Embedded problems: exploiting NULL pointer dereferences

Your device could be at risk

Choosing a cloud hosting partner with confidence

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.

On ARM9 and newer cores, the exception table can be relocated high by driving the HIVECS processor pin high. On the XScale core, the vectors can be relocated high by setting the Exception Vector Relocation Bit of the ARM control register to 1. The vectors will be mapped to address 0xFFFF0000.

Manufacturers of networked devices could issue a firmware update to remap the vectors to high memory.

I saw a thread on the dailydave mailing list, where Brad Spengler posted: "I submit for your record-keeping what I believe to be the first public exploit for a null ptr dereference bug in the Linux kernel." It seems they are talking about x86 too?

Barnaby Jack: There have been examples of NULL pointer exploitation on a variety of architectures and platforms, including a locally exploitable vulnerability in pt_chmod from 1994 found by 8lgm. More recently, skape wrote a paper on exploiting NULL pointers in Internet Explorer.

The Vector Rewrite Attack is not the first time a NULL pointer dereference has been manipulated to cause an exploitable vulnerability - but I haven't been made aware of any published vulnerabilities that use the NULL address to overwrite interrupt entries to gain execution control. What I like about the Vector Rewrite Attack, is the fact that it can be leveraged both locally and remotely and is 100 per cent reliable - it also affects the most popular embedded architectures.

Beginner's guide to SSL certificates

More from The Register

next story
Russian hackers exploit 'Sandworm' bug 'to spy on NATO, EU PCs'
Fix imminent from Microsoft for Vista, Server 2008, other stuff
FYI: OS X Yosemite's Spotlight tells Apple EVERYTHING you're looking for
It's on by default – didn't you read the small print?
Microsoft pulls another dodgy patch
Redmond makes a hash of hashing add-on
'LulzSec leader Aush0k' found to be naughty boy not worthy of jail
15 months home detention leaves egg on feds' faces as they grab for more power
Forget passwords, let's use SELFIES, says Obama's cyber tsar
Michael Daniel wants to kill passwords dead
Kill off SSL 3.0 NOW: HTTPS savaged by vicious POODLE
Pull it out ASAP, it is SWISS CHEESE
Facebook slurps 'paste sites' for STOLEN passwords, sprinkles on hash and salt
Zuck's ad empire DOESN'T see details in plain text. Phew!
prev story


Forging a new future with identity relationship management
Learn about ForgeRock's next generation IRM platform and how it is designed to empower CEOS's and enterprises to engage with consumers.
Cloud and hybrid-cloud data protection for VMware
Learn how quick and easy it is to configure backups and perform restores for VMware environments.
Three 1TB solid state scorchers up for grabs
Big SSDs can be expensive but think big and think free because you could be the lucky winner of one of three 1TB Samsung SSD 840 EVO drives that we’re giving away worth over £300 apiece.
Reg Reader Research: SaaS based Email and Office Productivity Tools
Read this Reg reader report which provides advice and guidance for SMBs towards the use of SaaS based email and Office productivity tools.
Security for virtualized datacentres
Legacy security solutions are inefficient due to the architectural differences between physical and virtual environments.