Last week, T-Mobile scrambled to patch a vulnerability uncovered by two University of California Berkeley students that made its Wi-Fi calling feature susceptible to man-in-the-middle attacks.
At issue in the students' research, published in full here (PDF), is the certificate implementation used in the feature. The now-patched bug in its Android feature used a certificate chain in which one certificate's name was the IP address of the server, and the second self-signed root certificate “is not included in standard Certificate Authority (CA) distributions”.
“This can mean that the root certificate was either built-in to T-Mobile’s client software, or they did not implement certificate validation correctly. In fact, the client does not seem to have any problems with sslsniff intercepting the connection, making us conclude the latter,” the students, Jethro Beekman and Christopher Thompson, write.
With a man-in-the-middle attack initiated, the researchers write, an attacker can capture the SIP message that provides the encryption key to be used for the calling session – allowing them to record all incoming and outgoing calls or SMS messages using the Wi-Fi calling feature.
“We verified the ability to record outgoing calls and incoming and outgoing text messages. We also verified the ability to change the destination phone number on outgoing calls by modifying sslsniff to change all occurrences of <sip: firstname.lastname@example.org>, replacing a single target phone number by a different one,” they continue.
The only vulnerable phones were those using the T-Mobile IMS stack, covering a number of Samsung and HTC phones. According to the researchers, T-Mobile claimed to have pushed an update to all affected users by 18 March. ®