@DTS Engineer , Thank you for your help. After I compared your code with mine, I realized that due to all the modifications I did in the code, I forgot to change the completion handler of the client challenge to .useCredential, that's why I was still seeing the issue. I was able able to remove NSAllowsArbitraryLoads, as well and the app connection worked as expected.

@DTS Engineer , thank you for your help, but I think there's still something wrong happening. I asked the guys from the backend to prevent the response 403 and if you run the CURL on terminal, you will get now a Hello world now. I'm still getting the same error when I debug my POC. Do you have any idea why is that happening?

@DTS Engineer , The print was actually working, I could see 🔐 Client cert challenge triggered in the logs. I also added the print command you asked and this is what I got: did receive challenge, method: NSURLAuthenticationMethodServerTrust, this one works did receive challenge, method: NSURLAuthenticationMethodClientCertificate, this one fails. I have created a github repository, where you can download the project and it also includes the certificates I'm using. Here's the link: https://github.com/renanstig/mtlspoc One more thing that might be interesting is that when I run this command here on my terminal, I get connected without any issues, that's how I know that my p12 is actually working fine. Obviously, I had to convert my ca.cer into a ca.pem to be able to run it. curl https://ss3.at.docu-tools.com \ --cert-type P12 \ --cert client_full.p12 \ --pass 1234 \ --cacert ca.pem \ -v

I'm also seeing the same issue with RealmSwift App Store Connect Operation Error The app references non-public selectors in Payload/Docu Tools 3.app/Frameworks/RealmSwift.framework/RealmSwift: get: App Store Connect Operation Error The app references non-public selectors in Payload/Docu Tools 3.app/Docu Tools 3: redoAction:, rotateAction:, saveAction:, transform:, undoAction: App Store Connect Operation Error The app references non-public selectors in Payload/Docu Tools 3.app/Frameworks/Realm.framework/Realm: authenticationDidCompleteWithError:, get: