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Verifying CRIME, SSLv2 and Plain Text TLS Injection with OpenSSL

If you are a system administrator or penetration tester, you need to be able to check for common vulnerabilities. When configured incorrectly, SSL/TLS has many. There are tons of SSL auditing tools out there, some with more functionality than others, but why add more tools when you can do it yourself?

So this is a tutorial on how to install Openssl from source on a Debain system with a few easy modifications so that you will be able to test for CRIME, SSL version 2 and TLS plain text injection.

First, you’ll need to make sure you have these programs installed:

$ sudo apt-get update && sudo apt-get install build-essential 	devscripts m4 quilt debhelper

Next, you’ll need the source code to OpenSSL. After this is downloaded, you’ll need to move into the openssl directory:

$ apt-get source openssl
	$ cd openssl-*/

SSLv2

SSL version two is an outdated SSL protocol that is filled with problems. For this reason, Debian and other Linux distributions disable SSLv2 in openssl by default. While this is a nice gesture to prevent users from making insecure ciphers, there are those of us that need to be able to check for said ciphers. So we’ll be adding the support back in.

First, from the openssl directory, remove all patches:

$ quilt pop -a

Now we need to remove where the no-sslv2 patch is mentioned inside of the files debian/patches/series and debian/rules.

In debian/patches/series, just take out the entire line where no-sslv2 is mentioned. In my case the file originally looked like:

33 default_bits.patch 
	34 ssltest_no_sslv2.patch 
	35 cpuid.patch

and after the change it became:

33 default_bits.patch 
	34 cpuid.patch 
	35 aesni-mac.patch

Also remove just the no-ssl2 from one line in debian/rules:

Original line:

22 CONFARGS  = --prefix=/usr --openssldir=/usr/lib/ssl –libdir=lib/$	(DEB_HOST_MULTIARCH) no-idea no-mdc2 no-rc5 zlib  enable-tlsext no-ssl2

After removal:

22 CONFARGS  = --prefix=/usr --openssldir=/usr/lib/ssl --libdir=lib/$	(DEB_HOST_MULTIARCH) no-idea no-mdc2 no-rc5 zlib  enable-tlsext

Now that all instances of the no-sslv2 patch have been removed, we can put the remaining patches back into place:

$ quilt push -a

If you were to stop at this point in the tutorial, upon installing you would be able to successfully issue the command:

$ openssl s_client -ssl2 -connect

If the connection is made, the server is configured to use SSL version 2.


TLS Plain Text Injection

TLS plain text injection is a vulnerability where if a command is injected in plain text before an encrypted authenticated session begins, the command is run after the encrypted session has started. Wietse Venema,describes it in detail here: http://www.postfix.org/CVE-2011-0411.html and gives a quick way to modify the openssl source to check for this issue. At this link, you can find out how to replace the -starttls smtp flag to test for TLS plain text injection. However by following that guide, you’ll be losing the previous functionality. So instead, I’ll be showing you how to add a few new commands: -starttls smtpi and -starttls ftpi that will add the a TLS injection check without losing any of the old openssl functionality.
Open apps/s_client.c in your favorite text editor (I’m rather partial to VIM). We are pretty much going to change the source of this file anywhere PROTO_FTP or PROTO_SMTP are used. So in vim I searched for PROTO_SMTP\|PROTO_FTP to highlight those areas and make them easy to find.

We need to add in the variables PROTO_SMTPI and PROTO_FTPI, just yank and put (copy / paste) the PROTO_SMTP and PROTO_FTP lines, stealing their format, and add the “I” to the end of them. Here is what it looks like on my screen (lines 550 and 554). The top part of the split screen is the original, the bottom is after the modification.

terminal_first

The next occurrence of our search term is where it checks what command line argument you passed to -starttls. We just need to copy the same format again and add options for smtpi and ftpi. Make sure to make the smtpi an else if, since there can only be one if. The following is what it should look like after you are done (new lines: 911, 912, 919, 920 )

terminal_second

Next up is a list of if statements to check which protocol you are using for -starttls so we can send the appropriate message to the server. This section is a bit more complicated, so play close attention to the code. For smtp the code looks like:

terminal_third

This means the same code will execute for smtp or smtpi. Now we just need to change the way the code in this if statement works:

terminal_fourth

So now, if starttls_proto==PROTO_SMTPI is given, it will append the harmless plain text command RSET to the STARTTLS command. By adding this, we will not lose our normal -starttls smtp functionality.

Nearly the same thing is done for the PROTO_FTP else if section, only instead of RSET you will use a NOOP command for no-operation.

termail_fifth

and

terimal_sixth

Go ahead and save the file. If you were to stop at this point in the tutorial, upon installing you would be able to successfully issue the command:

$openssl s_client -starttls ftpi -connect

Upon making the connection, openssl will print out information on the cipher. There will be a line that is just two dashes (“–”) and then a 220 response. The 220 response is saying the server connected to you and is happy. Now if there is another line that says “220 Command okay.” that means it also revived and executed your NOOP command which was sent in plain text, and thus the server is vulnerable.

CRIME

CRIME is a vulnerability in SSL compression. If a web server supports SSL compression then it is vulnerable to the CRIME attack. If you just do an apt-get install openssl you probably don’t have zlib enabled, so openssl will not try to use compression while connecting to a server. To make sure you have zlib, just configure with the the parameter “zlib” and “zlib-dynamic” as so:

$ ./config zlib zlib-dynamic

Now if you were to install at this point, whenever you use s_client to connect to a server, openssl will automatically try to use compression. If you see “Compression: gzip” then the server supports SSL compression and is vulnerable to CRIME. However, if you see “Compression: None” it is not vulnerable.

Installing
Run the following commands to install your changes. From the openssl directory do the following.

Update the source:

$ dch -n 'sslv2, tls injection and zilb'

Record the changes in the source tree:
 $ dpkg-source –commit

Build the package:


$ debuild -uc -us

Install:
$ cd ../
 $ sudo dpkg -i *ssl*.deb

That’s it! Now openssl can test for SSLv2, CRIME, SMTP TLS plain text injection, and FTP TLS plain text injection, all without breaking the other wonderful things we love about openssl (such as testing expired certificates, self-signed certificates, weak ciphers and more). One tool to rule them all.

Read the original blog entry...

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Christina O’Neill has been working in the information security field for 3 years. She is a board member for the Northern Ohio InfraGard Members Alliance and a committee member for the Information Security Summit, a conference held once a year for information security and physical security professionals.

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