path: root/2010/gsm_security-dc2010/gsm_security-openbsc.tex

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% Copyright 2004 by Till Tantau <tantau@users.sourceforge.net>.
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\title{GSM security nightmares}

\subtitle
{Running your own GSM network with OpenBSC}

\author{Harald Welte}

\institute
{gnumonks.org\\gpl-violations.org\\OpenBSC\\airprobe.org\\hmw-consulting.de}
% - Use the \inst command only if there are several affiliations.
% - Keep it simple, no one is interested in your street address.

\date[DORS/CLUC 2010] % (optional, should be abbreviation of conference name)
{DORS/CLUC 2010, May 2010, Zagreb/Croatia}
% - Either use conference name or its abbreviation.
% - Not really informative to the audience, more for people (including
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\subject{GSM Security}
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% Structuring a talk is a difficult task and the following structure
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\begin{frame}{About the speaker}
\begin{itemize}
	\item Using + playing with Linux since 1994
	\item Kernel / bootloader / driver / firmware development since 1999
	\item IT security specialist, focus on network protocol security
	\item Board-level Electrical Engineering
	\item Always looking for interesting protocols (RFID, DECT, GSM)
\end{itemize}
\end{frame}

\section{GSM/3G security}

\begin{frame}{GSM/3G protocol security}
\begin{itemize}
	\item Observation
	\begin{itemize}
		\item Both GSM/3G and TCP/IP protocol specs are publicly available
		\item The Internet protocol stack (Ethernet/Wifi/TCP/IP) receives lots of scrutiny
		\item GSM networks are as widely deployed as the Internet
		\item Yet, GSM/3G protocols receive no such scrutiny!
	\end{itemize}
	\item There are reasons for that:
	\begin{itemize}
		\item GSM industry is extremely closed (and closed-minded)
		\item Only about 4 closed-source protocol stack implementations
		\item GSM chipset makers never release any hardware documentation
	\end{itemize}
\end{itemize}
\end{frame}

\subsection{The closed GSM industry}

\begin{frame}{The closed GSM industry}{Handset manufacturing side}
\begin{itemize}
	\item Only very few companies build GSM/3.5G baseband chips today
	\begin{itemize}
		\item Those companies buy the operating system kernel and the protocol stack from third parties
	\end{itemize}
	\item Only very few handset makers are large enough to become a customer
	\begin{itemize}
		\item Even they only get limited access to hardware documentation
		\item Even they never really get access to the firmware source
	\end{itemize}
\end{itemize}
\end{frame}

\begin{frame}{The closed GSM industry}{Network manufacturing side}
\begin{itemize}
	\item Only very few companies build GSM network equipment
	\begin{itemize}
		\item Basically only Ericsson, Nokia-Siemens, Alcatel-Lucent and Huawei
		\item Exception: Small equipment manufacturers for picocell / nanocell / femtocells / measurement devices and law enforcement equipment
	\end{itemize}
	\item Only operators buy equipment from them
	\item Since the quantities are low, the prices are extremely high
	\begin{itemize}
		\item e.g. for a BTS, easily 10-40k EUR
	\end{itemize}
\end{itemize}
\end{frame}

\begin{frame}{The closed GSM industry}{Operator side}
\begin{itemize}
	\item Operators are mainly banks today
	\item Typical operator outsources
	\begin{itemize}
		\item Billing
		\item Network planning / deployment / servicing
	\end{itemize}
	\item Operator just knows the closed equipment as shipped by manufacturer
	\item Very few people at an operator have knowledge of the protocol beyond what's needed for operations and maintenance
\end{itemize}
\end{frame}

\subsection{Security implications}

\begin{frame}{The closed GSM industry}{Security implications}
The security implications of the closed GSM industry are:
\begin{itemize}
	\item Almost no people who have detailed technical knowledge outside the protocol stack or GSM network equipment manufacturers
	\item No independent research on protocol-level security
	\begin{itemize}
		\item If there's security research at all, then only theoretical (like the A5/2 and A5/1 cryptanalysis)
		\item Or on application level (e.g. mobile malware)
	\end{itemize}
	\item No open source protocol implementations
	\begin{itemize}
		\item which are key for making more people learn about the protocols
		\item which enable quick prototyping/testing by modifying existing code
	\end{itemize}
\end{itemize}
\end{frame}

\begin{frame}{Security analysis of GSM}{How would you get started?}
If you were to start with GSM protocol level security analysis, where and
how would you start?
\begin{itemize}
	\item On the handset side?
	\begin{itemize}
		\item Difficult since GSM firmware and protocol stacks are closed and proprietary
		\item Even if you want to write your own protocol stack, the layer 1 hardware and signal processing is closed and undocumented, too
		\item Known attempts
		\begin{itemize}
			\item The TSM30 project as part of the THC GSM project
			\item mados, an alternative OS for Nokia DTC3 phones
		\end{itemize}
		\item none of those projects successful so far
	\end{itemize}
\end{itemize}
\end{frame}

\begin{frame}{Security analysis of GSM}{How would you get started?}
If you were to start with GSM protocol level security analysis, where and
how would you start?
\begin{itemize}
	\item On the network side?
	\begin{itemize}
		\item Difficult since equipment is not easily available and normally extremely expensive
		\item However, network is very modular and has many standardized/documented interfaces
		\item Thus, if equipment is available, much easier/faster progress
	\end{itemize}
\end{itemize}
\end{frame}

\begin{frame}{Security analysis of GSM}{The bootstrapping process}
\begin{itemize}
	\item Read GSM specs day and night (> 1000 PDF documents) ;)
	\item Gradually grow knowledge about the protocols
	\item Obtain actual GSM network equipment (BTS)
	\item Try to get actual protocol traces as examples
	\item Start a complete protocol stack implementation from scratch
	\item Finally, go and play with GSM protocol security
\end{itemize}
\end{frame}


\subsection{The GSM network}

\begin{frame}{The GSM network}
  \begin{figure}[h]
  \centering
  \includegraphics[width=100mm]{gsm_network.png}
  \end{figure}
\end{frame}

\begin{frame}{GSM network components}
  \begin{itemize}
    \item The BSS (Base Station Subsystem)
    \begin{itemize}
      \item MS (Mobile Station): Your phone
      \item BTS (Base Transceiver Station): The {\em cell tower}
      \item BSC (Base Station Controller): Controlling up to hundreds of BTS
    \end{itemize}
    \item The NSS (Network Sub System)
    \begin{itemize}
      \item MSC (Mobile Switching Center): The central switch
      \item HLR (Home Location Register): Database of subscribers
      \item AUC (Authentication Center): Database of authentication keys
      \item VLR (Visitor Location Register): For roaming users
      \item EIR (Equipment Identity Register): To block stolen phones
    \end{itemize}
  \end{itemize}
\end{frame}

\begin{frame}{GSM network interfaces}
  \begin{itemize}
    \item Um: Interface between MS and BTS
    \begin{itemize}
	\item the only interface that is specified over radio
    \end{itemize}
    \item A-bis: Interface between BTS and BSC
    \item A: Interface between BSC and MSC
    \item B: Interface between MSC and other MSC
  \end{itemize}
  GSM networks are a prime example of an asymmetric distributed network,
  very different from the end-to-end transparent IP network.
\end{frame}


\subsection{The GSM protocols}

\begin{frame}{GSM network protocols}{On the Um interface}
  \begin{itemize}
    \item Layer 1: Radio Layer, TS 04.04
    \item Layer 2: LAPDm, TS 04.06
    \item Layer 3: Radio Resource, Mobility Management, Call Control: TS 04.08
    \item Layer 4+: for USSD, SMS, LCS, ...
  \end{itemize}
\end{frame}

\begin{frame}{GSM network protocols}{On the A-bis interface}
  \begin{itemize}
    \item Layer 1: Typically E1 line, TS 08.54
    \item Layer 2: A variant of ISDN LAPD with fixed TEI's, TS 08.56
    \item Layer 3: OML (Organization and Maintenance Layer, TS 12.21)
    \item Layer 3: RSL (Radio Signalling Link, TS 08.58)
    \item Layer 4+: transparent messages that are sent to the MS via Um
  \end{itemize}
\end{frame}


\section{Implementing GSM protocols} 

\subsection{Getting started}

\begin{frame}{Implementing GSM protocols}{How I got started!}
\begin{itemize}
	\item In September 2008, we were first able to make the BTS active and see it on a phone
	\begin{itemize}
		\item This is GSM900 BTS with 2 TRX at 2W output power (each)
		\item A 48kg monster with attached antenna
		\item 200W power consumption, passive cooling
		\item E1 physical interface
	\end{itemize}
	\item I didn't have much time at the time (day job at Openmoko)
	\item Started to read up on GSM specs whenever I could
	\item Bought a HFC-E1 based PCI E1 controller, has mISDN kernel support
	\item Found somebody in the GSM industry who provided protocol traces
\end{itemize}
\end{frame}

\subsection{Timeline}

\begin{frame}{Implementing GSM protocols}{Timeline}
\begin{itemize}
	\item In November 2008, I started the development of OpenBSC
	\item In December 2008, we did a first demo at 25C3
	\item In January 2009, we had full voice call support
	\item In June 2009, I started with actual security related stuff
	\item In August 2009, we had the first field test with 2BTS and > 860 phones
\end{itemize}
\end{frame}

\subsection{OpenBSC}

\begin{frame}{Security analysis of GSM}{OpenBSC}
What is OpenBSC
\begin{itemize}
	\item A {\em GSM network in a box} software
	\item Implements minimal subset of BSC, MSC, HLR, SMSC
	\item Is Free and Open Source Software licensed under GNU GPL
	\item Supports Siemens BS-11 BTS (E1) and ip.access nanoBTS (IP based)
	\item Has classic 2G signalling, voice and SMS support
	\item Implements various GSM protocols like
	\begin{itemize}
		\item A-bis RSL (TS 08.58) and OML (TS 12.21)
		\item TS 04.08 Radio Resource, Mobility Management, Call Control
		\item TS 04.11 Short Message Service
	\end{itemize}
\end{itemize}
\end{frame}

\section{Security analysis}

\subsection{Theory}

\begin{frame}{Known GSM security problems}{Scientific papers, etc}
\begin{itemize}
	\item No mutual authentication between phone and network
	\begin{itemize}
		\item leads to rogue network attacks
		\item leads to man-in-the-middle attacks
		\item is what enables IMSI-catchers
	\end{itemize}
	\item Weak encryption algorithms
	\item Encryption is optional, user does never know when it's active or not
	\item DoS of the RACH by means of channel request flooding
	\item RRLP (Radio Resource Location Protocol)
	\begin{itemize}
		\item the network can obtain GPS fix or even raw GSM data from the phone
		\item combine that with the network not needing to authenticate itself
	\end{itemize}
\end{itemize}
\end{frame}

\subsection{The Baseband}

\begin{frame}{Known GSM security problems}{The Baseband side}
\begin{itemize}
	\item GSM protocol stack always runs in a so-called baseband processor (BP)
	\item What is the baseband processor
	\begin{itemize}
		\item Typically ARM7 (2G/2.5G phones) or ARM9 (3G/3.5G phones)
		\begin{itemize}
			\item Runs some RTOS (often Nucleus, sometimes L4)
			\item No memory protection between tasks
		\end{itemize}
		\item Some kind of DSP, model depends on vendor
		\begin{itemize}
			\item Runs the digital signal processing for the RF Layer 1
			\item Has hardware peripherals for A5 encryption
		\end{itemize}
	\end{itemize}
	\item The software stack on the baseband processor
	\begin{itemize}
		\item is written in C and assembly
		\item lacks any modern security features (stack protection, non-executable pages, address space randomization, ..)
	\end{itemize}
\end{itemize}
\end{frame}

\subsection{Observations}

\begin{frame}{Interesting observations}{Learned from implementing the stack}
While developing OpenBSC, we observed a number of interesting
\begin{itemize}
	\item Many phones use their TMSI from the old network when they roam to a new network
	\item Various phones crash when confronted with incorrect messages.  We didn't even start to intentionally send incorrect messages (!)
	\item There are tons of obscure options on the GSM spec which no real network uses.  Potential attack vector by using rarely tested code paths.
\end{itemize}
\end{frame}

\subsection{GSM Protocol Fuzzing}

\begin{frame}{GSM Protocol Fuzzing}{Theoretical basis}
How to do GSM protocol fuzzing
\begin{itemize}
	\item From the handset to the network
	\begin{itemize}
		\item Basically impossible due to closeness of baseband
		\item However, some incomplete projects working on it
	\end{itemize}
	\item From the network side
	\begin{itemize}
		\item Easy in case of {\em rogue network} attacks
		\item Fuzzing target is the GSM stack in the baseband processor
	\end{itemize}
	\item As an A-bis man in the middle
	\begin{itemize}
		\item Needs access to an A-bis interface of an actual network
		\item Very attractive, since no encryption and ability to fuzz both network and handset
	\end{itemize}
\end{itemize}
\end{frame}

