MICR lines are the machine-readable characters printed along the bottom of many checks. MICR stands for Magnetic Ink Character Recognition, and the line usually contains the routing number, account number, and check number. Banks and payment processors use it to sort and verify paper checks quickly and accurately.
In cyber security, MICR lines matter because a clear scan or photo of a check can be enough to reproduce the line for fraud. Attackers may use stolen images from leaked case files, payment records, or document archives to create counterfeit checks, alter payment details, or support other financial crimes. Defenders protect against this by restricting access to check images, redacting MICR data when possible, monitoring for unusual check activity, and treating leaked financial documents as high-risk because they can be reused long after the original breach.