What are the best network vulnerability scanners for detecting and mitigating zero-day vulnerabilities?

In my opinion and with some experience over my shoulders I would say that the best vulnerability scanner is Nessus by far. I've been using that tool for more than 6 years and definitely is very accurate.

Avoid using digital devices. 🤷🏻♂️ 0-day vulnerabilities, by definition, are unknown. Contrary to common suggestions here, scanners won't help you - they're trained on known data. Your best bet is to limit the blast radius. Proactive measures are limited (perhaps zero-trust and micro-segmentation - boo buzzwords), but most strategies are reactive. Simple as that.

One of the most basic things you can do is build and maintain an inventory of your whole environment. Not just a simple count of servers but down to the level of OS version, RAM, EoL/S dates etc. Once you’ve got this, continually assess the environment against information from vendors or sites like OpenCVE. Vulnerability scanning is just part of the picture. There is a whole lot more that needs to be considered when protecting your environment.

Zero-day vulnerabilities are security holes for which there are no patches available because software makers are unaware of them. Traditional scanners rely on existing vulnerability databases, which makes it difficult to detect these flaws. Knowing that these vulnerabilities can be exploited before they are found and fixed is one of the fundamentals of zero-day vulnerabilities. Behavioural analysis, heuristic techniques, and anomaly detection are more effective at detecting zero-day exploits than signature-based strategies. Zero-day vulnerability detection can be improved by using cutting-edge security techniques that use AI and machine learning, as well as by keeping up with evolving threats.

Zero-Day itself indicates that there is no patches available or produced, so this is a loophole in security for which there is no defence available, also the owner of the product don’t know this is present in their product and they are having zero day to provide a solution on this.

A scanner will not fix the problem of a zero-day vulnerability. It is a constant Red and Blue Team responsibility to keep monitoring the network and especially the behavior of the network infrastructure. It is imperative to promote daily operations lead by the Red Team Department and also some Proof of concept. Although some successful exploitations ways will show up after the PoC, keep in mind that it is better than it was your Team who discovered it.

Being able to perform credential scans or agent based scans to aid in human operated threat hunting exercises is vital for a vulnerability scanner to be useful in identifying assets vulnerable to zero day vulnerabilities. Having the knowledge and capability of constructing testing plugins for the scanner should a goal of all vulnerability management teams. Waiting on scanner vendors to develop testing modules may not meet your risk appetite or threat model.

Regular updates are very important because when we do not update our scanners, then they are not updated with the new vulnerabilities that are coming day by day, So it is important to update the scanners that we use, So that the database should be updated and when we use our vulnerability scanners, they are able to detect the new vulnerabilities

I agree. With almost all security related tools, not updating them in a regular basis will prevent you from detecting known issues or testing known vulnerabilities or confirming recent patches have been deployed across your scanned environment.

There are some other circumstances that are necessary to fulfill in order to exploit a zero-day vulnerability. Most of the time, that particular vulnerability must be aligned with other deficiencies in the network or the asset. Keep in mind that a vulnerability scanner won't fix the problem, but will alert you to take action.