1. A system for assessing security risk associated with an electronic transaction, the system comprising at least one processor programmed to: analyze current user information indicative of at least one user behavior observed in connection with the electronic transaction; determine at least one user identifier associated with the electronic transaction; analyze history information associated with the at least one user identifier, the history information being indicative of at least one previously observed behavior associated with the at least one user identifier; analyze at least one risk factor associated with the electronic transaction; and provide a risk assessment for the electronic transaction at least in part by combining the current user information, the history information associated with the at least one user identifier, and the at least one risk factor associated with the electronic transaction; wherein the at least one user identifier comprises at least one network address; wherein the at least one processor is further programmed to: obtain a first risk value based on the current user information; obtain a second risk value based on the history information associated with the at least one network address; and obtain a third risk value based on the least one risk factor associated with the electronic transaction; and use a plurality of weights to combine the first, second, and third risk values to thereby obtain an overall risk value indicative of a level of risk associated with the electronic transaction, wherein the plurality of weights comprises first, second, and third weights applied respectively to the first, second, and third risk values.