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hacktricks/network-services-pentesting/pentesting-printers/firmware-updates.md
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The dangers of malicious firmware updates are well-known and have been discussed early by [1] and [2]. In contrast to other networked devices however, it is common for printers to deploy firmware updates as ordinary print jobs. This opens up a wide gateway for attackers because access to printing functionality is usually a low hurdle. One can only speculate about the motivation for such insecure design decisions but it seems logical that historic reasons play a role: Printers used to be connected by parallel or USB cable. Without network connectivity, security was less important and without a password-protected web server or similar functionality the printing channel was the only way to send data to the device.

Firmware modification attacks against network printers have been demonstrated by [3] for HP devices, by [4] for the Canon PIXMA series and by [5] and [6] for various Xerox models. As a countermeasure, printer manufacturer started to digitally sign their firmware [7].

Vendors

To give an overview of firmware deployment procedures 1,400 firmware files for the top 10 printer manufacturers have been downloaded and systematically categorized by [8]. The results are as follows.

HP

Firmware can be downloaded from support.hp.com or directly from ftp.hp.com via FTP. 419 files in HP's traditional remote firmware update `.rfu` format and 206 newer ‘HP FutureSmart’ binaries `.bdl` can be retrieved. The .rfu files contain proprietary PJL commands like @PJL UPGRADE SIZE=…, indicating that firmware updates are deployed as normal print jobs. This has been demonstrated by [3] and caused HP to digitally sign all their printer firmware since March 2012 [7].

Canon

Firmware is available at www.canon.com/support. Canon however requires a valid device serial number to download any firmware. According to [4], who were able to modify firmware for the Canon PIXMA series, ‘there is no signing the correct way to do it but it does have very weak encryption’. According to email correspondence with a Canon technical support representative, ‘firmware does have to be digitally signed by Canon in order for it to be accepted by the printer’.

Epson

Firmware can be downloaded from epson.com and via FTP from download.epson-europe.com. Files come as WinZip self-extracting .exe files and can be unpacked using unp[9]. The contained .efu files can be analyzed using Binwalk[10] which extracts the actual firmware. One can obtain 49 .rcx files of unknown format ‘SEIKO EPSON EpsonNet Form’ and nine .prn files containing PJL commands `@PJL ENTER LANGUAGE=DOWNLOAD`. Epson has not published any information on protection mechanisms. Firmware released before 2016 did not apply code signing and could be manipulated as shown by [11]. They ‘believe huge amounts of the devices produced since 1999 […] could be vulnerable’.

Dell

Firmware can be obtained from downloads.dell.com and from ftp.us.dell.com/printer. Files can be unpacked using unp and the included .zip files can be extracted with a variant of unzip. Dell does not produce any printing devices, but rebadges the products of other vendors. Therefore a wide variety of firmware files, including 18 .hd files containing @PJL FIRMWARE=…, 25 .prn files containing @PJL ENTER LANGUAGE=DOWNLOAD and 30 .fls/.fly files containing @PJL LPROGRAMRIP were found. Regarding protection mechanisms, Dell has not released any publicly available information.

Brother

Firmware cannot be easily downloaded. Instead a Windows binary needs to be run which checks for available printers and requests download links for the latest firmware from a web service. By guessing correct parameters one is able to get the links for 98 files. Firmware files do not need to be unpacked as they already come in raw format. 79 files have the extension .djf and contain @PJL EXECUTE BRDOWNLOAD, while 9 .blf files contain @PJL ENTER LANGUAGE=PCL. Brother has not released any publicly available information on protection mechanisms.

Lexmark

Firmware is available from support.lexmark.com and can be unpacked using unp. 63 fls files could be obtained containing the PJL header @PJL LPROGRAMRIP to install the firmware. Lexmark's security whitepaper claims ‘packages must be encrypted with a symmetric encryption algorithm through a key that is known only to Lexmark and is embedded securely in all devices. However, the strongest security measure comes from requiring that all firmware packages must include multiple digital 2048-bit RSA signatures from Lexmark. If these signatures are not valid [...] the firmware is discarded’ [12].

Samsung

Firmware can be downloaded from www.samsung.com/us/support/download. Retrieved files either come as zip archives or Windows executables which can be run in wine and further unpacked using unp. This way, 33 .hd files starting with @PJL FIRMWARE and associated .prn files containing @PJL DEFAULT SWUPGRADE=ON could be obtained. Samsung has not released any publicly available information on protection mechanisms.

Xerox

Firmware is publicly available at www.support.xerox.com. Downloaded files come in zip format and can be unpacked using unzip. Firmware files are in different formats: 16 .hd files including @PJL FIRMWARE=…, 36 PostScript files for older devices and 35 .dlm files which is the format used by currently used by Xerox and includes digital signatures. A flaw in the deployment process however was found by [5] and extended by [6], leading to remote code execution – the private key and the tool used for code signing was contained in the firmware itself.

Ricoh

The ‘Firmware Download Center’ at support.ricoh.com is not open to the general public. Fortunately the interweb contains direct links to a couple of driver/firmware download pages so one is able to obtain 31 firmware files using a simple Google search `site:support.ricoh.com firmware`. Files can be unpacked using unp. 14 .bin files contain @PJL RSYSTEMUPDATE SIZE=… while 15 .brn files are associated with a settings.ini, including @PJL FWDOWNLOAD and USERID=sysadm, PASSWORD=sysadm. Ricoh does not provide any up-to-date information on protection mechanisms. In a whitepaper dating back to 2007, Ricoh claims that ‘only service technicians have a password and dedicated account for making firmware updates’ [13].

