Many IoT devices (or Windows when the LAN network location is set to “Private”) expose a wider surface area to local network addresses. Having a competent firewall on your residential router is still useful, especially for those that have no idea how to configure their endpoints securely.

Comparing a residential router to a network operator’s router is spurious: those routers don’t perform any sort of filtering for the public internet traffic flowing through them.

Is there any residential router that exposes internal endpoints be default? I've yet to come across one that does not have a deny-any policy on it's WAN interface and has incoming destination NATs setup up.

What use is reducing the attack surface of a device which only ever initiates connections?

Edit: also there are network operators that block customer traffic on certain ports liike NetBIOS, SMB or SMTP to name a few.

Many happily do Network PNP, etc. which allows them to open ports on the public facing side of the router.

If your home router is compromised (which is what the parent comment was talking about, considering it mentioned CVEs) the attacker who now controls it can easily make connections to devices on your network via the router’s local address.

As for how the router that is theoretically not accepting incoming connections from the internet itself gets compromised in the first place: among other issues some routers can be RCEd by a webpage visited by someone inside the LAN[1]. That’s just one example, you can find tons of these if you search for router vulnerabilities. In practice out of date routers end up in botnets frequently.

It has nothing to do with network operators blocking SMB traffic; the attacker can communicate with the router via whatever C2 mechanism they put in the malware, which probably won’t even involve opening a port on the router. The SMB or what have you to the endpoint would be entirely within the LAN.

[1]: https://www.malwarebytes.com/blog/news/2023/02/arris-vulnera...

If the router were compromised, what use would "Having a competent firewall on your residential router" be?

The edit was in response to "network operator's routers [...] don't perform any sort of filtering" and had nothing to do with C2 traffic?

The original comment I responded to said “There are countless routers in between you and your destination which you can't audit anyway. End devices long since consider the routers to be compromised and have everything verified and encrypted in transit.” My point is that having a home router that does not allow incoming connections to the devices behind it - whether that’s due to compromise or misconfiguration - prevents substantial attacks. It’s hard to call a compromised router a “competent firewall”.

I mean, your router is the single key to your kingdom—-your local network. If you don’t treat all your local devices as hostile (which is a reasonable thing to do but almost no one does it), then having your router in shape is somewhere in the important to critical range.