Ramblings & ephemera

How the Greek cell phone network was compromised

From Vassilis Prevelakis and Diomidis Spinellis’ “The Athens Affair” (IEEE Spectrum: July 2007):

On 9 March 2005, a 38-year-old Greek electrical engineer named Costas Tsalikidis was found hanged in his Athens loft apartment, an apparent suicide. It would prove to be merely the first public news of a scandal that would roil Greece for months.

The next day, the prime minister of Greece was told that his cellphone was being bugged, as were those of the mayor of Athens and at least 100 other high-ranking dignitaries, including an employee of the U.S. embassy.

The victims were customers of Athens-based Vodafone-Panafon, generally known as Vodafone Greece, the country’s largest cellular service provider; Tsalikidis was in charge of network planning at the company.

We now know that the illegally implanted software, which was eventually found in a total of four of Vodafone’s Greek switches, created parallel streams of digitized voice for the tapped phone calls. One stream was the ordinary one, between the two calling parties. The other stream, an exact copy, was directed to other cellphones, allowing the tappers to listen in on the conversations on the cellphones, and probably also to record them. The software also routed location and other information about those phone calls to these shadow handsets via automated text messages.

The day after Tsalikidis’s body was discovered, CEO Koronias met with the director of the Greek prime minister’s political office. Yiannis Angelou, and the minister of public order, Giorgos Voulgarakis. Koronias told them that rogue software used the lawful wiretapping mechanisms of Vodafone’s digital switches to tap about 100 phones and handed over a list of bugged numbers. Besides the prime minister and his wife, phones belonging to the ministers of national defense, foreign affairs, and justice, the mayor of Athens, and the Greek European Union commissioner were all compromised. Others belonged to members of civil rights organizations, peace activists, and antiglobalization groups; senior staff at the ministries of National Defense, Public Order, Merchant Marine, and Foreign Affairs; the New Democracy ruling party; the Hellenic Navy general staff; and a Greek-American employee at the United States Embassy in Athens.

First, consider how a phone call, yours or a prime minister’s, gets completed. Long before you dial a number on your handset, your cellphone has been communicating with nearby cellular base stations. One of those stations, usually the nearest, has agreed to be the intermediary between your phone and the network as a whole. Your telephone handset converts your words into a stream of digital data that is sent to a transceiver at the base station.

The base station’s activities are governed by a base station controller, a special-purpose computer within the station that allocates radio channels and helps coordinate handovers between the transceivers under its control.

This controller in turn communicates with a mobile switching center that takes phone calls and connects them to call recipients within the same switching center, other switching centers within the company, or special exchanges that act as gateways to foreign networks, routing calls to other telephone networks (mobile or landline). The mobile switching centers are particularly important to the Athens affair because they hosted the rogue phone-tapping software, and it is there that the eavesdropping originated. They were the logical choice, because they are at the heart of the network; the intruders needed to take over only a few of them in order to carry out their attack.

Both the base station controllers and the switching centers are built around a large computer, known as a switch, capable of creating a dedicated communications path between a phone within its network and, in principle, any other phone in the world. Switches are holdovers from the 1970s, an era when powerful computers filled rooms and were built around proprietary hardware and software. Though these computers are smaller nowadays, the system’s basic architecture remains largely unchanged.

Like most phone companies, Vodafone Greece uses the same kind of computer for both its mobile switching centers and its base station controllers—Ericsson’s AXE line of switches. A central processor coordinates the switch’s operations and directs the switch to set up a speech or data path from one phone to another and then routes a call through it. Logs of network activity and billing records are stored on disk by a separate unit, called a management processor.

The key to understanding the hack at the heart of the Athens affair is knowing how the Ericsson AXE allows lawful intercepts—what are popularly called “wiretaps.” Though the details differ from country to country, in Greece, as in most places, the process starts when a law enforcement official goes to a court and obtains a warrant, which is then presented to the phone company whose customer is to be tapped.

Nowadays, all wiretaps are carried out at the central office. In AXE exchanges a remote-control equipment subsystem, or RES, carries out the phone tap by monitoring the speech and data streams of switched calls. It is a software subsystem typically used for setting up wiretaps, which only law officers are supposed to have access to. When the wiretapped phone makes a call, the RES copies the conversation into a second data stream and diverts that copy to a phone line used by law enforcement officials.

