The Credit Union, which offers savings schemes and and loans, has contacted its 15,000 members to inform them of the cyber attack, which happened in February but has only recently come to light.
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A Credit Union spokesman said: "Sheffield Credit Union was a victim of a cyber crime incident which is believed to have taken place on February 14, 2018, but only recently came to light, which has resulted in member data being accessed from our servers.
"This incident may expose you to text messaging, cold calling and attempts to defraud."
The organisation stressed that money has not been accessed.
The spokesman added: "We must emphasise that your money in the Credit Union is safe.
"Sheffield Credit Union takes the security of its members' data very seriously and this matter is being dealt with as our highest priority.
"We would like to reassure you that our IT security has been reviewed and increased to protect against future events of this nature and that the relevant statutory authorities have been contacted in respect of this crime."