OpenVPN Security Advisory: Dec 14, 2018
Action needed: Important update for OpenVPN Access Server

Revoking Certificates

Revoking a certificate means to invalidate a previously signed certificate so that it can no longer be used for authentication purposes.

Typical reasons for wanting to revoke a certificate include:

Example

As an example, we will revoke the client2 certificate, which we generated above in the “key generation” section of the HOW TO

First open up a shell or command prompt window and cd to the easy-rsa directory as you did in the “key generation” section above. On Linux/BSD/Unix:

. ./vars
./revoke-full client2

On Windows:

vars
revoke-full client2

You should see output similar to this:

Using configuration from /root/openvpn/20/openvpn/tmp/easy-rsa/openssl.cnf
DEBUG[load_index]: unique_subject = "yes"
Revoking Certificate 04.
Data Base Updated
Using configuration from /root/openvpn/20/openvpn/tmp/easy-rsa/openssl.cnf
DEBUG[load_index]: unique_subject = "yes"
client2.crt: /C=KG/ST=NA/O=OpenVPN-TEST/CN=client2/emailAddress=me@myhost.mydomain
error 23 at 0 depth lookup:certificate revoked

Note the “error 23” in the last line. That is what you want to see, as it indicates that a certificate verification of the revoked certificate failed.

The revoke-full script will generate a CRL (certificate revocation list) file called crl.pem in the keyssubdirectory. The file should be copied to a directory where the OpenVPN server can access it, then CRL verification should be enabled in the server configuration:

crl-verify crl.pem

Now all connecting clients will have their client certificates verified against the CRL, and any positive match will result in the connection being dropped.

CRL Notes

When the crl-verify option is used in OpenVPN, the CRL file will be re-read any time a new client connects or an existing client renegotiates the SSL/TLS connection (by default once per hour). This means that you can update the CRL file while the OpenVPN server daemon is running, and have the new CRL take effect immediately for newly connecting clients. If the client whose certificate you are revoking is already connected, you can restart the server via a signal (SIGUSR1 or SIGHUP) and flush all clients, or you can telnet to the management interface and explicitly kill the specific client instance object on the server without disturbing other clients.

While the crl-verify directive can be used on both the OpenVPN server and clients, it is generally unnecessary to distribute a CRL file to clients unless a server certificate has been revoked. Clients don’t need to know about other client certificates which have been revoked because clients shouldn’t be accepting direct connections from other clients in the first place.

The CRL file is not secret, and should be made world-readable so that the OpenVPN daemon can read it after root privileges have been dropped.

If you are using the chroot directive, make sure to put a copy of the CRL file in the chroot directory, since unlike most other files which OpenVPN reads, the CRL file will be read after the chroot call is executed, not before.

A common reason why certificates need to be revoked is that the user encrypts their private key with a password, then forgets the password. By revoking the original certificate, it is possible to generate a new certificate/key pair with the user’s original common name.

Share