Ericsson optionally provides an interception management system (IMS), through which lawful call intercepts are set up and managed. When a court order is presented to the phone company, its operators initiate an intercept by filling out a dialog box in the IMS software. The optional IMS in the operator interface and the RES in the exchange each contain a list of wiretaps: wiretap requests in the case of the IMS, actual taps in the RES. Only IMS-initiated wiretaps should be active in the RES, so a wiretap in the RES without a request for a tap in the IMS is a pretty good indicator that an unauthorized tap has occurred. An audit procedure can be used to find any discrepancies between them.

It took guile and some serious programming chops to manipulate the lawful call-intercept functions in Vodafone’s mobile switching centers. The intruders’ task was particularly complicated because they needed to install and operate the wiretapping software on the exchanges without being detected by Vodafone or Ericsson system administrators. From time to time the intruders needed access to the rogue software to update the lists of monitored numbers and shadow phones. These activities had to be kept off all logs, while the software itself had to be invisible to the system administrators conducting routine maintenance activities. The intruders achieved all these objectives.

The challenge faced by the intruders was to use the RES’s capabilities to duplicate and divert the bits of a call stream without using the dialog-box interface to the IMS, which would create auditable logs of their activities. The intruders pulled this off by installing a series of patches to 29 separate blocks of code, according to Ericsson officials who testified before the Greek parliamentary committee that investigated the wiretaps. This rogue software modified the central processor’s software to directly initiate a wiretap, using the RES’s capabilities. Best of all, for them, the taps were not visible to the operators, because the IMS and its user interface weren’t used.

The full version of the software would have recorded the phone numbers being tapped in an official registry within the exchange. And, as we noted, an audit could then find a discrepancy between the numbers monitored by the exchange and the warrants active in the IMS. But the rogue software bypassed the IMS. Instead, it cleverly stored the bugged numbers in two data areas that were part of the rogue software’s own memory space, which was within the switch’s memory but isolated and not made known to the rest of the switch.

That by itself put the rogue software a long way toward escaping detection. But the perpetrators hid their own tracks in a number of other ways as well. There were a variety of circumstances by which Vodafone technicians could have discovered the alterations to the AXE’s software blocks. For example, they could have taken a listing of all the blocks, which would show all the active processes running within the AXE—similar to the task manager output in Microsoft Windows or the process status (ps) output in Unix. They then would have seen that some processes were active, though they shouldn’t have been. But the rogue software apparently modified the commands that list the active blocks in a way that omitted certain blocks—the ones that related to intercepts—from any such listing.

In addition, the rogue software might have been discovered during a software upgrade or even when Vodafone technicians installed a minor patch. It is standard practice in the telecommunications industry for technicians to verify the existing block contents before performing an upgrade or patch. We don’t know why the rogue software was not detected in this way, but we suspect that the software also modified the operation of the command used to print the checksums—codes that create a kind of signature against which the integrity of the existing blocks can be validated. One way or another, the blocks appeared unaltered to the operators.

Finally, the software included a back door to allow the perpetrators to control it in the future. This, too, was cleverly constructed to avoid detection. A report by the Hellenic Authority for the Information and Communication Security and Privacy (the Greek abbreviation is ADAE) indicates that the rogue software modified the exchange’s command parser—a routine that accepts commands from a person with system administrator status—so that innocuous commands followed by six spaces would deactivate the exchange’s transaction log and the alarm associated with its deactivation, and allow the execution of commands associated with the lawful interception subsystem. In effect, it was a signal to allow operations associated with the wiretaps but leave no trace of them. It also added a new user name and password to the system, which could be used to obtain access to the exchange.

…Security experts have also discovered other rootkits for general-purpose operating systems, such as Linux, Windows, and Solaris, but to our knowledge this is the first time a rootkit has been observed on a special-purpose system, in this case an Ericsson telephone switch.

So the investigators painstakingly reconstructed an approximation of the original PLEX source files that the intruders developed. It turned out to be the equivalent of about 6500 lines of code, a surprisingly substantial piece of software.

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