Lucian Constantin … Malware Infection Forces Printers to Print Garbled Data, Researchers Say:
Printers connected to Windows computers infected with new variants of a malware program called Trojan.Milicenso, will automatically print out pages full of garbled data, according to security researchers from antivirus firm Symantec.
Trojan.Milicenso is distributed in several ways: as a malicious e-mail attachment, as a drive-by download launched from compromised websites or as a fake codec advertised by social engineering scams, the Symantec researchers said.
After it infects a computer, the malware drops a copy of Aware.Eorezo as a randomly named .spl file (Windows Printer Spool File) in the default Windows printer spool directory -- %SystemRoot%\system32\spool\printers. Despite the .spl extension, the rogue file is actually an executable one.
The spool directory temporarily holds copies of files that printers are scheduled to print. Even though some printers allow users to specify a custom spool directory, many configurations use the default Windows one.
This causes printers attached to computers infected with new Trojan.Milicenso variants to automatically print the contents of the rogue .spl file, sometimes until their paper runs out.