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Embedded problems: exploiting NULL pointer dereferences

Your device could be at risk

Which router manufacturers should be interested in your research?

Barnaby Jack: This reaches further than router manufacturers. Any manufacturer that uses an ARM or XScale, and even PowerPC processor, should take an interest in this research. All of these architectures allow remapping of the vectors to a high address, which essentially protects from this vulnerability class. This attack is not going to be the next stack overflow, but the simple truth is, these flaws do exist - and prevention is quick and simple. In the case of router manufacturers, I can only speak for ourselves - at Juniper, all research is distributed internally before being released to the public. Our engineers take this research and apply any necessary changes to our product line.

PowerPC. Does this mean that XBox360 and Playstation3 and Nintendo Wii might be vulnerable to this vector attack? Considering that they can be connected to the internet...

Barnaby Jack: All three consoles are using PowerPC based cores. If an exploitable NULL dereference was found that allowed an attacker to overwrite the NULL address, and vectors were mapped low, then an attack would be possible. From the little I have read, at least concerning the Xbox360 and its hypervisor mode, I would consider an attack unlikely. I don't know enough about the other game console architectures and their memory management implementations to say specifically if they would be vulnerable.

We have already seen some attacks against bug in wireless drivers for common OSes. I am wondering if this vector might help attackers exploit bugs in drivers included in embedded systems.

Barnaby Jack: Definitely. The wireless interface on embedded devices is generally the first place I would hunt for vulnerabilities. There are ample opportunities to find exploitable bugs. The same (or at least very similar) vector used to exploit wireless drivers under common OS's can be applied to embedded systems with a wireless stack. The most interesting aspect of the wireless interface is that many products will integrate a System-On-Chip design that has the wireless code built into the chip, and the developer will call the API provided by the chip manufacturer. These SoC designs are heavily used in all manner of consumer, and high-end devices. An exploitable flaw within a popular SoC would affect many devices.

Thinking at something like a blackberry... How do you extract the code from one of those embedded devices?

Barnaby Jack: There are a number of methods to retrieve the firmware image. If a JTAG connection can be made to the device, then the firmware image can be read off the flash chip via JTAG. There are both commercial and free products available that support reading and writing flash memory via the JTAG port. A popular opensource project for interfacing with JTAG enabled devices is jtagtools. The jtagtools software can program and read various external flash chips.

Often, the firmware can be downloaded from the vendors site. In many cases there will be no compression or encryption, or the firmware will be compressed with a known compression scheme. If a firmware image is encrypted or compressed with a proprietary scheme, the firmware image may contain an unencrypted copy of the bootloader - the bootloader can be reversed to find the compression algorithm, and a custom decoder can be written.

If connecting a JTAG isn't feasible and the firmware image is not available for download, then a last ditch effort is to physically remove the flash memory chip from the device, and read the image in an external reader. The simplest way to remove the chip from the device is by using a hot air rework tool. Apply hot air to re-flow the solder, and remove the chip. The chip can then be read externally in an eeprom reader. Another option is to use the "Chip Quik" product. It is a low melting point alloy, that when heated and fused with soldered joints, forms a new alloy that will stay in a molten state long enough for components to be removed.

Should vendors encrypt the firmwares available on their websites?

Barnaby Jack: Many vendors will encrypt or compress their firmware images. If the bootloader code is unencrypted in the image then the compression/encryption scheme can be reversed and a custom tool written to decrypt the image. If you have access to the device, and a debug session either over serial or JTAG, you can employ a trick that's been used since the dawn of executable packing - and that is to simply dump the image when it decrypts itself in memory. Once the image has decrypted itself in memory, the processor can be halted, and the decrypted code can be dumped to file. Then it's a simple matter of loading the code into your favorite disassembler.

Should final products ship with debugging features disabled?

Barnaby Jack: Ideally, yes. Many vendors will remove resistors leading from the JTAG port, or in the case of the ARM architecture, may drive the TRST pin low. Neither of these methods is sufficient to disable the JTAG functionality. Resistors are easily replaced, and pins can be pulled high. The ideal choice is to remove the JTAG traces altogether. The main argument against removing JTAG and UART ports is the cost factor.

Manufacturing boards is expensive, and the cost involved to manufacture both prototype and production boards may not be warranted. Debugging functionality may be needed in some production devices, for technicians to service the device, for example.

If the JTAG port is removed, it's certainly a roadblock for an attacker, but it will not stop someone who is determined to have access to the code - other methods such as tapping data traces and re-socketing chips can be employed.

In the end it all boils down to writing secure software. If the software on the device is vulnerable, removing debugging functionality won't change that fact.

Federico Biancuzzi is freelancer. In addition to SecurityFocus he also writes for ONLamp, LinuxDevCenter, and NewsForge.

This article originally appeared in Security Focus.

Copyright © 2007, SecurityFocus

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