singapore/seoul series recent cyber attack net north korean hackers millions dollars virtual currencies like bitcoin attack expect international sanction drive country seek new source cash researchers say north korea government-backed hackers blame rise number cyber attack include so-called wannacry cyber attack cripple hospitals bank company across globe year analysts say explosive growth value bitcoin make cryptocurrencies attractive target north korea become increasingly isolate international sanction impose nuclear weapons missile program bitcoin trade 19,104 per bitcoin one point tuesday less 1,000 begin 2017 accord coinmarketcap.com researchers south korea host world busy virtual currency exchange account 15 25 percent world bitcoin trade give day say attack year exchange like bithumb coinis youbit digital fingerprint hackers north korea researchers find independently verify north korea reject accusations involve hack spokesman south korea unification ministry handle north korean affairs say monday government consider countermeasures include sanction cyber attack representatives bithumb coinis decline comment monday youbit spokeswoman tell reuters company target north korean hackers tuesday company announce suffer another cyber attack cost 17 percent assets force exchange halt operations file bankruptcy hackers behind second attack identify one cyber security researcher say authorize speak matter investigate say similarities youbit hack report tuesday early attack company link north korea another researcher work youbit first hack april say company since experience consistent string attack use malicious code previously use north korea south korea intelligence service report 7.6 billion million worth cryptocurrencies steal previous attack multiple exchange accord south korea chosun ilbo newspaper amount could worth 90 billion korean 82 million moonbeom park researcher korea internet security agency tell reuters malicious code use attack summer virtually identical previous attack connect north korea say attack year begin target company steal customers personal information include name email address park say customers target so-called spearphishing email infect email design look south korea taxation agency korean national tax service say researchers say attackers impersonate official body email tell recipient agency conduct tax investigation user attach document however korean-language file infect trojan horse program would exploit vulnerability hanword korean-language word process software allow hackers remotely control user computer park say attackers would access user bitcoin wallet either computer bitcoin exchange server say researchers say exchange also attack use fake email account cristiana brafman kittner principal analyst cybersecurity firm fireeye say could confirm whether north korea actually steal virtual currencies say hackers link target multiple exchange past six nine months believe criminal activity observe originate north korea result regime look alternative source revenue say north korean cyber threat actors present immediate risk financial service sector worldwide multi-stage cyber attack net north korea millions virtual currencies researchers