Feeds

Embedded problems: exploiting NULL pointer dereferences

Your device could be at risk

Top 5 reasons to deploy VMware with Tegile

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.

Security for virtualized datacentres

More from The Register

next story
'Kim Kardashian snaps naked selfies with a BLACKBERRY'. *Twitterati gasps*
More alleged private, nude celeb pics appear online
Home Depot ignored staff warnings of security fail laundry list
'Just use cash', former security staffer warns friends
Hackers pop Brazil newspaper to root home routers
Step One: try default passwords. Step Two: Repeat Step One until success
UK.gov lobs another fistful of change at SME infosec nightmares
Senior Lib Dem in 'trying to be relevant' shocker. It's only taxpayers' money, after all
Who.is does the Harlem Shake
Blame it on LOLing XSS terroristas
Snowden, Dotcom, throw bombs into NZ election campaign
Claim of tapped undersea cable refuted by Kiwi PM as Kim claims extradition plot
Freenode IRC users told to change passwords after securo-breach
Miscreants probably got in, you guys know the drill by now
THREE QUARTERS of Android mobes open to web page spy bug
Metasploit module gobbles KitKat SOP slop
BitTorrent's peer-to-peer chat app Bleep goes live as public alpha
A good day for privacy as invisble.im also reveals its approach to untraceable chats
prev story

Whitepapers

Providing a secure and efficient Helpdesk
A single remote control platform for user support is be key to providing an efficient helpdesk. Retain full control over the way in which screen and keystroke data is transmitted.
A strategic approach to identity relationship management
ForgeRock commissioned Forrester to evaluate companies’ IAM practices and requirements when it comes to customer-facing scenarios versus employee-facing ones.
Saudi Petroleum chooses Tegile storage solution
A storage solution that addresses company growth and performance for business-critical applications of caseware archive and search along with other key operational systems.
WIN a very cool portable ZX Spectrum
Win a one-off portable Spectrum built by legendary hardware hacker Ben Heck
The next step in data security
With recent increased privacy concerns and computers becoming more powerful, the chance of hackers being able to crack smaller-sized RSA keys increases.