I have an active account at ebay-kleinanzeigen.de (like Craigslist in the US).
Today I got a valid email from them that my email address was changed.
On my mac, I opened up my ebay-kleinanzeigen account in my Chrome browser and was logged in with the hacker's email address.
I understand how the attacker could have cracked the password and change my email address in the account. Once you have access to the ebay-kleinanzeigen account, changing the email address is just two clicks. But how could I had been logged in with his address in my own browser?
I also recently turned on a very old Windows 7 machine and logged into my Google account. Hence the question if the attacker compromised my browser and the login could have been synced across the browsers of the different machines?
What is the most probable scenario and what are my next steps?