oday the German hacker group “The Hacker’s Choice” officially released a new DDoS tool. The tool exploits a weakness in SSL to kick a server off the Internet.
The THC-SSL-DOS tool allows a single computer with a modest internet connection to crash a much more powerful server with vastly more bandwidth, but only when the server supports what’s known as SSL renegotiation, Monday’s postings claimed. Renegotiation is used to establish a new secret key securing communications after an encrypted session has already commenced. Renegotiation was at the heart of a flaw in the SSL protocol discovered in 2009 that allowed attackers to inject text into encrypted traffic passing between two endpoints.
Technical details can be found at http://www.thc.org/thc-ssl-dos.
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. Download: Windows binary:
Unix Source :
thc-ssl-dos-1.4.tar.gz Use “./configure; make all install” to build.
./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
Comparing flood DDoS vs. SSL-Exhaustion attack:
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).
Tips & Tricks for whitehats
1. The average server can do 300 handshakes per second. This would require 10-25% of your laptops CPU.
2. Use multiple hosts (SSL-DOS) if an SSL Accelerator is used.
3. Be smart in target acquisition: The HTTPS Port (443) is not always the best choice.
Other SSL enabled ports are more unlikely to use an SSL Accelerator (like the POP3S, SMTPS, … or the secure database port). Counter measurements: No real solutions exists. The following steps can mitigate (but not solve) the problem: 1. Disable SSL-Renegotiation 2. Invest into SSL Accelerator Either of these countermeasures can be circumventing by modifying THC-SSL-DOS. A better solution is desireable. Somebody should fix this.