Note: TMG can also be configured to request a client certificate but this does not seem to need SSL renegotiation. Setting the DisableRenegoOnServer registry entry to 1 on TMG mitigates against the SSL DoS renegotiation attack but TMG will not be able to handle the above mentioned scenario.IPS) in front of TMG you may create(if possible) a rule to mitigate against the SSL DoS renegotiation attack. TMG in secure web server publishing scenarios using client certificate authentication is vulnerable to the SSL DoS renegotiation attack no current work around on TMG itself exist.TMG, in its default configuration, is vulnerable to the SSL DoS renegotiation attack.
FOREFRONT TMG 2010 FEATURES WINDOWS
The Windows Schannel currently(to my knowledge) does not provide separate registry entries for the above functions. server side initiated renegotiation –> what we need enabled as we want the server(TMG) to be able to initiate renegotiation requests to clients in secure web server publishing scenarios using client certificate authentication.client side initiated renegotiation –> what we want to disable as we don’t want the server to respond to renegotiation requests from the client due to the SSL Reneg DoS issue.With the DisableRenegoOnServer registry entry on the TMG(server) we can control two separate functions: if the client presents the correct cert will get access if not an error (HTTP) message will be presented by TMG.TMG renegotiates asking for a client cert.between the client and TMG a SSL connection is established without the server asking for a client cert.What is needed is the server side initiated renegotiation(for client certificate authentication), meaning allow TMG to initiate SSL renegotiation to client.įrom what I have seen, on TMG when you use client certificate authentication: This is not needed for secure web server publishing rules even with client certificate authentication. browser) is allowed to initiate SSL renegotiation to the server(TMG). TMG’s NIS currently does not include a signature to mitigate against the SSL Renegotiation DoS.īy client side initiated renegotiation we understand that the client(e.g.TMG makes use of the (limited) Schannel so you cannot disable the client side initiated renegotiation on TMG if you use client certificate authentication.during the DoS attack hundreds of SSL handshakes are triggered within the same TCP connection nullifying TMG’s flood mitigation features.SSL client side initiated renegotiation is enabled by default(even when you don’t needed) I’ve notified about this Microsoft Security Response Center on.It has if you published a secure server with TMG using the web server publishing rules because: What has this to do with Forefront TMG 2010 ? More technical details about the DoS can be found on Vincent Bernat’s blog. December 2011 adrian Forefront TMG, SSL (2)Īs you may be aware some time ago a tool to exploit a known SSL Renegotiation DoS issue was released.