If you are a concerned Yahoo email account holder
The latest reports say that Yahoo lost data for more than one billion users back in August 2013 and that the data is suspected to contain names, email addresses, hashed passwords, security questions and associated answers. In addition, Yahoo has stated that the attackers have accessed Yahoo proprietary code used to generate cookies for user access without credentials.
This breach raises a number of questions, including: Why did it take so long to identify and notify authorities about it? What are the implications for Yahoo users? What might this mean for Yahoo going forward?
Yahoo appears to have been informed by law enforcement that the breach may have occurred, indicating that its internal detective controls have been, and may continue to be, inadequate. This is reinforced by a statement from Bob Lord (Yahoo's CISO) who stated "we have not been able to identify the intrusion associated with this theft." (https://yahoo.tumblr.com/post/154479236569/important-security-information-for-yahoo-users).
Although Yahoo claims that this notification is distinct from the 2014 breach (reported in September 2016), it raises questions as to why this more significant breach was not identified during earlier investigations. Forensic investigations may have been either too focussed on the 2014 breach, or incomplete, preventing identification of this earlier and more significant breach. To add balance to this argument, it should be stated that it is not clear at this time if the breached systems were related, however following the 2014 breach, Yahoo should certainly have considered further investigations to identify if any wider breaches had occurred.
So what are the implications for Yahoo users? Considering that this breach constitutes approximately one-third of Yahoo’s user base, it would be a fair assumption for all Yahoo users that their accounts have been compromised. The data set reported to be compromised includes both username and passwords, and whilst the passwords are reportedly hashed, the weak algorithm in use leaves them wide open to abuse (see our earlier blog post on password hashing - https://www.cyberis.co.uk/2012/06/adding-pinch-of-salt.html)
Cyberis advises Yahoo users, and users of related services such as Flickr and Tumblr, to change their passwords with immediate effect. If you have used your Yahoo password with any other service, you should also change these passwords. If you have registered for a website using a Yahoo email account, you should also consider resetting your password for these services, especially if you haven't used them for some time. Password reset services often use email addresses to manage a password change or forgotten password function. Anyone with access to the breached data could have potentially used this information to access any site associated with your Yahoo email account.
Given that Yahoo has announced that proprietary data was accessed, the breach is currently assumed to extend to Yahoo internal systems. This could suggest a highly skilled and motivated adversary, potentially even a state-sponsored hacking group. Access to millions of email accounts would be a clear motivation to many different threat actors of course, including foreign intelligence services and governments. We fully expect that further information about the extent of the breach will be released in the near future, but in the meantime, it’s certainly not looking good for Yahoo.