On April 21, 2020, OpenSSL announced the vulnerability of OpenSSL 1.1.1d, 1.1.1e, and 1.1.1f and released a security update for it. As a result of this vulnerability, services using both TLS 1.3 and these OpenSSL versions may be vulnerable to Denial of Service (DoS) attacks.

In this article, we provide more information about this vulnerability and explain how to protect your VPSagainst it.

The vulnerability

When establishing an SSL connection, a 'handshake' procedure is performed to establish a secure connection between the client and server.

If you use TLS 1.3 in OpenSSL 1.1.1d, 1.1.1e and 1.1.1f, a specific function (SSL_check_chain) can be called during or after this handshake process. If an attacker sends an invalid or empty signature and the SSL_check_chain function is called, a 'NULL pointer dereference' occurs. The SSL_check_chain function then 'breaks', causing the application that uses it to crash.

Check if your VPS is vulnerable

You can easily check your VPS for this OpenSSL vulnerability by checking the version number of OpenSSL. To do this, use the following commands:

openssl version

The output looks like the example below. Is version number 1.1.1d, 1.1.1e, or 1.1.1f shown here?Then your VPS is vulnerable.

Debian 10 Buster

In Debian Buster stable, you'll always see 1.1.1d as version. For Debian Buster, check if you've installed update 1.1.1d-0+deb10u3 using the command:

apt-cache policy openssl 

The output will look as follows. If you don't see version 1.1.1d-0+deb10u3 or newer here, your VPS is vulnerable.

Securing your VPS

Are you using OpenSSL version number 1.1.1d, 1.1.1e or 1.1.1f?Then you just need to update OpenSSL to 1.1.1g to fix the problem.For example, in Debian 10 / Ubuntu 20.04 you do this with the command:

sudo apt -y update && sudo apt -y upgrade

Finally, check whether the update to OpenSSL 1.1.1.g was successful with the command :

openssl version

Or in Debian 10:

apt-cache policy openssl 

That's it! Your VPS can now no longer be exploited through this vulnerability in OpenSSL.For more general tips for securing your VPS, see this article.

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