So I have a restful API that is responsible for handling the licenses for my app, it records
Currently what happens is the user authenticate via a third party auth service (firebase) then the app takes the user ID from firebase auth response and get the license info without any authorization on the license API. Essentially if you have a user ID you can get all the info mentioned above
My question: Is this a security flaw? or is this acceptable considering that it is very hard for a malicious user to obtain another user's ID and even if he obtains the ID the info is not really that useful
This is a data protection question, not a programming question and you watch it from the wrong perspective in the comments. Information security and data protection and totally different jobs with different mindset.
User ID - personal data
User email - personal data
Username - personal data
license expiry date - probably company secret
What do we protect here?
Some not that sensitive personal data like username and email, which can be guessed by checking the companies webpage or linkedin. The license expiry date is not that important I guess, maybe the company wants to keep it in secret, who knows, better to ask them. The user ID if it is used only here is not a big deal.
What happens when somebody gets the user ID?
They can watch the upper data 24/7 until you add proper authentication or change the User ID. It does not matter how they get it, let's assume they can. They can watch HTTP packets, or an employee sends it by accident to somebody, or they steal this part from the database or they get it from HTTP server logs when you throw out a faulty HDD, who cares?!
What would be the effect to your company?
If your customers don't care about it, then ok. If they would terminate the contract immediately and report the data protection incident to an authority, you will be sued, etc. then it is an issue...
What else can happen?
Do you have support? Can somebody use the User ID and the upper infos to trick your support into terminating the contract and sending back money to the bank account they give? If so then it is an issue either.
Risk assessment is about checking the probabilities, motivations, consequences, etc. of something like the upper and mitigate them if necessary with risk reducing protective measures. Such a thing can be for example encrypting and signing the User ID and the session expiration date before sending it to your API, if it is possible to send encrpyted signed JWT by FireBase.