In a statement by the Information Commissioner's Office (ICO) on 4 October 2017, it was announced that Yahoo! has indicated that significantly more people may have been affected by the data breach reported in December 2016 than initially thought and that all user accounts may have been affected by the August 2013 theft. This is understood to include all UK Yahoo! account holders at the time.
Yahoo! is still uncovering additional problems despite the length of time since the breach occurred. The ICO is currently investigating Yahoo! and has advised them to contact all customers affected as soon as possible.