The SSL version 3.0 POODLE (Padding Oracle On Downgraded Legacy Encryption) vulnerability (CVE-2014-3566) was officially released on October 14, 2014 by OpenSSL. The flaw was disclosed by a team of Google researchers including Bodo Möller, Thai Duong, and Krzysztof Kotowicz. This vulnerability is a consequence of an implementation flaw associated with the use of block cipher encryption in SSLv3. Block ciphers encrypt data in fixed-length blocks. If the plain text value to be encrypted is not a multiple of the defined block size, the cipher will apply padding to the data to increase the size, so that it can be converted to cipher text. The concern is that the Message Authentication Code (MAC) does not cover the block cipher padding and when the message is decrypted, the integrity of the padding cannot be verified. This can allow an attacker to decrypt cipher text, one byte at a time.
This vulnerability only affects the SSLv3 protocol, which is rarely used by modern web browsers that prefer the usage of TLSv1 encryption. However, due to the widespread support for SSLv3 on both servers and web browsers, an attacker can still leverage this vulnerability by using it in conjunction with a downgrade attack. A downgrade attack could be accomplished by intercepting and manipulating traffic associated with the SSL/TLS cipher suite negotiation, conducted between the client and server.
In the original disclosure article, B.Möller, T. Duong, and K. Kotowicz succinctly illustrate the impact of this vulnerability, referencing a scenario in which it could be used to compromise secure session tokens within the context of a web application (p.2, https://www.openssl.org/~bodo/ssl-poodle.pdf).
Due to the fact that this vulnerability must be exploited within a chosen-plaintext context, the only probable exploitation scenario with any significant impact is within a web context. For an attacker to successfully exploit this vulnerability, multiple highly specific conditions must exist. These conditions include the following:
- The attacker must be able to intercept and manipulate traffic between the client and server (as in a Man-in-the-Middle scenario)
Despite the special circumstances and high level of skill required to exploit this vulnerability, the impact of a successful attack would be significant. Successful exploitation could result in an attacker gaining access to small pieces of highly sensitive encrypted traffic such as session tokens. Acquisition of these session tokens could be used in session hijacking attacks to completely take over a victim’s session within the context of the web application.
Server Testing with OpenSSL Client:
To determine if a particular service is vulnerable, use the SSL client in SSLv3 mode and supply the server name or IP address in conjunction with the port number of the service in question. If the connection succeeds then SSLv3 is enabled:
openssl s_client -connect <server>:<port> -ssl3
openssl s_client -connect google.com:443 -ssl3
Server Testing with Nmap:
The SSL-enum Nmap Scripting Engine (NSE) script can also be used to determine if servers are vulnerable. Nmap should be executed with the syntax provided below:
nmap <server> –script ssl-enum-ciphers -p <port>
nmap google.com –script ssl-enum-ciphers -p 443
If the scan returns a list of support ciphers under the SSLv3 header, then SSLv3 is enabled.
| TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA – strong
| TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA – strong
| TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA – strong
| TLS_ECDHE_RSA_WITH_RC4_128_SHA – strong
| TLS_RSA_WITH_3DES_EDE_CBC_SHA – strong
| TLS_RSA_WITH_AES_128_CBC_SHA – strong
| TLS_RSA_WITH_AES_256_CBC_SHA – strong
| TLS_RSA_WITH_CAMELLIA_128_CBC_SHA – strong
| TLS_RSA_WITH_CAMELLIA_256_CBC_SHA – strong
| TLS_RSA_WITH_RC4_128_MD5 – strong
| TLS_RSA_WITH_RC4_128_SHA – strong
Server Testing with SSLLabs:
Qualys has made a web-based testing utility available at the URL listed below. This can be used to test public facing servers.
If the scan returns indication that there is still support for SSLv3, then the server is vulnerable.
Browser Client Testing with Poodle Test:
A web-based test has been constructed to test client browsers and determine if they are vulnerable to the POODLE attack.
If your browser is vulnerable, the site will display the following image:
Unfortunately, there is no patch remediation for this vulnerability. However, SSLv3 is a depreciated protocol and should be disabled on both servers and clients (browsers). Further, both Mozilla and Google have posted that they will be updating both FireFox and Google Chrome, in the coming months, to disable SSLv3 support. However, it should be noted that disabling SSLv3 could potentially break some websites or legacy web applications that support SSLv3.
