BadUSB — On accessories that turn evil by Karsten Nohl
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The document discusses the vulnerabilities and attack scenarios associated with BadUSB exploits on USB devices, emphasizing the ease with which these devices can be reprogrammed for malicious purposes. It outlines various attack demonstrations, including USB attacks affecting Windows, Linux, and Android systems, as well as the challenges of detecting and mitigating such threats. The authors call for better security measures and awareness due to the growing prevalence of exploitable USB peripherals, while highlighting the lack of effective defenses from manufacturers and OS vendors.
BadUSB — On accessories that turn evil by Karsten Nohl
- 1. SRLabs Template v12 BadUSB — On accessories that turn evil Karsten Nohl <nohl@srlabs.de> Sascha Krißler <sascha@srlabs.de> Jakob Lell <jakob@srlabs.de>
- 2. 2 Demo 1 – USB s&ck takes over Windows machine
- 3. Agenda 3 § USB background § Reprogramming peripherals § BadUSB aLack scenarios § BadUSB exposure § Defenses and next steps
- 4. USB devices are recognized using several idenPfiers 4 USB devices Connectors + hubs Host Root hub Examples USB thumb drive 8 – Mass Storage AA627090820000000702 0 – Control 1 – Data transfers Interface class End points Iden&fier a. 1 – Audio b. 14 – Video Webcam Serial number (opPonal) 0258A350 0 – Control 1 – Video transfers 6 – Audio transfers 7 – Video interrupts
- 5. USB devices are iniPalized in several steps 5 Devices can have several iden&&es § A device indicates its capabiliPes through a descriptor § A device can have several descriptors if it supports mulPple device classes; like webcam + microphone § Device can deregister and register again as a different device Power-‐on + Firmware init Load driver Register Set address Send descriptor Set configuraPon Normal operaPon Register again … OpPonal: deregister Load another driver USB device USB plug-‐and-‐play
- 6. USB devices include a micro-‐controller, hidden from the user 6 8051 CPU Bootloader USB controller Controller firmware Mass storage Flash The only part visible to the user
- 7. Agenda 7 § USB background § Reprogramming peripherals § BadUSB aLack scenarios § BadUSB exposure § Defenses and next steps
- 8. Reversing and patching USB firmware took 2 months 8 1. Find leaked firmware and flash tool on the net 2. Sniff update communicaPon using Wireshark 3. Replay custom SCSI commands used for updates 4. (Reset bricked devices through short-‐circuiPng Flash pins) Document firmware update process Patch firmware Reverse-‐engineer firmware 1. Load into disassembler (complicaPon: MMU-‐like memory banking) 2. Apply heurisPcs: – Count how olen funcPon starts match up with funcPon calls for different memory locaPon guesses; the most matches indicate that you guessed right – Find known USB bit fields such as descriptors 3. Apply standard solware reversing to find hooking points 1. Add hooks to firmware to add/change funcPonality 2. Custom linker script compiles C and assembly code and injects it into unused areas of original firmware Other possible targets We focused on USB sPcks, but the same approach should work for: § External HDDs § Webcams, keyboards § Probably many more … A B C
- 9. Agenda 9 § USB background § Reprogramming peripherals § BadUSB aKack scenarios § BadUSB exposure § Defenses and next steps
- 10. 10 Demo 2 – Windows infects USB s&ck which then takes over Linux machine
- 11. Keyboard emulaPon is enough for infecPon and privilege escalaPon (w/o need for solware vulnerability) 11 Challenge – Linux malware runs with limited user privileges, but needs root privileges to infect further sPcks Approach – Steal sudo password in screensaver Restart screensaver (or policykit) with password stealer added via an LD_PRELOAD library § User enters password to unlock screen § Malware intercepts password and gains root privileges using sudo
- 12. 12 Demo 3 – Android phone changes DNS sePngs in Windows
