Why Yahoo and Facebook are warning some users about foreign hackers

Yahoo , +0.76% has become the latest big tech company to say it will warn users whose accounts may be the target of hackers from another country. The company said this week that it will send users notifications if "we strongly suspect that you may have been a target of an attack, and want to encourage you to take steps to secure your online presence." Google , +0.04% began issuing such warnings in 2012, and Facebook , -0.83% and Twitter , +0.49% do so as well. Hackers in this case, those linked to another country, such as China or Russia who cannot crack corporate or government agency networks often seek to infiltrate an organization through employees' personal accounts. If the individual uses the same password across accounts, or logs onto work-related portals from home, criminals can approach their targets through those gateways.