A website has been found displaying the phone numbers and passport details of more than 1.5 million MTS users.
A resident of Ufa stumbled upon the site when it appeared as a spam link on the Russian social networking site V Kontakte.
He found his old phone number and the current numbers of several of his friends on the database.
The Federal Services have been alerted and are now trying to shut down the website.
The leaked details belong to MTS users in St. Petersburg and Bashkiriya. The leak at MTS, which has 70 million customers in Russia, took place in 2006.
This is not the first time that data held by Russian mobile phone operators has ended up online. This past July, thousands of personal SMS text messages became publicly available on Russia’s top search engine.
By entering a particular phone number, one could see the full texts of personal text messages and the phone numbers of recipients, as well as the date they were sent.
Some confidential banking account information also became available, making many of the operator’s clients vulnerable to fraud.