A system includes first and second subsystems. The first subsystem receives a validation number request, transmitted by a first device in response to the entry of an account number into a first field of a webpage. In response to receiving the request, the first subsystem randomly generates the validation number, stores a copy in memory, and transmits it to a second device. The second subsystem receives a transaction request that includes the validation number from the first device, and transmits the received number to the first subsystem. The first device transmitted the request in response to the second device receiving the validation number and its subsequent entry into a second field of the webpage displayed on the first device. In response to receiving the validation number, the first subsystem determines that it matches the stored copy and transmits a message to the second subsystem authorizing the transaction.