A representative of the company said the flaws were fixed last month by making changes on the company's Yahoo Mail servers.
"We were alerted of it at the end of May, early June," spokeswoman Mary Osako said. "There ended up being two variations of the issue: One which we could reproduce in a few days and the other which took a lot of effort to reproduce."
The vulnerabilities are of a type known as cross-site scripting flaws, which typically take advantage scripting languages and misconfigured Web servers to launch attacks against a user's computer. The attacks typically redirect the user to another Web site, allow access to the user's cookies or, sometimes, allow the attacker to run code on the victim's computer.
Yahoo fixed the flaws in its server code. No patch is required by the Yahoo Mail users.