I think I can add something to this article https://bestaucasinosonline.com/safe-and-secure-online-casinos/, it doesn't feel complete. So I need your opinion on the criteria of site's safety aside obvious things like HTTPS protocol
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Safety and security are defined differently around the world. If I were to rank secure sites if the meet, for example, Mozilla privacy and security standards (https://foundation.mozilla.org/en/privacynotincluded/about/methodology/).
Ideally, I would like to see that they have security certifications (ISO27001, SOC2 Type 2, etc) a bug bounty programme or some sort of third-party verification (pentesting, daily vulnerability scans, etc.)
There are many criteria that can be applied, so if you don't want to do the checks yourself (or have an advisor to do it), perhaps you can rely on certifications if they're available.
There are many ways to secure a website but my quick advise is to implement security at the early stages of your development. I've worked with a lot of developers and shown them how they can leverage tools like Snyk to review their code and provide fixes.
Most of the time security is done after the code is written and this causes delays. Some other times code reaches servers and then are scanned for vulnerabilities which is the same thing that hackers are doing to find exploits so I highly recommend DevSecOps as a start!