1. A server system for monitoring user authenticity during user activities in user sessions on at least one application server, the server system comprising: one or more processors; and at least one computer readable storage memory comprising program code that, when executed by the one or more processors, causes the one or more processors to: perform a user-modeling process in which a user model is adapted session-by-session to user activities, wherein the user model includes a plurality of adaptive feature-specific user-behavior models that are associated with features indicative of user behavior; and perform a user-verification process comprising: determining a plurality of feature-specific risk-score values, wherein determining each feature-specific risk-score value comprises comparing at least one of the adaptive feature-specific user-behavior models with each respective feature extracted from one or more of the user activities during one of the user sessions on the at least one application server, determining a total risk-score value indicative of user non-authenticity by: a) weighting and combining the feature-specific risk-score values, wherein the feature-specific risk-score values are combined by a multi-criteria decision analysis technique, and b) weighting and combining pre-combined risk-score values, wherein the pre-combined risk-score values are determined by combining a portion of the feature-specific risk-score values using multi-criteria decision analysis, and in response to the total risk-score value exceeding a given threshold, performing a corrective action selected from the group consisting of (i) signing out the user, (ii) requesting a two-factor authentication from the user, (iii) locking the user, and (iv) initiating an alert function, wherein the user model is adapted on a user-model server based on the user activities associated with different applications on the at least one application server, and the adaptive feature-specific user-behavior models are specific for the different applications.