Is your smartphone telling every website you visit your telephone number?

O2 mobile users in the UK are venting on Twitter today, fuming at their discovery that their phone number is being shared with every website that they visit over the network.

I found a colleague who owns an iPhone on the O2 network, and we tried it out for ourselves. Making sure we turned off his WiFi connection, we used the O2 mobile network to access the web.

Sure enough, his mobile number was being secretly communicated to websites he visited, embedded inside an http header called HTTP_X_UP_CALLING_LINE_ID.

O2’s response so far is to tell concerned Twitter users that it is investigating the issue.

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