Con esto en el web.config de cada aplicacion se puede autenticar con un solo formulario de login.aspx en varias aplicaciones :
<authentication mode="Forms">
<forms
loginUrl="Http://localhost/Seguridad/Login.aspx"
enableCrossAppRedirects ="true"
name=".ASPXAUTH"
protection="All"
timeout="30"
path="/"
>
</forms>
</authentication>
<authorization>
<deny users="?"/>
</authorization>
<!-- Validation and decryption keys must exactly match and cannot
be set to "AutoGenerate". The validation algorithm must also
be the same. -->
<machineKey
validationKey="C50B3C89CB21F4F1422FF158A5B42D0E8DB 8CB5CDA1742572A487D9401E3400267682B202B746511891C1 BAF47F8D25C07F6C39A104696DB51F17C529AD3CABE"
decryptionKey="8A9BE8FD67AF6979E7D20198CFEA50DD3D3 799C77AF2B72F"
validation="SHA1" />