What does it really take to exploit a printer?

Printer Hack: Researchers Can Set Media’s Pants on Fire

In the past couple of weeks, there has been quite a bit of press and blogging about a security vulnerability in HP printers that was discovered by researchers in the Intrusion Detection Lab at Columbia University.

In a nutshell, the researchers found a way to replace the operating firmware on an HP printer with firmware of their own design that can do bad things, and they also found a way to do it to a printer that is on a private network behind a firewall.

MSNBC ran an “exclusive” story about it calling it a “devastating attack” to which “millions of printers” could be subjected. Its lede suggested that hackers could cause the printer to catch fire, or be used for identity theft, or be used to take control of entire networks.

In practice, this isn’t an easy vulnerability to exploit on a large scale.

Let me explain:

First, you need to target a printer that supports PJL and its largely undocumented remote firmware update (RFU) function. Many printers support PJL, but RFU is less commonly supported. Many printers don’t have any mechanism for remote updates, and many others use something other than PJL’s RFU function for remote updates.

Once you've found a printer that supports PJL and its RFU function, you'll need to make sure that it will apply a firmware update without checking its authenticity. I can’t speak for other manufacturers, but my employer’s products have been using digital signature verification for firmware updates for at least the seven plus years that I have worked for them.

Next, you need to be able to create new firmware to do your bidding. To do that, you need to know what is the manufacturer and model of your target. The researchers demonstrated exploitation of a victim’s printer that was on a private, firewalled network, but didn’t mention how they determined which make and model of printer would be used by a particular victim. They would need to know that in order to send the correct firmware image to the victim.

And then there is the matter of reverse-engineering printer firmware. It is certainly possible, but not very practical when you consider that there are thousands of different printer models to contend with.

The researchers say that “rewriting the printer’s firmware takes only about 30 seconds”, but they are referring to the time it takes for the printer to update its flash memory and not how long it takes for someone to reverse-engineer a printer to do something malevolently useful.

Next, you need to get the victim to print a document that contains the firmware update code, and of course they need to print it on the printer that you targeted. I don’t know if it is possible to embed an RFU in a printable document in such a way that isn’t obvious when the document is viewed, as most people do before they print something. Perhaps they will disclose that detail at the Chaos conference.

Now, finally, you own the victim’s printer.