entry points."

BT has a tie-up with Yahoo! and the tipster pointed us towards tales of webmail account hacking woe from the telco's customers, including one from someone who works for a support organisation and another from a computer company in Devon. The latter ruled out a virus infection at their end and suggested a security flaw in the Yahoo! mobile app might be to blame for the problem.

The thread on the BT support forum, running to many pages, and the posts on a broadband support forum both suggest that Yahoo! webmail hijack problems flared up last weekend.

Back in January, Yahoo! said that it had squashed a cross-site scripting (XSS) vulnerability in its webmail service which was blamed for a spate of account hijackings. The compromised accounts were used to send spam.

The vulnerability, which was triggered by tricking users into clicking on a malicious link, appears to be the same bug offered for sale for about $700 on an underground forum in November. Offensive Security warned at the time that Yahoo!'s fix was ineffective, leaving the bug in play.

Meanwhile, last month security firm BitDefender warned of an apparently separate attack aimed at harvesting Yahoo! webmail account cookies that relied in part on a buggy version of WordPress on Yahoo! Developers Blog, malicious JavaScript and cross-site scripting flaws. It's not clear if the two events were connected.

We asked Yahoo! about the recent upsurge in reports of Yahoo! webmail takeovers. In response, it fingered an XSS problem, but did not confirm if it was the same problem it told us it had squashed in January. We'll update you when we have an answer.

Yahoo! added that it is working hard to seek out and restore and hijacked accounts.

The XSS flaws reported to Yahoo! have been fixed and we continue to aggressively investigate reports of any email accounts exhibiting anomalous behaviour. We're committed to protecting our users and their data. We strongly urge our users to change their passwords frequently and to use unique, alphanumeric passwords for each online site they visit.

Separately, another Reg reader, who asked not to be named, complained that his email account archives had been deleted.

"I am a Yahoo! Mail Plus subscriber," our source explained. "I signed up six months ago for a year. This means that I will avoid deactivation for not signing in for four months. However, on March 1st 2013 my account was deactivated, and all my previous emails were deleted without warning."

Posts on a Yahoo! support forum Down Under in New Zealand suggest this incident is far from isolated.

The situation remains confusing. TheNextWeb reports the continuation of Yahoo! webmail account hijacking despite attempts by Yahoo! to plug holes in its systems. The website carries a timeline of the problem, which remains an issue 10 weeks after it was first reported in early January. ®

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