A merchant plug-in (MPI) is a software module designed to facilitate 3-D Secure verifications to help prevent credit card fraud.
[1] The MPI identifies the account number and queries the servers of the card issuer (Visa, MasterCard, or JCB International) to determine if it is enrolled in a 3D-Secure program and returns the web site address of the issuer access control server (ACS) if it is found.
[3] Each card issuer is required to maintain an ACS used to support cardholder authentication.
The plug-in verifies the ACS signature and decides if it wishes to proceed with the transaction.
Commercial MPI software is available from a number of vendors.