Other user Yahoo user logged in on browser w/ no physical access to computer

[Serpico780]

Hi,

I've got a real stumper here. This last week I've had two unknown yahoo users logged in on my computer. The first time I noticed this a user by the name of michael was logged in when I went to yahoo.ca. I was unable to access the account and was prompted to log in. I contacted yahoo email help and was given a canned reply which was really no help. I logged in today with no concerns, checked my email, and when I went back into yahoo to check my mail again, a user by the name of mariette.foxcroft was signed in. This time I took a screen shot and promptly called yahoo email tech support. I was told that it's impossible for someone else to be logged into their account on my computer without physical access, which is what I've been taught, and have always believed to be dogma. I told them I had not left my computer unattended from the first time I logged into yahoo up to the time this other user was logged in. I was hung up on twice. I've done a malware scan, security essentials is up to date and no problems are evident using OTS security software.

How is this possible and what is it called?? Any thoughts?

Thanks