Security Notice: THC-SSL-DOS, Lotus Domino and SSL Regegotiation

A group called www.thc.org released a tool called THC-SSL-DOS.  Here's a clip from their site:

THC-SSL-DOS is a tool to verify the performance of SSL.

Establishing a secure SSL connection requires 15x more processing
power on the server than on the client.

THC-SSL-DOS exploits this asymmetric property by overloading the server and knocking it off the Internet.

This problem affects all SSL implementations today. The vendors are aware
of this problem since 2003 and the topic has been widely discussed.

This attack further exploits the SSL secure Renegotiation feature
to trigger thousands of renegotiations via single TCP connection.

Its also been covered in various places on the web, like here or here.

While it doesn't look like there is much that can be done to mitigate it, you may get some relief for your Lotus Domino servers (and other software that uses Domino as a platform) by disabling SSL Renegotiation.

It's an option available in the following releases of Lotus Domino:

• Lotus Domino 8.0.2 Fix Pack 6 +


• Lotus Domino 8.5.1 Fix Pack 4 +


• Lotus Domino 8.5.2 +


• Lotus Domino 8.5.3 +

The fix is the Notes.ini parameter below.

SSL_DISABLE_RENEGOTIATE=1

This Notes.ini parameter was originally released in response to CVE-2009-3555 which detailed a Man-In-The-Middle attack that would allow the attacker to insert data in HTTPS sessions, and possibly other sessions secured by SSL.   (I'm looking at you LDAP, SMTP, and POP3)