washington democratic senator mark warner monday ask u.s. securities exchange commission investigate whether yahoo yhoo.o senior executives fulfil obligations inform investors public hack attack affect 500 million user account disclosure foundation federal securities laws public company require disclose material events shareholders know warner say letter sec chairwoman mary jo white yahoo face point question exactly know 2014 cyber attack announce last week expose email credentials half billion account critical issue company seek prevent breach affect pending takeover core business verizon inc vz.n warner also ask sec probe whether yahoo make complete accurate representations security information technology systems agency evaluate current thresholds company need report material data breach although sec longstanding guidance publicly trade company report hack incidents company experience know breach often omit detail regulatory file accord 2012 reuters investigation reut.rs/2dblx5s sept. regulatory file sec yahoo state knowledge incidents third party claim allege ... unauthorized access personal data customers could material adverse effect verizon acquisition establish yahoo liable damage sec rule pretty high bar data breach case say robert cattanach lawyer dorsey whitney specialize cyber security yahoo additionally protect liability give relative lack sensitivity data compromise cattanach say though say sec federal trade commission likely open investigations least one state massachusetts also seek information yahoo breach spokesperson state attorney general tell reuters monday yahoo far provide clear detail timeline make aware breach announce thursday cyber security service firm stroz friedberg hire yahoo help investigate breach firm spokeswoman carolyn vadino say fbi also investigate hack yahoo blame state-sponsored actor although company provide technical information support claim u.s. senator seek sec probe yahoo disclosure hack