A method for client authentication, the method comprising: receiving, by a server from a client device, a request for identity authentication corresponding to a user account; acquiring, by the server, a social relationship similarity as a result of matching social relationship data of the client device and stored social relationship data of the user account, the matching being performed by the client device or the server; determining whether the social relationship similarity meets a preset passing condition and if affirmative allowing the client device to pass the identity authentication, wherein the preset passing condition is met if the social relationship similarity is equal to or exceeds a passing threshold similarity; upon determining that the social relationship similarity fails to meet the preset passing condition, determining whether the social relationship similarity meets a preset alert condition, wherein the preset alert condition is met if the social relationship similarity falls below an alert threshold similarity, the alert threshold similarity corresponding to a threshold degree of similarity less than that of the passing threshold similarity, and wherein the determining whether the social relationship similarity meets the preset passing condition and whether the preset alert condition is met are performed either: i) both by the server if the matching is performed by the server; or ii) both by the server or both by the client device if the matching is performed by the client device, and: if affirmative, denying the identity authentication and taking an alert action by the server; or if negative, such that the social relationship similarity falls between the passing threshold similarity and the alert threshold similarity, denying the identity authentication without taking the alert action, acquiring, by the server, another social relationship similarity as a result of matching another social relationship data of the client device and the stored social relationship data of the user account, and upon determining, by the server, that the other social relationship similarity meets the preset passing condition, allowing the client device to pass the identity authentication. receiving, by a server from a client device, a request for identity authentication corresponding to a user account; acquiring, by the server, a social relationship similarity as a result of matching social relationship data of the client device and stored social relationship data of the user account, the matching being performed by the client device or the server; determining whether the social relationship similarity meets a preset passing condition and if affirmative allowing the client device to pass the identity authentication, wherein the preset passing condition is met if the social relationship similarity is equal to or exceeds a passing threshold similarity; upon determining that the social relationship similarity fails to meet the preset passing condition, determining whether the social relationship similarity meets a preset alert condition, wherein the preset alert condition is met if the social relationship similarity falls below an alert threshold similarity, the alert threshold similarity corresponding to a threshold degree of similarity less than that of the passing threshold similarity, and wherein the determining whether the social relationship similarity meets the preset passing condition and whether the preset alert condition is met are performed either: i) both by the server if the matching is performed by the server; or ii) both by the server or both by the client device if the matching is performed by the client device, and: if affirmative, denying the identity authentication and taking an alert action by the server; or if negative, such that the social relationship similarity falls between the passing threshold similarity and the alert threshold similarity, denying the identity authentication without taking the alert action, acquiring, by the server, another social relationship similarity as a result of matching another social relationship data of the client device and the stored social relationship data of the user account, and upon determining, by the server, that the other social relationship similarity meets the preset passing condition, allowing the client device to pass the identity authentication. if affirmative, denying the identity authentication and taking an alert action by the server; or if negative, such that the social relationship similarity falls between the passing threshold similarity and the alert threshold similarity, denying the identity authentication without taking the alert action, acquiring, by the server, another social relationship similarity as a result of matching another social relationship data of the client device and the stored social relationship data of the user account, and upon determining, by the server, that the other social relationship similarity meets the preset passing condition, allowing the client device to pass the identity authentication.