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T-Mobile Fixes Man-in-the-Middle Vulnerability in Wi-Fi Calling App

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The default “Wi-Fi Calling” feature on T-Mobile devices that lets milllions of Android users make phone calls over a wireless Internet connection contained a vulnerability that could have been exploited to perform man-in-the-middle (MiTM) attacks.

Graduate students Jethro Beekman and Christopher Thompson from the Electrical Engineering and Computer Sciences department at the University of California Berkeley uncovered the issue and reported it to T-Mobile’s security team in December. T-Mobile's senior manager for Mobile Assurance and Product Security, Darren Kress, said that as of yesterday the vulnerability had been resolved for all devices.

Beekman and Thompson found that the Wi-Fi Calling feature did not properly validate the transport layer security (TLS) certificate from the server with which it must communicate. Because of this, the researchers claim attackers could potentially forge themselves counterfeit certificates that would allow them to perform MiTM attacks by impersonating the T-Mobile server that handles the Wi-Fi Calling application. Attackers that perform a proper exploit could intercept, spy on, decrypt, and otherwise modify voice calls, text messages, or any other traffic transmitted via T-Mobile’s Wi-Fi Calling feature.

In a technical analysis of the exploit, the Berkeley graduate students examined the certificate chain that T-Mobile’s server was sending to their device. Two anomalies stood out: the name of the first cert was merely the IP address of the server, and the self-signed cert was not included in standard certificate authority distributions (nor was it recognized in various Web searches), which ended up meaning that T-Mobile hadn’t implemented certificate validation correctly and that their certificates could be easily spoofed.

From here they noticed that a session initiation protocol dialogue pops up when a TLS connection is established between T-Mobile and the device. The device authenticates itself by sending its phone number, International Mobile station Equipment Identity (IMEI), and International Mobile Subscriber Identity (IMSI) to the server. The server then responds with an INVITE message containing an encryption key that lets an attacker decrypt the SIP dialogue, which an attacker can use to record incoming and outgoing calls and texts, record, block, and reroute SIP traffic, spoof sender identification or message content, and impersonate incoming and outgoing calls.

The most effective way for an attacker to exploit this vulnerability is by being on the same, open wireless network as his or her victim.


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