Kyocera

Kyocera does not release firmware to end-users. In a publicly available Kyocera dealer forum however, firmware downloads for various models are linked: ftp.kdaconnect.com. Files can be unpacked using unp and contain mountable cramfs[14] and squashfs[15] images as well as proprietary binary formats. Firmware is deployed as a print job with !R! UPGR'SYS';EXIT; prepended – the upgrade command of the PRESCRIBE page description language [16]. Kyocera has not released any publicly available information on protection mechanisms.

Konica

Although not actively promoted, firmware for Konica Minolta printers can be downloaded from download6.konicaminolta.eu. Newer Internet-connected devices have the capability to perform firmware updates themselves. Compressed files come in different formats and can be unpacked using unp, unzip and tar which results in 38 proprietary .bin files, 20 PostScript based ‘softload printer modules’ for older devices and 14 files of different extensions containing PJL commands like @PJL ENTER LANGUAGE=FIRMUPDATE. The Konica Minolta security whitepaper claims that firmware is verified using a ‘hash value’ [17]. It may be doubted that such a scheme is cryptographically secure.

Results

Out of ten analyzed manufacturers, nine use PJL commands for all or at least some of their firmware update procedures which is a strong indicator that updates are deployed as ordinary print jobs. The remaining manufacturer – Kyocera – applies the PRESCRIBE page description language. One can therefore claim that it is common in the printing industry to install new firmware over the printing channel itself and name a major design flaw present in almost any printer device: data and code over the same channel. Exploitation of this issue however is hard as for most manufacturers no reasoned statement on protection mechanisms can be made. An in-depth analysis of firmware modification attacks should therefore be part of future research. A summary of file headers or types for all obtained firmware files is given below:

Vendor Extension Quantity File header or type
HP rfu 419 @PJL UPGRADE SIZE=…
bdl 206 FutureSmart binary format
Epson rcx 49 SEIKO EPSON EpsonNet Form
prn 9 @PJL ENTER LANGUAGE=DOWNLOAD
brn 7 Unknown binary, includes config file
Dell fls, fly 30 @PJL LPROGRAMRIP
prn 25 @PJL ENTER LANGUAGE=DOWNLOAD
hd 18 @PJL FIRMWARE=…
brn 3 Unknown binary, includes config file
ps 2 PostScript title: Firmware Update
pjl 1 @PJL ENTER LANGUAGE=FLASH
Brother djf 79 @PJL EXECUTE BRDOWNLOAD
blf 9 @PJL ENTER LANGUAGE=PCL
Lexmark fls 63 @PJL LPROGRAMRIP
bin, fls 6 Unknown binary format
Samsung hd 33 @PJL FIRMWARE=…
fls, hd0 4 @PJL DEFAULT P1284VALUE=…
Xerox ps 36 PostScript title: Firmware Update
dlm 35 Xerox Dynamic Loadable Module
prn, bin 20 @PJL ENTER LANGUAGE=DOWNLOAD
hd 16 @PJL FIRMWARE=…
brn 10 Unknown binary, includes config file
bin 10 @PJL SET JOBATTR="@SWDL"
fls, hd, hde 8 @PJL DEFAULT P1284VALUE=…
fls, xfc 4 @PJL ENTER LANGUAGE=XFLASH
pjl 3 @PJL FSDOWNLOAD [name].rpm
axf 3 RISC OS AIF executable
Ricoh brn 15 @PJL FWDOWNLOAD…
bin 14 @PJL RSYSTEMUPDATE SIZE=…
fls 4 @PJL LPROGRAMRIP
Kyocera cramfs, img 98 cramfs image
bin, squashfs 79 squashfs image
bin, kmmfp 41 u-boot legacy uImage
efi, kmpanel 13 proprietary image format
Konica Minolta bin 38 unknown binary, additional checksum file
ps 20 PostScript title: Softload printer modules
ftp, prn 11 @PJL ENTER LANGUAGE=FIRMUPDATE
upg 1 @PJL ENTER LANGUAGE=UPGRADE

How to test for this attack?

The security of code signing is based on keeping the private key a long-term trade secret. There are however still printers in the wild which are potentially vulnerable to malicious firmware – either because they have not yet been updated or because proprietary checksum algorithms are sold as cryptographically secure digital signature schemes. It certainly must be pointed out that analyzing firmware can be hard if vendors do not document their firmware formats and update routines. Usually this requires some reverse engineering. Testing the feasibility of firmware modification attacks therefore is not trivial. In a simple test, one can flip a single bit and check if the modified firmware is still accepted by the printer device. If not, either a checksum or a digital signature is verfied by the printer. Finding the difference is not always easy and writing malicious firmware with a correct checksum can be a time-consuming project.

Other attack scenarios include:

  • Even if the firmware is signed, one may be able to downgrade to a certain signed firmware version which has known security weaknesses.
  • Even if the firmware is signed, it can sometimes be mounted to gain further information especially Konica Minolta firmware is easly mountable.
  • Just because firmware is signed doesn't mean its secure. Using binwalk/grep etc. one may find components with known vulnerabilities like CVE-2015-7547.
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