To configure the security appliance to automatically pass user login credentials for Clientless SSL VPN connections on to internal servers, use the auto-signon command in any of three modes: webvpn configuration, webvpn group configuration, or webvpn username configuration mode. The authentication method can be NTLM (includes NTLMv1 and NTLMv2), HTTP Basic authentication, or both.
http://www.cisco.com/en/US/docs/security/asa/asa80/command/reference/a2.html#wp1630522