Rule Definition
Using LAContext evaluatePolicy: method provides a callback reply block reply:(void (^)(BOOL success, NSError *error))reply. The success returned value must be checked properly to avoid any misuse of the authentication check.

Remediation
Make sure you have implemented the check of success variable in the reply block callback

Violation Code Sample

LAContext *myContext = [[LAContext alloc] init];
NSError *authError = nil;
NSString *myLocalizedReasonString = <#String explaining why app needs authentication#>;

if ([myContext canEvaluatePolicy:LAPolicyDeviceOwnerAuthenticationWithBiometrics error:&authError]) {
    [myContext evaluatePolicy:LAPolicyDeviceOwnerAuthenticationWithBiometrics
                  localizedReason:myLocalizedReasonString
                            reply:^(BOOL success, NSError *error) {
            
           NSLog(@"Is it ok ?"); // not sure it is...
        }];
}
} else {
    // Could not evaluate policy; look at authError and present an appropriate message to user
}

Fixed Code Sample

LAContext *myContext = [[LAContext alloc] init];
NSError *authError = nil;
NSString *myLocalizedReasonString = <#String explaining why app needs authentication#>;
 
if ([myContext canEvaluatePolicy:LAPolicyDeviceOwnerAuthenticationWithBiometrics error:&authError]) {
    [myContext evaluatePolicy:LAPolicyDeviceOwnerAuthenticationWithBiometrics
                  localizedReason:myLocalizedReasonString
                            reply:^(BOOL success, NSError *error) {
            if (success) {
                // User authenticated successfully, take appropriate action
            } else {
                // User did not authenticate successfully, look at error and take appropriate action
            }
        }];
} else {
    // Could not evaluate policy; look at authError and present an appropriate message to user
}

https://developer.apple.com/library/ios/documentation/LocalAuthentication/Reference/LocalAuthentication_Framework/