In a comment on my entry on generating self-signed TLS certificates today, James suggested:

My go-to tool is OpenVPN's EasyRSA. Admittedly that creates a CA which you can then sign certificates with, but for your internal hosts it would mean you could install said CA into your browser and then trust them all.

Superficially, this is certainly an appealing idea. If you have a fleet of IPMIs or other internal websites that need TLS certificates and that have names where you can't get public certificates, you can avoid everyone having to trust them one by one. Just set up a local CA, sign all the internal website certificates with them, add the local CA certificate to your browser, and you're done.

Unfortunately if you do this, you have just loaded a fairly large security-defeating gun and pointed it straight at your face. It's not just that your local CA can be attacked to sign certificate for any host, not just your internal ones; more importantly, certificates signed by a manually added CA specifically bypass all of the modern TLS protections built into browsers. This isn't just things like HTTP Public Key Pinning headers that your browser may have memorized, it's also even critically important pinned keys hard-coded into browsers themselves. A certificate signed by a manually added CA bypasses all of those checks.

(For all of this we may blame HTTPS interception middleware. Browser vendors have extremely reluctantly bowed to the demands of businesses that want to deploy them and have them intercept absolutely everything, partly because businesses basically hold the cards here if they're willing to go far enough.)

As far as I know there's no way in either Firefox or Chrome to constrain a manually added CA to only have its certificates accepted for certain (sub)domains. This means that no matter what you want, your local CA intended for intranet websites has just as much TLS interception ability as the TLS CA for a mandatory HTTPS middleware box. If an attacker can compromise it, they gain complete HTTPS interception capabilities for web browsing, both internal and external. None of the usual precautions and warnings will protect you in the least.

This means that a local CA that you have people's browsers trust is a very big deal, even (or especially) if only the sysadmins are trusting it. If you're going to have one at all, I think that it should involve some sort of hardware security module, even a simple and cheap one. If you are not willing to strongly protect a local CA, at least to the level of buying basic HSM hardware for it, then you should not even think of having one; it's simply far too dangerous in the event of a serious attacker. Even if you buy HSM hardware for it, I think that the balance of risks versus gains are often not going to be in favour of a local CA.

(To be clear, all of this is specific to local CAs that you will have your browsers trust. There are perfectly sensible and not particularly dangerous uses for a local CA outside of this. The general way to know if you're safe is that every operation that is supposed to use the local CA should have to explicitly trust the local CA's root certificate, whether that's through a command-line option or a specific configuration file setting. You should never add a local CA to your general trust roots, whether those are the browser trust roots or the system's generic trust roots.)

(Years ago I sort of wrote about this here, but I didn't take it anywhere near far enough and in particular I didn't think of what an attacker could do with access to your local or organizational CA. Not that overzealous security people aren't a serious risk in and of themselves, and it's not as if middleware HTTPS interception has a good reputation. Rather the contrary.)