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Base Jumping

Attacking the GSM baseband and base station

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Tuesday, 20 July 2010

mailto:[email protected]

mailto:[email protected]

Overview

!GSM!Base Station!Base Band!Conclusion

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GSM: The Protocol

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Documents

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!Dozens of docs!Thousands of pages!Important one (defines L3)

!GSM 04 08


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Logical Channels

Broadcast Channels (BCH)!!!!!Broadcast Control Channel (BCCH)!!!!!Frequency Correction Channel (FCCH)!!!!!Synchronization Channel (SCH)!!!!!Cell Broadcast Channel (CBCH)


Logical Channels, cont.! Common Control Channels (CCCH)!!!!!Paging Channel (PCH)!!!!!Random Access Channel (RACH)!!!!!Access Grant Channel (AGCH)

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Logical Channels, cont.

Standalone Dedicated Control Channel (SDCCH)!!!!!Associated Control Channel (ACCH)!!!!!Fast Associated Control Channel (FACCH)!!!!!Slow Associated Control Channel (SACCH)

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GSM Channels

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!Opening a channel is slow!Can take seconds

!Specific channels for specific uses


Opening a channel

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RACH


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RACH

AGCH


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RACH

AGCH

LCH


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PCH


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RACH

PCH


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RACH

PCH

AGCH


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RACH

PCH

AGCH

LCH


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MS

BTS

BTSBSCMSC

ARFCN


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Base Transceiver StationBTS

Base StationController

BSC

Mobile StationController

MSC

Mobile StationMS

Base Station Sub-SystemBSS


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MSBSSMSCHLR

VLR


Mobile Identifiers

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IMSI


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IMSI

IMEI


GSM Attacks

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RACHell

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!Request channel allocation!Flood the BSS with requests!First announced by Dieter Spaar at DeepSec

!Prevent everyone from using that cell


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RACHell


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?

RACHell


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Our Target


Demo - RACHell

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IMSI Flood

!Send IMSI ATTACH messages!pre-authentication!Overload the HLR/VLR infrastructure!Prevent everyone using the network

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IMSI Flood


How hard to get an IMSI?

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IMSI DETACH

!Send multiple Location Update Requests including a spoofed IMSI!Unauthenticated

!Prevent SIM from receiving calls and SMS

!Discovered by Sylvain Munaut

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IMSI DETACH


Baseband Fuzzing

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=+

How to make a smartphone


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Two separate computers


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Baseband

!Controls the radio!Separate CPU and code base!RTOS!Written in C!Typically legacy code base (decades)


Coseinc GSM FuzzFarm!OpenBTS based fuzzer delivery engine

!Targetting! iPhone!HTC (Android)!Palm Pre!Blackberry!Nokia

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Conclusion

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GSM Trouble

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!GSM is no longer a walled garden!GSM spec has security problems!Expect many more issues as OSS reduces costs for entry


Future work

!More GSM stack fuzzing!Next gen protocol stacks

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Thanks to

Harald Walte, Osmocom-bb & OpenBTS


Questions?

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