SK Telecom shares plunged on Monday amid growing concern about potential damage from a hacking incident that exposed users’ SIM card information. 

According to the Korea Exchange, SK Telecom shares fell six-point-75 percent from the previous trading day to close at 53-thousand-900 won, or around 37 U.S. dollars and 32 cents.

On Tuesday last week, the nation’s largest mobile carrier said it verified the data breach on April 18 after a malware attack.

Meanwhile, shares of SK Telecom rivals LG Uplus and KT rose three-point-75 and one-point-79 percent, respectively, with some one-thousand-600 SK Telecom subscribers changing carriers on Saturday, about eight times the expected number.

Shares of information security firms and SIM makers surged by up to 30 percent.