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Embedded problems: exploiting NULL pointer dereferences
Your device could be at risk
By Federico Biancuzzi, SecurityFocus
Interview Barnaby Jack developed a method for exploiting certain NULL pointer dereferences on the ARM and XScale architectures (and likely PowerPC). This method affects a lot of devices since most mobile phones and PDA are ARM based, and high-end routers often use the XScale architecture.
Could you introduce yourself?
Barnaby Jack: I'm a staff security researcher at Juniper Networks. I've been involved in computer security for a number of years, mostly dealing with operating system internals, reverse engineering, and anything low-level. I've recently started to focus some of my research efforts into embedded systems - I'm having fun with it. I'm a kiwi born and bred, but these days I'm living way across the pond up in the bay area.
Could you describe the vector rewrite attack you have developed?
Barnaby Jack: The Vector Rewrite Attack is a method for exploiting certain NULL pointer dereferences on the ARM and XScale architectures. In general, NULL pointer dereference flaws are considered non-exploitable. On the XScale and ARM architectures the memory address 0 is mapped, and also holds the exception vector table. The exception vector table is a set of branch instructions that correspond to different exceptions, such as software and hardware interrupts. When a case arises that writes to the 0 address with user-defined source data, it is possible to gain execution control by rewriting the exception table.
On many embedded devices, execution is running in Supervisor (SVC) mode so memory access is unrestricted. The PowerPC architecture also stores the vector table at a low address, and is likely vulnerable to this same attack. Research into the PPC architecture is ongoing.
A short paper describing the attack is available here (pdf).
There were some comments around the net about your attack and its link with the JTAG interface. Could you please explain us how you used JTAG and the link with your attack?
Barnaby Jack: The JTAG interface is a hardware interface that when used in conjunction with a hardware debugging probe, allows live debugging of the embedded processor. JTAG is simply used as a debugging mechanism. JTAG is in no way required for an attack, and is used for exploit development in the same way a debugger such as ollydbg would be used on a PC. Most modern cores have JTAG support built into the processor design.
Which architectures are affected?
Barnaby Jack: ARM and XScale architectures, and likely the PowerPC architecture. The MIPS processor maps the vectors to a high address, and is not susceptible to this exploitation method. Any architecture that stores the vector table at 0x0 would be vulnerable to this attack.
Can we consider this a hardware design problem?
Barnaby Jack: This could be considered more of a problem in the architecture design. The MIPS architecture for example bases the exception vectors at a high address, at 0x8000xxxx. Thankfully, with ARM, XScale, and PowerPC - there is an option to map the vectors to a high address.
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This is yet another weapon to add to an attacker's arsenal. In this case, prevention is fairly simple.
Vendors, take note.