I was answering a question on StackOverflow. The questioner has implemented Proxy TLS Interception which is very dangerous. I am reposting this thread on my website to share the details of my answer.
The Answer: Never Implement Proxy TLS Interception
I am adding this comment to scare the beeswax out of management.
TLS Interception is so dangerous that no reasonable company would implement it if they read the following.
The scenario in this example. I am an IT person responsible for a corporate proxy. The company has implemented TLS Interception and I control the proxy. I have no access to Google Cloud resources for my company. I am very smart and I understand Google Cloud IAM and OAuth very well. I am going to hack my company because maybe I did not get a raise (invent your own reason).
I wait for one of the managers who has an organization or project owner/editor level permissions to authenticate with Google Cloud. My proxy logs the HTTPS headers, body and response for everything going to
https://www.googleapis.com/oauth2/v4/token and a few more URLs.
Maybe the proxy is storing the logs on a Google Cloud Bucket or a SAN volume without solid authorization implemented. Maybe I am just a software engineer that finds the proxy log files laying about or easily accessed.
The corporate admin logs into his Google Account. I capture the returned OAuth Access Token. I can now impersonate the org admin for the next 3,600 seconds. Additionally, I capture the OAuth Refresh Token. I can now recreate OAuth Access Tokens at my will anytime I want until the Refresh Token is revoked which for most companies, they never do.
For doubters, study my Golang project which shows how to save OAuth Access Tokens and Refresh Tokens to a file for any Google Account used to authenticate. I can take this file home and be authorized without any authentication. This code will recreate the Access Token when it expires giving me almost forever access to any account these credentials are authorized for. Your internal IT resources will never know that I am doing this outside of your corporate network.
Note: Stackdriver Audit logging can capture the IP address, however, the identity will be the credentials that I stole. To hide my IP address, I would go to Starbucks or a public library a few hours drive from my home/job and do my deeds from there. Now figure out the where and the who for this hacker. This will give a forensics expert heartburn.
Note: This problem is not an issue with Google OAuth or Google Cloud. This is an example of a security problem that the company has deployed (TLS Interceptions). This style of technique will work for almost all authentication systems that I know of that do not use MFA.