A German group of Hackers known as Hackers Choice have released a program they assert will allow a single computer to take down a Web server using a secure connection

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.

./thc-ssl-dos 127.3.133.7 443

Handshakes 0 [0.00 h/s], 0 Conn, 0 Err

Secure Renegotiation support: yes

Handshakes 0 [0.00 h/s], 97 Conn, 0 Err

Handshakes 68 [67.39 h/s], 97 Conn, 0 Err

Handshakes 148 [79.91 h/s], 97 Conn, 0 Err

Handshakes 228 [80.32 h/s], 100 Conn, 0 Err

Handshakes 308 [80.62 h/s], 100 Conn, 0 Err

Handshakes 390 [81.10 h/s], 100 Conn, 0 Err

Handshakes 470 [80.24 h/s], 100 Conn, 0 Err

A traditional flood DDoS attack cannot be mounted from a single DSL connection. This is because the bandwidth of a server is far superior to the bandwidth of a DSL connection: A DSL connection is not an equal opponent to challenge the bandwidth of a server.

This is turned upside down for THC-SSL-DOS: The processing capacity for SSL handshakes is far superior at the client side: A laptop on a DSL connection can challenge a server on a 30Gbit link. 

Traditional DDoS attacks based on flooding are sub optimal: Servers are prepared to handle large amount of traffic and clients are constantly sending requests to the server even when not under attack. 

The SSL-handshake is only done at the beginning of a secure session and only if security is required. Servers are _not_ prepared to handle large amount of SSL Handshakes.

The worst attack scenario is an SSL-Exhaustion attack mounted from thousands of clients (SSL-DDoS).

No real solutions exists. The following steps can mitigate (but not solve) the problem:

Either of these countermeasures can be circumventing by modifying THC-SSL-DOS. A better solution is desireable. Somebody should fix this.

You can download THC-SSL-DOS here:

Windows: thc-ssl-dos-1.4-win-bin.zip

Linux: thc-ssl-dos-1.4.tar.gz