- 13. Network traffic can also be diverted by “DHCP on USB” 13 AKack steps 1. USB sPck spoofs Ethernet adapter 2. Replies to DHCP query with DNS server on the Internet, but without default gateway Result 3. Internet traffic is sPll routed through the normal Wi-‐Fi connecPon 4. However, DNS queries are sent to the USB-‐supplied server, enabling redirecPon aLacks DNS assignment in DHCP over spoofed USB-‐Ethernet adapter All DNS queries go to aLacker’s DNS server
- 14. “Can I charge my phone on your laptop?” – Android phones are the simplest USB aLack plaworm 14 Prepara&on – Android comes with an Ethernet-‐ over-‐USB emulaPon needing liLle configuraPon AKack – Phone supplies default route over USB, effecPvely intercepPng all Internet traffic DHCP overrides default gateway over USB-‐Ethernet Computer sends all Internet traffic through phone Hacked by the second factor? Using keyboard emulaPon, a virus-‐infected smartphone could hack into the USB-‐ connected computer. This compromises the “second factor” security model of online banking. Proof-‐of-‐concept released at: srlabs.de/badusb
- 15. Bonus: Virtual Machine break-‐out 15 Malicious VM Host 1. VM tenant reprograms USB device (e.g., using SCSI commands) 3. USB device spoofs key strokes, changes DNS, … 2. USB peripherals spawns a second device that gets connected to the VM host
- 16. Boot-‐sector virus, USB style 16 Hide rootkit from OS/AV. When an OS accesses the sPck, only the USB content is shown Infect machine when boo&ng. When the BIOS accesses the sPck, a secret Linux is shown, booPng a root kit, infecPng the machine, and then booPng from hard disk Fingerprint OS/BIOS. Patched USB sPck firmware can disPnguish Win, Mac, Linux, and the BIOS based on their USB behavior USB content, for example Linux install image Secret Linux image
- 17. 17 Demo 4 – USB thumb drive emulates keyboard and second drive to infect computer during boot
- 18. Family of possible USB aLacks is large 18 More aKack ideas Effect § External storage can choose to hide files instead of delePng them § Viruses can be added to files added to storage § First access by virus scanner sees original file, later access sees virus § Emulate a keyboard during boot and install a new BIOS from a file in a secret storage area on a USB sPck § Emulate a USB display to access security informaPon such as Captchas and randomly arranged PIN pads AKacks shown Emulate keyboard Hide data on s&ck or HDD Rewrite data in-‐flight Update PC BIOS Spoof display Spoof network card “USB boot-‐ sector” virus
- 19. Agenda 19 § USB background § Reprogramming peripherals § BadUSB aLack scenarios § BadUSB exposure § Defenses and next steps
- 20. We analyzed the possible reach of BadUSB from two perspecPves 20 Top-‐down analysis BoKom-‐up analysis § Start from largest USB controller vendors § Find their chip families for popular use cases § Analyze datasheets and web sites for whether chips can be reprogrammed § Start from actual hardware § Open device to find which chips are used § Determine whether bootloader and firmware storage (e.g. SPI flash) are available § Try to find firmware update tools for their chips § 5 device classes: Host, Hub, Charger, Storage, Peripheral § From top 8 chip vendors § Totaling 52 chip families (not every vendor serves each class) § Analyzed 33 devices from six device classes: Hub, Input/HID, Webcam, SD adapter, SATA adapter § Results released at opensource.srlabs.de
- 21. Both analyses suggest that up to half of USB chips are BadUSB-‐vulnerable 21 4 6 1 4 8 2 4 4 5 5 4 4 1 Peripheral Storage Charger Hub Host 1 4 1 2 3 3 2 4 3 4 1 5 SATA adapter SD adapter Webcam Input Probably vulnerable Top-‐down: Perhaps vulnerable, depends on design / configuraPon; BoLom-‐up: more research needed Unlikely vulnerable Top-‐down analysis BoKom-‐up analysis
- 22. Small hardware design differences can determine BadUSB-‐ vulnerability 22 These USB hubs both contain the same controller chip Only one of them also contains an SPI flash that can store BadUSB modificaPons
