crush depth

Let's Encrypt For Woe And Loss

In a crippling bout of sinusitis, and after reading that Chrome is going to mark http sites as insecure, I decided to put good sense aside and deploy Let's Encrypt certificates on the io7m servers.

I've complained about the complexity of this before, so I started thinking about how to reduce the number of moving parts, and the number of protection boundary violations implied by the average ACME setup.

I decided the following invariants must hold:

  • The web server must not have write access to its own configuration, certificates, or logs. This is generally a given in any server setup. Logging is actually achieved by piping log messages to a log program such as svlogd which is running as a different user. In my case, I can actually go much further and state that the web server must not have write access to anything in the filesystem. This means that if the web server is compromised (by a buffer overflow in the TLS library, for example), it's not possible for an attacker to write to any data or code without first having to find a way to escalate privileges.

  • The web server must have read access to a challenge directory. This is the directory containing files created in response to a Let's Encrypt (LE) challenge.

  • The acme client must have read and write access to the certificates, and it must have write access to a challenge directory, but nothing else in the filesystem. Specifically, the client must not have write access to the LE account key or the directory that the web server is serving. This means that if the client is compromised, it can corrupt or reissue certificates but it can't substitute its own account key and request certificates on behalf of someone else.

  • The acme client must not have any kind of administrative access to the web server. I don't want the acme client helpfully restarting the web server because it thinks it's time to do so.

There are some contradictions here: The acme client must not be able to write to the directory that the web server is serving, and yet the web server must be able to serve challenge responses to the LE server. The acme client must not be able to restart the web server, and yet the web server must be restarted in order to pick up new certificates when they're issued.

I came up with the following:

  • An httpd-reloader service sends a SIGHUP signal to the web server every 30 minutes. This causes the web server to re-read its own configuration data and reload certificates, but does not kill any requests that are in the process of being served and specifically does not actually restart the web server.

  • The acme client writes to a private challenge directory, and a private certificates directory. It doesn't know anything about the web server and is prevented from accessing anything other than those directories via a combination of access controls and chrooting.

  • The web server reads from a read-only nullfs mount of a wwwdata directory, and the challenge directory above is placed in wwwdata/.well-known/acme-challenge via another read-only nullfs mount. The web server also reads from a read-only nullfs mount of the certificates directory above in order to access the certificates that the acme client creates.

  • The acme client is told to update certificates hourly, but the acme client itself decides if it's really necessary to update the certificates each time (based on the time remaining before the certificates expire).

  • The intrusion detection system has to be told that the web server's certificates are permitted to change. The account key is never permitted to change. I don't want notifications every ~90 days telling me the certificates have been modified.

Data flow

I set up all of the above and also took some time to harden the configuration by enabling various HTTP headers such as Strict-Transport-Security, Content-Security-Policy, Referrer-Policy, etc. I'm not going to require https to read and I'm not going to automatically redirect traffic from the http site to the https site. As far as I'm concerned, it's up to the people reading the site to decide whether or not they want https. There are plenty of browser addons that can tell the browser to try https first, and I imagine Chrome is already doing this without addons.

The Qualys SSL Labs result:


Now we can all sleep soundly in the knowledge that a third party that we have no reason whatsoever to trust is telling us that is safe.