Monday, October 31, 2011

SSL DoS, Snort, and You

Upon hearing of the release of THC SSL DoS tool, we decided to download it and look at it in our lab. The idea was intriguing and we were curious to see it in action.

If you are unfamiliar with the method utilized, the THC SSL DoS tool seeks to issue a Denial of Service (DoS) against hosts that offer SSL/TLS encrypted services. Unlike SSL flooding techniques of the past, this attack does not do this with rapid connections. Instead it makes a small number of connections and then rapidly renegotiates the SSL handshake inside those same connections.

The problem is that an attacker no longer needs a large amount of bandwidth or to mount a distributed attack to be able to successfully perform an SSL DoS attack. By utilizing a single SSL connection to a server, thousands of SSL handshake renegotiation requests can be performed very quickly.

To quote THC:
"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."
So, what can Snort do for you? We knew that with the default configuration on Snort's SSL preprocessor we were not going to see the renegotiation happening. The reason is that once a successful SSL connection is made, without an SSL decryption appliance (Sourcefire sells them), Snort will ignore the rest of the conversation - the logic being that, since it's now encrypted, we can't do any detection on the traffic anyway. However, all hope is not lost. If you are in a position in which you need to detect this, there is a way. This detection behavior is controlled by the SSL preprocessor option "noinspect_encrypted"; removing that keyword will cause Snort to continue inspection after a session goes encrypted.
So, what next? First, let's look at the SSL/TLS record layer content types:

Hex Dec Type
0x14 20 ChangeCipherSpec
0x15 21 Alert
0x16 22 Handshake
0x17 23 Application


Since the attack utilizes handshake renegotiations, we are interested in the Handshake (0x16). Now we are interested in the two bytes of SSL/TLS version information:

Major Minor Version Type
3 0 SSL 3.0
3 1 TLS 1.0
3 2 TLS 1.1
3 3 TLS 1.2


So, if we take the content type, major version, and minor version within the first three bytes, we can get a pretty decent match. Next we sprinkle in a detection_filter to track the number of renegotiations, and finally we remove the noinspect_encrypted from the SSL preprocessor, and there it is ... SSL negotiation DoS detection.

This isn't a configuration I would recommend unless you've got a good reason because there will be a performance penalty. However, if you need it, you've got it. Run the rules that match your environment, adjust the ports as needed, and tweak the detection_filter to taste. The rules will be released in the next SEU.

This blog post has been brought to you by the letters V, R, T.

alert tcp $EXTERNAL_NET any -> $HOME_NET [443,465,587,995,993] (msg:"DOS multiple SSLv3 Encrypted Handshake messages - THC-SSL tool, potential DoS"; flow:established,to_server; ssl_state:!client_hello; content:"|16 03 00|"; depth:3; detection_filter:track by_src,count 25, seconds 2; reference:url,www.thc.org/thc-ssl-dos/; classtype:attempted-dos; sid:20436;)

alert tcp $EXTERNAL_NET any -> $HOME_NET [443,465,587,995,993] (msg:"DOS multiple TLSv1 Encrypted Handshake messages - THC-SSL tool, potential DoS"; flow:established,to_server; ssl_state:!client_hello; content:"|16 03 01|"; depth:3; detection_filter:track by_src,count 25, seconds 2; reference:url,www.thc.org/thc-ssl-dos/; classtype:attempted-dos; sid:20437;)

alert tcp $EXTERNAL_NET any -> $HOME_NET [443,465,587,995,993] (msg:"DOS multiple TLSv1.1 Encrypted Handshake messages - THC-SSL tool, potential DoS"; flow:established,to_server; ssl_state:!client_hello; content:"|16 03 02|"; depth:3; detection_filter:track by_src,count 25, seconds 2; reference:url,www.thc.org/thc-ssl-dos/; classtype:attempted-dos; sid:20438;)

alert tcp $EXTERNAL_NET any -> $HOME_NET [443,465,587,995,993] (msg:"DOS multiple TLSv1.2 Encrypted Handshake messages - THC-SSL tool, potential DoS"; flow:established,to_server; ssl_state:!client_hello; content:"|16 03 03|"; depth:3; detection_filter:track by_src,count 25, seconds 2; reference:url,www.thc.org/thc-ssl-dos/; classtype:attempted-dos; sid:20439;)

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