app rejected with "Guideline 5.2.3 - Legal"

Guideline 5.2.3 - Legal We still found that your app provides potentially unauthorized access to third-party audio or video streaming, catalogs, and discovery services. Your app and its contents should not infringe upon the rights of another party. In the event your app infringes another party’s rights, you are responsible for any liability to Apple because of a claim. Next Steps To resolve this issue, please attach documentary evidence in the App Review Information section in App Store Connect evidencing that you have all necessary rights or permissions to the third-party audio or video streaming, catalogs, and discovery services in your app. In accordance with section 3.2(f) of the Apple Developer Program License Agreement, you acknowledge that submitting falsified or fraudulent documentation can result in the termination of your Apple Developer Program account and the removal of your apps from the App Store. Once Legal has reviewed your documentation and confirms its validity, we will proceed with the review of your app. Resources Review Guideline 5.2 Intellectual Property for more information about appropriate uses of third-party content and services on the App Store Review App Store Connect Help for more information on including notes in the App Review Information section of App Store Connect

The rejection is about providing a gateway for commercial copyright content to be used without permission by the user. Unless your app provides disclaimers against the use of copyright material or uses an algorithm to detect such things the rejection is pretty much outlining those potentials.

Doesn't every app that connects to the internet have potential for being a gateway for commercial content?

Sure, but it seems that reviewer has noticed that you don't just provide Internet access, but a focused or even direct access (e.g., providing links) to some content ("your app provides potentially unauthorized access to third-party audio or video streaming, catalogs, and discovery services") and that this access is in fact not casual but really part of the value of the app. As we don't see your code or your app, so it is hard to say more.

Anyway, the reviewer will have the final word. So you need to ask yourself, putting in their shoes (they rapidly see the app, get info from automatic code analysis, have not your level of knowledge of the app) … : what can make him / her think so ?

From there, either solve it or find arguments to prove the contrary, maybe on a very specific example from your app.

Did you solve the issue? I had the same problem in my radio-app. I added a customer written statement in AppStore Connect:

Hello iTunes Connect Team, 

This is to state that [Customer Name] has authorized [Your Name] from [Your Company Name] for its Mobile Application development and deployment. 

[Your Name] from [Your Company Name] have all the rights to publish the app under the name of [Product Name]. 

Our registration at Buma/Stemra is: [number] and at Sena is: [number] 

Please treat this document as a proof which indicates our rights. 

Thank you for your anticipation. 

Name: [Customer Name]
Autograph: ***