Vodafone India is exposing website surfer/user's phone number to the partner website when using mobile data plan. It might be offering hidden API to those partner websites to uniquely identify users. I have noticed it around 1-year before, thought about blogging, but forgot it at that time. Now, I have got some time and so thought of explaining the issue.
How I noticed the issue?
Sometime ago, I've bought SanDisk microSD card. In the wrapper, they've mentioned about some offer about hungama (Still available in SanDisk website at www.sandisk.in/campaign/landing/hungama/offer_three). The URL mentioned was offers.hungama.com/sandisk3/ and when I opened that in my mobile browser, it was prompting with Welcome, 91-98XXXXXXXX.
I was shocked as I never seen anything like that before. So, I immediately opened the same URL in my desktop browser and got completely different page:
|Desktop version of the same page|
And, again I tried to open the same page in mobile browser with Wi-Fi alone (by disabling Mobile Data), and now also they couldn't identify my mobile number. So, I realized that it might be something to do with the network (Vodafone). Initially I thought that they may be decoding the phone number from IP address. So, I asked Heleena (my wife) to figure out some pattern for that. Then it turn out that there is no definite pattern. I have also analyzed HTTP headers when browsing through Vodafone internet. Then, realized that hungama might be utilizing some hidden API from Vodafone to decode user's IP to mobile number.
At least for me, this looks like a serious privacy issue. hungama at least openly exposes the phone number to users, so that users can know that hungama can read their numbers/identity. But, there may be many sites--especially many social media sites such as Facebook, Twitter, Google, etc may still be uniquely identifying users without revealing it at all.