A new phishing attack hops from one Gmail account to the next by searching through compromised users' previous emails for messages with attachments, then replies them from the compromised account, replacing the link to the attachment with a lookalike that sends you to a fake Google login page (they use some trickery to hide the fake in the location bar); the attackers stand by and if you enter your login/pass, they immediately seize control of your account and attack your friends.
I've long thought that the best phishing attack would look through compromised users' email archives and reply to recent threads with malware attachments and messages like, "Does this explain my point?" or "Have a look at this, it really shows how wrong you were," etc.
Once you complete sign-in, your account has been compromised. A commenter on Hacker News describes in clear terms what they experienced over the holiday break once they signed in to the fake page:
"The attackers log in to your account immediately once they get the credentials, and they use one of your actual attachments, along with one of your actual subject lines, and send it to people in your contact list.
For example, they went into one student's account, pulled an attachment with an athletic team practice schedule, generated the screenshot, and then paired that with a subject line that was tangentially related, and emailed it to the other members of the athletic team."
The attackers signing into your account happens very quickly. It may be automated or they may have a team standing by to process accounts as they are compromised.
Once they have access to your account, the attacker also has full access to all your emails including sent and received at this point and may download the whole lot.
Wide Impact: Highly Effective Gmail Phishing Technique Being Exploited [Mark Maunder/Wordfence]