Welcome back! This is the third post in the reverse engineering series. The first post was reverse engineering Soundcloud API and the second one was reverse engineering Facebook API to download public videos. In this post we will take a look at downloading private videos. We will reverse engineer the API calls made by Facebook and will try to figure out how we can download videos in the HD format (when available).
Step 1: Recon
The very first step is to open up a private video in an incognito tab just to make sure we can not access it without logging it. This should be the response from Facebook:
This confirms that we can not access the video without logging in. Sometimes this is pretty obvious but it doesn’t hurt to check.
We know of our first step. It is to figure out a way to log-into Facebook using Python. Only after that can we access the video. Let’s login using the browser and check what information is required to log-in.
I won’t go into much detail for this step. The gist is that while logging in, the desktop website and the mobile website require roughly the same POST parameters but interestingly if you log-in using the mobile website you don’t have to supply a lot of additional information which the desktop website requires. You can get away with doing a POST request to the following URL with your username and password:
https://m.facebook.com/login.php
We will later see that the subsequent API requests will require a fb_dtsg parameter. The value of this parameter is embedded in the HTML response and can easily be extracted using regular expressions or a DOM parsing library.
Let’s continue exploring the website and the video API and see what we can find.
Just like what we did in the last post, open up the video, monitor the XHR requests in the Developer Tools and search for the MP4 request.
Next step is to figure out where the MP4 link is coming from. I tried searching the original HTML page but couldn’t find the link. This means that Facebook is using an XHR API request to get the URL from the server. We need to search through all of the XHR API requests and check their responses for the video URL. I did just that and the response of the third API request contained the MP4 link:
#api #coding #yasoob #engineering facebook
