Mainstreet Credit Union Data Breach Impacts 4,123 Members

Mainstreet Credit Union, a member-owned credit union based in Lenexa, Kansas, disclosed a data breach that affected 4,123 individuals in the United States.

On Jan. 19, 2026, Mainstreet Credit Union received an alert from its Managed Security Operations Center that identified suspicious activity related to an employee's email account. The credit union initiated its incident response procedure and secured the compromised account.

On Jan. 22, 2026, Mainstreet Credit Union engaged the law firm Baker & Hostetler LLP to provide legal advice about the incident. Baker & Hostetler then hired CRA, a forensic investigation firm, to review evidence from the credit union's email environment and determine whether personal information had been accessed.

The investigation confirmed that an unauthorized actor accessed one Microsoft 365 email account between Jan. 12 and Jan. 20, 2026. However, the credit union was unable to determine which specific emails and attachments the threat actor viewed during that time, according to the notification.

Because the credit union could not pinpoint exactly which messages were opened, it conducted a thorough review of all emails and attachments stored in the compromised account. The review was completed on April 6, 2026, and revealed that one or more of the emails or attachments contained personal information belonging to affected individuals.

The types of personal information remain publicly unconfirmed at this time but could include Social Security numbers, names, financial account information and driver's license numbers.

The breach was disclosed to the Indiana Attorney General on May 4, 2026. Mainstreet Credit Union began notifying consumers on May 4, 2026.

Mainstreet Credit Union's response to the breach

Mainstreet Credit Union is offering 12 months of complimentary identity monitoring services through Kroll to affected individuals. The monitoring package includes three services: single bureau credit monitoring, fraud consultation and identity theft restoration.

Affected individuals can activate their monitoring services by visiting Kroll's enrollment website and entering the membership number included in their notification letter. Each letter includes a deadline by which recipients must enroll.

The credit union has also set up a dedicated phone line for questions about the incident. Individuals can call 844-403-4628, Monday through Friday, 8:00 a.m. to 5:30 p.m. Central Time, excluding some major U.S. holidays.