Fixing BEAST Attacks

G5 Cyber Security

2 months ago

TL;DR

BEAST (Browser Exploits Against SSL/TLS) is an old but still relevant attack that can allow someone to decrypt secure web traffic. This guide shows you how to disable vulnerable TLS versions and ciphers in your server configuration to protect against it.

What is BEAST?

BEAST exploits a weakness in older SSL 3.0 and TLS 1.0 protocols when used with certain cipher suites. It’s complex, but essentially allows an attacker to potentially decrypt HTTPS sessions if they can intercept the traffic.

How to Protect Against BEAST

Disable SSL 3.0: This is the most important step. SSL 3.0 is very old and has known vulnerabilities, including being susceptible to BEAST attacks.

Apache: Edit your Apache configuration file (usually httpd.conf or a virtual host file). Add or modify the following line:
```
SSLProtocol -all +TLSv1.2 +TLSv1.3
```
This disables all protocols except TLS 1.2 and TLS 1.3.
Nginx: Edit your Nginx configuration file (usually nginx.conf or a virtual host file). Add or modify the following line:
```
ssl_protocols TLSv1.2 TLSv1.3;
```
This also disables all protocols except TLS 1.2 and TLS 1.3.
IIS: Use the IIS Manager to disable SSL 3.0 in the SSL Settings for your website. You’ll find this under Server Certificates, then Bindings.

Disable Weak Cipher Suites: Some cipher suites are more vulnerable than others. Remove or disable those.

Apache: Add or modify the following line in your Apache configuration:
```
SSLCipherSuite ECDHE-RSA-AES256-GCM-SHA384:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-SHA384:ECDHE-RSA-AES128-SHA256:ECDHE-RSA-AES256-SHA:ECDHE-RSA-AES128-SHA
```
This specifies a set of strong cipher suites. Adjust this list based on your security requirements.

Nginx: Add or modify the following line in your Nginx configuration:

ssl_ciphers ECDHE-RSA-AES256-GCM-SHA384:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-SHA384:ECDHE-RSA-AES128-SHA256:ECDHE-RSA-AES256-SHA;

Again, adjust this list as needed.

Enable TLS 1.2 and TLS 1.3: Ensure these modern protocols are enabled. They are much more secure than older versions.

This is usually done in conjunction with disabling SSL 3.0 as shown above.

Restart Your Server: After making changes to your configuration, you *must* restart your web server for the changes to take effect.

Apache:
```
sudo systemctl restart apache2
```
(or similar command depending on your Linux distribution)
Nginx:
```
sudo systemctl restart nginx
```
IIS: Restart the website in IIS Manager.

Test Your Configuration: Use an online SSL testing tool to verify that SSL 3.0 is disabled and that you are using strong cipher suites.

Some popular tools include SSL Labs Server Test and TestSSL.sh

Important Considerations

Browser Compatibility: While rare, very old browsers might not support TLS 1.2 or TLS 1.3. Consider the needs of your users when making changes. However, security should generally take priority over supporting outdated software.
Regular Updates: Keep your web server and operating system up to date with the latest security patches.