To be fair, Rails has a lot more baked in base security than erlang did. Yes Rails had it's fair share of vulnerabilities. I remember there was a major eval issue in some deserializer code, but things such as sql injections or using POST over query strings is just basic knowledge almost every mediocre+ rails dev knows. They picked erlang for their scalability, but as a result had to write a lot of other things from scratch. WhatsApp is still an interesting engineering case study.

it's not a rails vulnerability though. This would happen irrespective of the language/framework if the developer decide to forego basic sanity checks. This would have been prevented with basic input validations (in any of the layers before it hit model, and model itself), or the organization did basic security testing.

You miss the point. If you use rails provided primitives it absolutely wouldn't happen. So it's fair to categorize this as a rookie coding mistake. The author took code that had these security primitives baked in the framework and REMOVED them, replacing it with insecure custom sql, which in turn led to the vulnerability.

Hah! Too great. Ok, so no good code in startups.