Internet security firm Trend Micro warned on Friday that cyber attackers have attempted to infiltrate Web-based email services run by Microsoft and Yahoo! as well as Google.
As US federal agents investigated a Gmail spying campaign uncovered by Google, Trend Micro said that Hotmail and Yahoo! Mail have been similarly targeted.
"There has been a variety of recent attacks on popular Webmail platforms," Trend Micro senior threat researcher Nart Villeneuve said in an online post.
"In addition to Gmail, Hotmail and Yahoo! Mail have also been targeted. While the attacks appear to have been separately conducted, these have some significant similarities," Villeneuve said.
Google said Wednesday that a cyber spying campaign originating in China had targeted Gmail accounts of senior US officials, military personnel, journalists, Chinese political activists, and officials in several Asian countries, mainly in South Korea.
"We recently uncovered a campaign to collect user passwords, likely through phishing," Google security team engineering director Eric Grosse said in a blog post.
"The goal of this effort seems to have been to monitor the contents of these users' emails, with the perpetrators apparently using stolen passwords," he said.
The campaign appeared to originate in Jinan, capital of the eastern Chinese province of Shandong, Grosse said, and targeted the personal Gmail accounts of hundreds of users of Google's free Web-based email service.
Attacks on Hotmail and Yahoo! Mail also appeared aimed at being able to secretly read messages and possibly find ways into other people's accounts, according to Trend Micro.
Along with monitoring accounts, attackers appeared intent on mining computers to find out what kind of software was used.
"Once the attackers know what software are installed on a target's computer, including antivirus products, they can craft a precise attack targeting any vulnerable software," Villeneuve said. "Such an attack will then have a high probability of success."
Explore further: Digital dilemma: How will US respond to Sony hack?