Remediation on Microsoft Internet Explorer:
Click the Settings button at the top-right corner of the browser, and then select ‘Internet Options’. Then browse to the ‘Advanced’ tab. In the Settings menu, scroll to the bottom and uncheck the box labeled ‘Use SSL 3.0’. Once completed, click ‘Apply’ then ‘OK’.
Remediation on Mozilla FireFox:
In the URL address bar, browse to ‘about:config’. You will then be given a warning, indicating that you should only modify these settings if you know what you are doing. We do, so click the button to disregard the warning and proceed. Then, in the Search bar, type ‘security.tls.version.min’. Double-click the setting with that Preference Name and then change the integer value from 0 to 1. Once this change has been made, click ‘OK’. This will disable SSLv2 and SSLv3, and only allow the browser to support TLSv1 and later.
Remediation on Google Chrome:
Ironically, despite the fact that it was a Google team that identified this vulnerability, Chrome’s GUI management interface offers no option to disable support for SSLv3. A common workaround is to start Chrome from a shortcut that leverages the command line argument to disable support for SSLv3.
To do this, right-click your Google Chrome shortcut and select ‘Properties’. Then, append the command line argument ‘ –ssl-version-min=tls1’ to the end of the value in the Target field (as seen in the provided image). Click ‘Apply’ and then ‘OK’. Once this modification has been made, support for any versions prior to TLSv1 is disabled anytime the browser is started from this Shortcut.
Remediation on Apache Server:
Modify the SSLProtocol directive in the server’s ssl.conf file to disable support for versions earlier than TLSv1 on Apache. The location of this file may vary depending on the build of the server.
For Ubuntu, the file can be modified with:
sudo nano /etc/apache2/mods-available/ssl.conf
If mod-ssl is enabled, the location will be:
sudo nano /etc/apache2/mods-enabled/ssl.conf
For CentOS, the file can be modified with:
sudo nano /etc/httpd/conf.d/ssl.conf
In the configuration file, modify the SSLProtocol directive to include the following:
SSLProtocol All -SSLv2 -SSLv3
To verify the configuration change, use the following:
Once support for SSLv2 and SSLv3 has been disabled, the Apache service will need to be restarted. This can be done with the following command:
sudo service apache2 restart
Remediation on IIS:
To disable support for SSLv3 on Microsoft IIS, a registry tweak is required. Open the registry editor (with command ‘regedit’) and then browse to the following key:
Inside the Protocols key, there should be a key called ‘SSL 3.0’ and inside that key, there should be a key called ‘Server’. If these keys do not exist, create them. Then, inside the ‘Server’ key, create a DWORD value called ‘Enabled’ and then leave its value at 0 (default). Once completed, restart the server to implement the new changes.
Remediation on NGINX:
Modify the ssl_protocols directive in the nginx.conf file to disable support for versions earlier than TLSv1 on Nginx. This file is located at /etc/nginx/nginx.conf and can be modified with:
sudo nano /etc/nginx/nginx.conf
Modify the ssl_protocols directive in the file to include the following:
ssl_protocols TLSv1 TLSv1.1 TLSv1.2;
To verify the configuration change, use the following:
sudo nginx -t
Once support for SSLv2 and SSLv3 has been disabled, the nginx service will need to be restarted. This can be done with the following command:
sudo service nginx restart
What is TLS_FALLBACK_SCSV?
In the event that you are not prepared to disable the use of SSLv3, downgrade attacks can be alternatively mitigated in some distinct scenarios by using a browser that supports a new cipher suite value called TLS_FALLBACK_SCSV. In the event that both the server and client browser support this option, a more secure negotiation process is used that prevents downgrading to a protocol or cipher that is less secure than the highest mutually supported option.
Unfortunately, at this time, limited support on the server-side and limited adoption by client browsers has made this an ineffective, comprehensive solution for this problem.
Presently, TLS_FALLBACK_SCSV is only supported by Google Chrome 33.0.1750 (February 2014 Build) and later. Other major web browsers will likely adopt support in the following months.
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