Biblio

This paper presents the results of a cryptographic analysis of the protocols used by the European Rail Traffic Management System (ERTMS). A stack of three protocols secures the communication between trains and trackside equipment; encrypted radio communication is provided by the GSM-R protocol, on top of this the EuroRadio protocol provides authentication for a train control application-level protocol. We present an attack which exploits weaknesses in all three protocols: GSM-R has the same well known weaknesses as the GSM protocol, and we present a new collision attack against the EuroRadio protocol. Combined with design weaknesses in the application-level protocol, these vulnerabilities allow an attacker, who observes a MAC collision, to forge train control messages. We demonstrate this attack with a proof of concept using train control messages we have generated ourselves. Currently, ERTMS is only used to send small amounts of data for short sessions, therefore this attack does not present an immediate danger. However, if EuroRadio was to be used to transfer larger amounts of data trains would become vulnerable to this attack. Additionally, we calculate that, under reasonable assumptions, an attacker who could monitor all backend control centres in a country the size of the UK for 45 days would have a 1% chance of being able to take control of a train.

Certificate verification is a crucial stage in the establishment of a TLS connection. A common security flaw in TLS implementations is the lack of certificate hostname verification but, in general, this is easy to detect. In security-sensitive applications, the usage of certificate pinning is on the rise. This paper shows that certificate pinning can (and often does) hide the lack of proper hostname verification, enabling MITM attacks. Dynamic (black-box) detection of this vulnerability would typically require the tester to own a high security certificate from the same issuer (and often same intermediate CA) as the one used by the app. We present Spinner, a new tool for black-box testing for this vulnerability at scale that does not require purchasing any certificates. By redirecting traffic to websites which use the relevant certificates and then analysing the (encrypted) network traffic we are able to determine whether the hostname check is correctly done, even in the presence of certificate pinning. We use Spinner to analyse 400 security-sensitive Android and iPhone apps. We found that 9 apps had this flaw, including two of the largest banks in the world: Bank of America and HSBC. We also found that TunnelBear, one of the most popular VPN apps was also vulnerable. These apps have a joint user base of tens of millions of users.