- 23. Recent trends suggest that BabUSB-‐exposure is further growing 23 Some device types appear more reprogrammable / BadUSB-‐vulnerable: § The early devices of a new standard (e.g. the first available USB 3 devices) § Peripherals with special funcPonality (e.g. SATA adapter that can copy disks) § High-‐end peripherals § Custom-‐tailored chips in high-‐volume devices were tradiPonally less likely to be reprogrammable; probably because mask ROMs are cheaper than Flash § Many such use cases are increasingly served with reprogrammable mulP-‐ purpose chips, that realize economies of scale by combining applicaPons § USB controllers found not to be reprogrammable were missing an essenPal component for upgrades, such as bootloader or Flash to store the update § All those controllers that bring the essenPals seem to be upgradable § ProtecPon from malicious updates is very rare: Only one (large) chip family brings fuse bits; none implement firmware signing Trend 1 – Newer and more complex devices are more vulnerable Trend 2 – Chips become more versa&le, and thereby more vulnerable Trend 3 – Most controllers that can be programmed are vulnerable Insight
- 24. Agenda 24 § USB background § Reprogramming peripherals § BadUSB aLack scenarios § BadUSB exposure § Defenses and next steps
- 25. No effecPve defenses from USB aLacks exist 25 Protec&on idea § USB devices do not always have a unique serial number § OS’s don’t (yet) have whitelist mechanisms Limita&on § The firmware of a USB device can typically only be read back with the help of that firmware (if at all): A malicious firmware can spoof a legiPmate one Block cri&cal device classes, block USB completely § Obvious usability impact § Very basic device classes can be used for abuse; not much is lel of USB when these are blocked § ImplementaPon errors may sPll allow installing unauthorized firmware upgrades § Secure cryptography is hard to implement on small microcontrollers § Billions of exisPng devices stay vulnerable Whitelist USB devices Scan peripheral firmware for malware Use code signing for firmware updates Disable firmware updates in hardware § Simple and effec&ve (but mostly limited to new devices)
- 26. Responsibility for BadUSB miPgaPon is unclear 26 BadUSB malware becomes more realis&c Fixes are not yet in sight No response from chip vendors § Sample exploit code for Phison USB 3 controllers was released by Adam Caudill and Brandon Wilson at Derbycon in September § Only miPgaPon aLempts right now are quick fixes such as GData’s Keyboard Guard § Phison, the mostly discussed vendor, notes that they are already offering beLer chips. Their customers don’t seem to chose them olen § Other affected vendors have stayed quiet No response from peripheral vendors § No affected vendor offers patches or a threat advisory § OS implementers do not appear to work on soluPon; with one excepPon: FreeBSD adds an opPon to switch off USB enumeraPon No OS vendor response vs.
- 27. § Use the reprogrammable chips for other applicaPons than USB storage § The flowswitch / phison project, for example, aims for a low-‐cost USB 3 interface for FPGAs USB peripherals can also be re-‐programmed for construcPve purposes 27 Idea 2 – Repurpose cheap controller chips Idea 1 – Speed up database queries § Data can be parsed on the sPck before (or instead of) sending it back to the host § Our original moPvaPon was to speed up of A5/1 rainbow table lookups
- 28. Take aways 28 QuesPons? usb@srlabs.de § USB peripherals provide for a versaPle infec&on path § As long as USB controllers are re-‐ programmable, USB peripherals should not be shared with others § Once infected – through USB or otherwise – malware can use peripherals as a hiding place, hindering system clean-‐up
- 29. Scope of top-‐down analysis The USB microcontroller market is split among many vendors 29 Microchip (SMSC) 10% Cypress 8% Alcor 7% Renesas 6% Genesys 5% ASMedia 5% Phison 5% FTDI 4% ST-‐E 4% JMicron 3% TI 3% Silicon MoPon 3% Silicon Labs 3% Exar 2% Displaylink 2% Fresco 1% PLX 1% Via Labs 1% Others 26% Wired USB Market Share (2012 Cypress Shareholders MeePng) Source: goo.gl/NtN0cf