Error on a plugin execution : SecurityPermission exception

Hello,

When I click on a button, i call a workflow which launchs a plugin. I meet the error below :

Unexpected exception from plug-in (Execute): Custom_WFA_Annuler_Terminer_DE.CallWsAnnulerTerminer: System.Exception: Request for the permission of type 'System.Security.Permissions.SecurityPermission, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' failed.

Thanks for your help,

Paul