The European Commission said that hackers used a spoof European Union website to try and steal information that would allow them to profit from the EU’s cap-and-trade program.
German authorities said hackers stole pollution permits worth €3m on January 28 when they set up a fake site designed to look like a European Commission page.
They sent emails to EU government users, asking them to log on at the site.
The hackers then stole seven usernames and passwords and used them to access the German database with information on permits used in the EU’s carbon trading programme.
Carbon trading allows factories and power stations to make money by selling unused quotas of how much greenhouse gas they can release.
Germany stopped some trading last week and warned users to ignore any emails with a link to the trading database asking them to re-enter their password.
The EU executive said it would upgrade security and that EU governments were investigating whether they had also been attacked.
It claimed that hackers had not been able to access a key EU database with data on actual greenhouse gas emissions that the trading system relies on.