Error OpenSSL:0A00018E:SSL OpenVPN 3 LINUX

[ketues]

Hello

I write in the Windows forum, because there is no Linux.

I use various OpenVPN 2.7 clients on Windows, Android, and Linux to connect to my OpenVPN server.

I have updated my Linux Mint and now it only lets me install version 3 of the OpenVPN client, but this version is not compatible with my server. I get the error of: "OpenSSL: error:0A00018E:SSL routines::ca md too weak""

How can I install OpenVPN client 2.7 on Linux Mint? Another way to fix the problem? In the rest of Windows and Android clients I can connect without problem.

Thank you

[openvpn_inc]

Hello ketues,

You might be able to get OpenVPN2 installed from https://community.openvpn.net/openvpn/w ... twareRepos, and if necessary, select an older version from there.

But really, you should be solving the security problem on your setup. Using an MD5 signed CA is not secure anymore. You should look into updating that to something more secure.

Simply put, you are getting this error because you're using certificates that are signed in such a way that is so out of date and dangerous to use, that we no longer want to support this. This should be a clear signal to you to do something about it, instead of just reverting to older software and ignoring the problem. You should be looking into replacing your certificate infrastructure to solve this issue.

Kind regards,
Johan

[ketues]

Thank you for answering so quickly.

On the server side SHA512 is used, not MD5, but it still gives me the error.

The server is an ASUS RT-AC85P Router Updated to the latest firmware version.

The OpenVPN server configuration is:
RSA Encryption: 2048 bit
Encryption: AES-256-CBC
HMAC Authentication: SHA-512
Authorization mode mode: TLS

I understand the security problem that this implies, but the OpenVPN server runs on an ASUS-RT router from 1 year ago, the manufacturer has not updated the firmware to be compatible with the new versions of OpenVPN. Can you do something on the ASUS router to make it compatible with the newer versions of OpenVPN?

For Windows, the download of the 2.7 client is available on this website, but not for Linux.

Thank you

[openvpn_inc]

Hello ketues,

VPN encryption is not an easy subject but here are a few things I know about this situation;

OpenVPN can use certificates to do identity verification. Basically the certificates provide a way to prove who you are. The certificate infrastructure starts with a private key for the certificate authority (CA), and a CA certificate signed with that. When a client or server certificate is generated this is done by creating a certificate signing request first and then having the CA sign it. That signing process is these days normally done using sha256. In the past it used to be sha1, or before that, md5. This is the situation you're in, since OpenVPN is complaining that the CA is signed with MD, which is not safe anymore. There is no way to easily fix this. You should just start a new certificate structure and this time make it use either secp384r1 or rsa2048 with sha256.

The server configuration you mention is about the handshake and encryption, and is less than ideal. You have chosen to use AES-256-CBC with a separate step to sign and verify each packet that gets sent and received by the OpenVPN tunnel processes. In theory this is great but there are a few problems. First off you can use AES-256-GCM, and hopefully your OpenVPN clients will switch to that automatically, if they can. With AES-256-GCM, the step of encrypting and then signing the packets is done in one phase instead of two separate ones. This is more efficient, while keeping the same level of encryption strength. Another point is that you have chosen to do sha512 for the separate signing and verification step. This is unnecessarily strong and actually can slow things down significantly.

People tend to think stronger is always better, but this is not always the case, especially when it's only used for an HMAC, where, ironically, MD5 is actually still fine to use for the packet signing and verifying, unlike with the certificate signatures, where the strength really does matter.

See also this article why MD5 support is removed;
https://openvpn.net/faq/md5-signature-a ... m-support/

I am sorry to hear that your ASUS-RT router, despite being fairly new, is apparently still using MD5 signed certificates. I understand that you can't fix their mistake.

You keep referring to a '2.7' client, I must assume this is OpenVPN Connect v2.7 which is deprecated. Sorry to say, but, this was only ever made for Windows and macOS, there is no Linux version. Since it is deprecated and not being updated anymore, this is the reason why such outdated security is still allowed on that older client. On newer versions by default it is not. For Linux there is open source community OpenVPN2 linux software at this address;
https://community.openvpn.net/openvpn/w ... twareRepos

There is a special flag in OpenVPN for setting the security level of the certificates being used which could still allow MD5 signed certificates to be used. I am not guaranteeing success but you could try adding this to the client profile:

tls-cert-profile insecure

The tls-cert-profile insecure setting is the least secure option.

Kind regards,
Johan