WASHINGTON (Reuters) - Yahoo's disclosure that hackers stole user data from at least 500 million accounts in 2014 has highlighted shortcomings in U.S. rules on when cyber attacks must be revealed and their enforcement.<div><a href="http://feeds.reuters.com/~ff/reuters/technologyNews?a=hxFHfLFTjAo:eDSAUmFysys:yIl2AUoC8zA"><img src="http://feeds.feedburner.com/~ff/reuters/technologyNews?d=yIl2AUoC8zA" border="0" /></a> <a href="http://feeds.reuters.com/~ff/reuters/technologyNews?a=hxFHfLFTjAo:eDSAUmFysys:V_sGLiPBpWU"><img src="http://feeds.feedburner.com/~ff/reuters/technologyNews?i=hxFHfLFTjAo:eDSAUmFysys:V_sGLiPBpWU" border="0" /></a> <a href="http://feeds.reuters.com/~ff/reuters/technologyNews?a=hxFHfLFTjAo:eDSAUmFysys:-BTjWOF_DHI"><img src="http://feeds.feedburner.com/~ff/reuters/technologyNews?i=hxFHfLFTjAo:eDSAUmFysys:-BTjWOF_DHI" border="0" /></a></div><img src="http://feeds.feedburner.com/~r/reuters/technologyNews/~4/hxFHfLFTjAo" height="1" width="1" alt="" />