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Ports that are blocked by browsers (2023)
Today I was experimenting with a small project using Docker and Flask. I just created two identical services and exposed them on two different ports: nothing fancy. However, to my surprise, only one of the two seemed to be working.
Through this vulnerability, an attacker could forge malicious HTML code to send data to other services used by the victim (for example crafting spam emails, or printing through a network printer). The wording in Firefox’s message allows us to understand one more thing: the request we’re sending to port 6000 never reaches the webserver, because it’s being immediately canceled by the browser. As you can see, this request doesn’t get blocked and it correctly shows the output produced by the Python webserver.
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