"Both Google and Yahoo have 30-year cookies. So where's the letter for Yahoo from the Working Group? And isn't 14 years from Microsoft excessive?" Sullivan wrote.
Yahoo and Microsoft both say they take privacy very seriously.
"Microsoft has a long-term commitment to providing customers with control over the collection, use and disclosure of their personal information. While we have not received formal communication from the Article 29 Working Party, we recognize that online search is creating legitimate concerns about privacy and are actively engaged with data protection authorities around the world to ensure that our practices meet the highest standards when it comes to protecting privacy," Microsoft said in a statement.
"Our users' trust is one of Yahoo's most valuable assets. That's why maintaining that trust and protecting our users' privacy is paramount to us. Our data retention practices vary according to the diverse nature of our services," Yahoo said.