Yahoo fixes e-mail service security flaw

Yahoo Inc. has fixed a flaw in its Web-based e-mail service that exposed Yahoo Mail users to serious attacks, including potential interception of personal data, security company Finjan Software Inc. said Wednesday.

Yahoo's mail service contained a so-called malicious script execution flaw, according to Finjan. An attacker could send an e-mail containing malicious script and that code would automatically run once the user opened the message, the company said.

If successful, an attacker could delete files on a victim's computer, steal personal information such as usernames and passwords, credit card numbers and any other information a user inputs on his computer, according to Finjan.

"The potential was huge, you basically could infect millions of Yahoo users within an hour," said Shlomo Touboul, Finjan's chief executive officer. When the malicious script runs, the attacker gains full control of a user's machine, he said.

The script code can be written in various programming languages, including Java, JavaScript, VB Script, Active X and HTML (Hypertext Markup Language), according to Finjan. Yahoo has now taken steps to block most of the scripts in its e-mail service, Touboul said.

Yahoo spokeswoman Mary Osako said the company takes security very seriously and employs rigorous and aggressive measures to help protect users. "Yahoo was informed of an issue in Yahoo Mail on Nov. 11 and quickly implemented a server-side fix which did not require users to take any action. We are unaware of any users who were affected by the issue which no longer affects Yahoo Mail," she said in a statement sent via e-mail.

Yahoo has been in the security spotlight recently. Last week researchers warned of a buffer overrun flaw in the company's instant messaging product that could allow attackers to run their own code on computers running the software.