How Can I Stop my Printer Printing Random Characters During a Scan?

If you have scanned a network containing printers with a vulnerability scanner you may have witnessed your printer printing pages of seemingly random characters.  This is a result of insecure features of modern printers reacting to data sent by the scanner.

One way to avoid this is to remove the printers from the list of scan targets.  The simplest way to do this is to remove their address from the list of scan targets.  Alternatively, if scanning a range of addresses and you only wish to exclude a subset of them you can add their addresses to the "Denied Targets" box at the top of your scan configuration and deselect the option "Automatically perform a passive web app scan against any discovered web applications" within the Infrastructure Scanning settings (Infrastructure Scanner Settings -> Vulnerability Scanner -> Options -> Advanced Settings).

It may be possible to include the printers in the scan without triggering printing.  If you wish to try this consider the following actions:

1. Make sure your printer's firmware is up to date.  Updates may include security fixes or bug fixes which prevent the random printing triggered by scanning (of course, keeping your devices up-to-date is good security practice anyway).
2. Enable secure or authenticated printing to block unauthorized printing.  How you do this will depend on your printer, so consult your printer's documentation for instructions.
3. Disable non-essential features of your printer, such as cloud printing or support for printing from mobile devices.  Even if you wish to use such features, it may be worth running a scan with them disabled to test whether these are the features triggering the printing during a scan.