\begin{frame}{A-bis injection}{for A-bis over IP}
How to do inject messages into A-bis over IP?
\begin{itemize}
	\item Problem
	\begin{itemize}
		\item A-bis/IP uses one TCP connection for OML and RSL messages
		\item OML initialization is essential for BTS to become operational
		\item TCP makes insertion of additional messages relatively hard
	\end{itemize}
	\item Solution: Build an {\em A-bis injection proxy}
	\begin{itemize}
		\item Transparently pass OML and RSL packets between BTS and BSC
		\item Add additional stateless UDP sockets for injecting messages, one socket each for
	\end{itemize}
	\begin{itemize}
		\item injecting OML/RSL to the network
		\item injecting OML/RSL to the BTS
	\end{itemize}
\end{itemize}
\end{frame}

\begin{frame}{A-bis Injection Proxy}{Principle of operation}
\begin{itemize}
	\item Proxy needs to be brought between BTS and BSC
	\item Luckily, A-bis/IP SSL support not always used
	\item Thus, physical access to the Ethernet link sufficient
	\item Configure system with two interfaces
	\begin{itemize}
		\item BSC-facing interface has IP of BTS
		\item BTS-facing interface has IP of BSC / default gw
	\end{itemize}
	\item BTS will make TCP connection to proxy
	\item proxy will make independent TCP connection to BSC
\end{itemize}
\end{frame}

\begin{frame}{scapy GSM support}{The actual fuzzing}
How to actually craft the packets for the fuzzing
\begin{itemize}
	\item GSM has many, many protocols
	\item Writing custom code will be a hard-coded special case for each of them
	\item Solution: Use scapy and implement the GSM protocols as scapy "layers"
	\begin{itemize}
		\item IPA protocol header
		\item RSL protocol layer
		\item RLL data indication / data request
		\item GSM 04.08 RR / MM / CC messages
	\end{itemize}
\end{itemize}
\end{frame}

\begin{frame}{OpenBSC silent calls}{A more elegant fuzzing interface}
\begin{itemize}
	\item Injection at the A-bis level has many problems
	\begin{itemize}
		\item you can only do it while a call is active
		\item you simply piggy-back on existing RR connections
	\end{itemize}
	\item The OpenBSC {\em silent call} feature can help
	\begin{itemize}
		\item we use OpenBSC to establish a RR connection
		\item in the GSM master/slave model, the phone will not close a connection unless told to do so
		\item we then send arbitrary data to the phone and receive its responses
		\item this currently only works from within OpenBSC, but we'll provide UDP injection sockets soon
	\end{itemize}
\end{itemize}
\end{frame}

\section{Summary}

\subsection{What we've learned}

\begin{frame}{Summary}{What we've learned}
\begin{itemize}
	\item The GSM industry is making security analysis very difficult
	\item It is well-known that the security level of the GSM stacks is very low
	\item We now have multiple solutions for sending arbitrary protocol data
	\begin{itemize}
		\item From a rogue network to phones (OpenBSC, OpenBTS)
		\item From an a-bis proxy to the network or the phones
	\end{itemize}
	\item There is ongoing work for a phone-based tool to fuzz the network
\end{itemize}
\end{frame}

\subsection{Where we go from here}

\begin{frame}{TODO}{Where we go from here}
\begin{itemize}
	\item The tools for fuzzing mobile phone protocol stacks are available
	\item It is up to the security community to make use of those tools (!)
	\item Don't you too think that TCP/IP security is boring
	\item Join the GSM protocol security research projects
	\item Boldly go where no man has gone before
\end{itemize}
\end{frame}

\subsection{Where we go from here}

\begin{frame}{Current Areas of Work / Future plans}
\begin{itemize}
	\item Packet data (GPRS/EDGE) support in OpenBSC
	\begin{itemize}
		\item GPRS/EDGE is used extensively on modern smartphones
		\item Enables us to play on IP level with those phones without a heavily filtered operator network
	\end{itemize}
	\item UMTS(3G) support in OpenBSC
	\item Access to MS side layer 1
	\item Playing with SIM Toolkit from the operator side
	\item Playing with MMS
	\item More exploration of RRLP
\end{itemize}
\end{frame}

\subsection{Further Reading}

\begin{frame}{Further Reading}
\begin{itemize}
	\item http://openbsc.gnumonks.org/
	\item http://airprobe.org/
	\item http://openbts.sourceforge.net/
	\item http://bb.osmocom.org/
\end{itemize}
\end{frame}

\end{document}