The union was contacted by the police on November 29 regarding the suspected trespassing.
"An initial investigation indicates that the hacker network behind the attack has retrieved a significant amount of data from our servers, therefore we consider our members' personal data to have been leaked," writes the union for the forestry, wood and graphic industries.
Members are urged to exercise extra caution in contacts made via, for example, text messages, emails, telephone or social media.
"Right now, there is an active police investigation underway, with which we are fully cooperating," says Julius Petzäll Mendonca, communications manager for the GS union.
