Tech company Yahoo has warned it had uncovered yet another massive cyber attack, saying data from more than 1 billion user accounts was compromised in August 2013.

The stolen information may have included personal data, hashed passwords and in some cases, encrypted or unencrypted security questions and answers. Yahoo said payment card data and bank account information were not stored in the system believed to be affected.

The number of affected accounts from this new report is double the scope of an attack in 2014 disclosed earlier in September. Yahoo said it believed the incident reported on Wednesday was likely distinct from September's disclosure.