Is there anything inherently insecure about an expired cert other than your browser just complaining about it?

Expiries are a defence-in-depth that exist primarily for crypt hygiene, for example to protect from compromised keys. If the private key material is well protected, the risk is very low.

However, an org (particularaly a .mil) not renewing its TLS certs screams of extreme incompetence (which is exactly what expiries are meant to protect you from.)

the idea behind expiration is:

- TLS certificates do leak, not just due to worst case bugs like heart blead

- revocation does not work well in practice

- affected operators aren't always aware if a certificate leaked

so by having expiration (and in recent years increasingly short validity duration) you reduce how the consequences of an leak, potentially to nothing if the attacker only gets their hand on the cert after it expired

this also has the unintended consequence that a long time expired certificate leaking isn't seen as a security issues, nor will you revoke it (it's already invalid).

But if you visit site with expired certificates you have the problem that you only know it had been valid in the past. You don't know if it was leaked after it became invalid or similar. I.e. you can't reasonable differentiate anymore between "forgotten to renew" and MITM attack. At which point it worth pointing out that MITM attacks aren't just about reading secrets you send, but can also inject malicious JS. And browser sandbox vulnerabilities might be rare but do exist.

A more extremem case of this dynamics are OIDC/OAuth access tokens. Which are far more prone to leak then certs, but in turn are only valid for a short time (max 5min) and due to that normally don't have a revocation system. (Thinks are different for the refresh token you use to get the access token, but the refresh token also is only ever send to the auth server which makes handling that way easier.)

Its more or less the same as using expired in person documents.

Instrinsically no, but at the same time its generally easier to source an expired identity document. If its long expired the verification standards might be different. People dont really put as much effort into destroying or revoking expired documents. The info might no longer be accurate.

The only addition in the computer context is it trains users to blindly click through warnings. If this becomes a pattern they will eventually not realize it if the error becomes something more serious.

No, but it reflects poorly on the maintainer. Plus, any browser complaint contributes to error fatigue. Users shouldn't just ignore these, and we shouldn't encourage them to ignore them just because we fail at securing our websites.

Certificate revocations are not required to be reported after the expiration date, so you can no longer reliably check if a certificate has been revoked (e.g., because its underlying key was exfiltrated or because it was misissued).

Yes. Visitors to the site are vulnerable to Man in the Middle (MitM) attacks, IF they click past the warning (which many people do)

To prevent abuse, for example to prevent an old owner of domain to have a valid certificate for the domain indefinitely after transfer.

Its a signifier of other problems.

but, in terms of security, wrong cert looks the same to most people as wrong domain. So once people get used to the cert being wrong and click through anyway, its easy to switcheroo and do a half arsed man in the middle.

But, it signifies that people are not monitoring the outside of the network. We have cert checks which alert if they are less than 5 days from expiry, as they should have been renewed and restarted automatically. If you're not doing that, what else are you halfarsing?

You're training users to click away error messages.

Not inherently, but it can introduce risk. Such as a bad actor using an old expired certificate it was able to acquire to play man-in-the-middle. But if that is happening you have bigger problems.

I'm not actually sure if browsers validate expired certificates. I couldn't find out if an invalid cert authority and an invalid date would look different than just the error for an invalid date. Educated guess says so, but that's just a guess.

The biggest concern is you'd hope the DoD (/DoW) is on top of their stuff, especially the DISA. This is a sign they are not. This is something that should never happen.

But then, there's this message:

> DoD Cyber Exchange site is undergoing a TSSL Certification renewal resulting in download issues for some users. Users on civilian network can continue downloads through the Advance tab in the error message.

Uh oh!!! This is concerning because (1) "Ignore SSL errors" is something you should never be telling users to do and (2) this is extra concerning because whoever wrote this does not seem to have a grasp on the English language:

- "TSSL Certification renewal" should be TLS/SSL Certificate renewal. (Caveat: Defense is full of arcane internal acronyms and TSSL could just be one of them.)

- "Users on civilian network" should be "Users on civilian networks", or "Users on a civilian network".

- "Advance tab" should be "Advance button".

So, we have three glaring red flags. Expired certs, telling users to ignore cert warnings, and various spelling and grammar mistakes.

People are citing the short 40-day certificate renewal window, but that's not the problem here. It's not a case of administration transition either. This cert was issued 2025-Mar-20 and was valid for 1 year. But IdenTrust DoD certs can't be renewed after they expire, so that might be why this is so broken.

In the most generous interpretation, the once-responsible party was cut with the huge DOGE cuts back in May 2025, and this failure of web administration is just one visible sign of the internal disarray you'd expect with losing 10% of your workforce.

It's a pretty dopey thing to miss.

Well it means that you can MITM a user and they won't know the difference (an expired cert is an expired cert, whether it's self-signed or not, the user clicks through anyway). It also means nobody is doing the regular maintenance to rotate keys and do upgrades/patches/etc.

Can you live without your immune system? Sure, for a little while. It’s the defense against man in the middle and many other things.

Inherently, not really. An expired, self-signed or even incorrect (as in, the wrong domain is listed) certificate can be used to secure a connection just as well as a perfectly valid certificate.

Rather, the purpose of all of these systems (in theory) is to verify that the certificate belongs to the correct entity, and not some third party that happens to impersonate the original. It's not just security, but also verification: how do I know that the server that responds to example.com controls the domain name example.com (and that someone else isn't just responding to it because they hijacked my DNS.)

The expiration date mainly exists to protect against 2 kinds of attacks: the first is that, if it didn't exist, if you somehow obtained a valid certificate for example.com, it'd just be valid forever. All I'd need to do is get a certificate for example.com at some point, sell the domain to another party and then I'd be able to impersonate the party that owns example.com forever. An expiration date limits the scope of that attack to however long the issued certificate was valid for (since I wouldn't be able to re-verify the certificate.)

The second is to reduce the value of a leaked certificate. If you assume that any certificate issued will leak at some point, regardless of how it's secured (because you don't know how it's stored), then the best thing you can do is make it so that the certificate has a limited lifespan. It's not a problem if a certificate from say, a month ago, leaks if the lifespan of the certificate was only 3 days.

Those are the on paper reasons to distrust expired certificates, but in practice the discussion is a bit more nuanced in ways you can't cleanly express in technical terms. In the case of a .mil domain (where the ways it can resolve are inherently limited because the entire TLD is owned by a single entity - the US military), it's mostly just really lazy and unprofessional. The US military has a budget of "yes"; they should be able to keep enough tech support around to renew their certificates both on time and to ensure that all their devices can handle cert rotations.

Similarly, within a network you fully control, the issues with a broken certificate setup mostly just come down to really annoying warnings rather than any actual insecurity; it's hard to argue that the device is being impersonated when it's literally sitting right across from you and you see the lights on it blink when you connect to it.

Most of the issues with bad certificate handling come into play only when you're dealing with an insecure network, where there's a ton of different parties that could plausibly resolve your request... like most of the internet. (The exception being specialty domains like .gov/.mil and other such TLDs that are owned by singular entities and as a result have secondary, non-certificate ways in which you can check if the right entity owns them, such as checking which entity the responding IP belongs to, since the US government literally owns IP ranges.)

“It’s an older code but it checks out.”

Anything that relies on certificate validation might not be working because of it.

I didn't see it mentioned, so I'll add the Broken Windows Theory. Simply put, this is at the very least, indicative of serious failures in other areas.

MITM