Physical damage
Long-term settings for printers and other embedded devices are stored in non-volatile memory (NVRAM) which is traditionally implemented either as EEPROM or as flash memory. Both components have a limited lifetime. On early HP LaserJets ‘flash chips would only sustain about 1000-2000 cycles of re-writing’ [1]. Today, vendors of flash memory guarantee about 100,000 rewrites before any write errors may occur. This number sounds large, but PJL and PostScript print jobs themselves can change long-term settings like paper tray media sizes or control panel passwords. Doing this a lot of times on purpose can be a realistic attack scenario leading to physical destruction of the NVRAM. Note that printing functionality itself is not affected but fixed settings containing wrong values can make the device practically unusable.
PJL
For a practical test to destroy NVRAM write functionality one can continuously set the long-term value for the number of copies with different values for X:
@PJL DEFAULT COPIES=X
In an evaluation with 20 laser printers, eight devices indicated a corrupt NVRAM within 24 hours [2]. Some gave EEPROM error codes, while others completely refused to set any long-term values anymore. The impact of such physical NVRAM destruction, however, is limited for two reasons: First, NVRAM parameters were not frozen at their current state (which would have been a random number of copies) but instead fixed to the factory default value. Secondly, all variables could still be changed for the current print job using the @PJL SET... command. Only the functionality to change long-term settings was broken.
Testing for the Attack
The feasibility of this attack, which has been implemented as the destroy command in PRET can be tested as follows:
./pret.py -q printer pjl Connection to printer established Welcome to the pret shell. Type help or ? to list commands. printer:/> destroy Warning: This command tries to cause physical damage to the printer NVRAM. Use at your own risk. Press CTRL+C to abort. Starting NVRAM write cycle loop in... 10 9 8 7 6 5 4 3 2 1 KABOOM! Dave, stop. Stop, will you? Stop, Dave. Will you stop, Dave? [... wait for about 24 hours ...] I'm afraid. I'm afraid, Dave. Dave, my mind is going... NVRAM died after 543894 cycles, 18:46:11
Required Privileges
The attack can only be performed by an attacker who has the capability to establish various network connections over a long period of time. A local attacker sneaking into a copy room usually does not have enough time to send a continuous datastream for nearly a day. [3] However, she can use an axe or a hammer to cause physical damage. In a cross-site printing scenario, the victim would have to keep an attacker-controlled web site open for hours which may also be considered unrealistic. [4]
PostScript
For PostScript, one needs to find an entry in the currentsystemparams dictionary which survives a reboot (and therefore must be stored in some kind of NVRAM). A good candidate would be a PostScript password, as discussed in credential disclosure. System parameters can be incremented in a PostScript loop, as show below, which can lead to a large number of NVRAM write cycles per second if the printer's hardware is implemented to write values directly instead of caching them:
/counter 0 def
{ << /Password counter 16 string cvs
/SystemParamsPassword counter 1 add 16 string cvs
>> setsystemparams /counter counter 1 add def
} loop
- ↑ Counting Pages in Printer Data Streams, J. Deußen, 2011, p. 36
- ↑ Exploiting Network Printers, J. Müller, 2016, p. 41
- ↑ Note that it might theoretically be possible to start a large print job – approximately several hundred megabytes of malicious PJL commands – from USB stick on a Friday afternoon and just walk away.
- ↑ Unless you find XSS on Facebook, in which case the impact of broken printers may be negligible.
- ↑ New PostScript Virus!?, CompuServe Desktop Publishing Forum (via archive.org), 1990
- ↑ Viruses and the Macintosh, D. Harley, 2000
