Difference between revisions of "Credential disclosure"

Printers are commonly deployed with a default password or no initial password at all. In both cases, end-users or administrators have to actively set a password to secure the device. This article discusses generic brute-force attacks against PJL and PostScript passwords as well as model specific password disclosure.

Brute-Force Attacks

Besides credentials leaked from sources like file system or memory access, SNMP and the printer's embedded web server, printing languages offer limited passwords protection mechanisms themselves. Breaking such mechanisms has a priority in this wiki because it focuses on printer-specific weaknesses. Furthermore, whilst the routines to set the password for a printer's embedded web server differ from model to model they are standardized for both PJL and PostScript. Although it is not very common for end-users or even administrators to set or actually know about these passwords, if enabled they can disable some of the attacks discussed in this wiki. Attackers should therefore have a motivation to crack or bypass them if necessary.

PJL

PJL offers the possibility to set a password to lock access to the printer's hard disk and/or control panel. PJL disk lock as shown below is the defense mechanism propagated by HP against PJL file system access, including its known path traversal vulnerabilities [1]:

@PJL JOB PASSWORD=0
@PJL DEFAULT DISKLOCK=ON
@PJL DEFAULT CPLOCK=ON


PJL passwords however are vulnerable to brute-force attacks because of their limited 16 bit key size as demonstrated by [2] who were able to unlock the disk protection within 6 hours in the worst case. With PJL interpreters having gotten faster while the PJL standard was never updated and still limits passwords to numerical values ranging from 1 to 65535 [3], cracking time has efficiently decreased. In a test with 20 devices, between 50 and 1,000 passwords could be evaluated per second leading to average cracking times between 30 seconds and 10 minutes.

While PJL passwords can be set on various devices, actual disk lock and/or control panel lock is only supported by few printers. It is unclear if the password has any undocumented, proprietary effects on these machines or is just a dummy variable. Furthermore, non-compliant with the PJL standard, Brother based devices do not even verify the password to lock or unlock the control panel, rendering it practically useless.

How to test this attack?

The lock and unlock commands of PRET can be used to test brute-force attacks against PJL passwords:

./pret.py -q printer pjl
Connection to printer established

Welcome to the pret shell. Type help or ? to list commands.
printer:/> lock 999
PIN protection:  ENABLED
Panel lock:      ON
Disk lock:       ON
printer:/> unlock
No PIN given, cracking.
PIN protection:  DISABLED
Panel lock:      OFF
Disk lock:       OFF


Who can perform this attack?

Anyone who can print, for example through USB drive or cable, Port 9100 printing or Cross-site printing. Feedback from the printer is not required because attackers can blindly remove the password protection by including all 65535 possible combinations in a single print job.

PostScript

PostScript offers two types of passwords: The SystemParamsPassword is used to change print job settings like paper size, while the StartJobPassword is required to exit the server loop and therefore permanently alter the PostScript environment. The checkpassword operator which takes either an integer or a string as input checks for both passwords at once [4]. The key size is very large: PostScript strings can contain arbitrary ASCII characters and have a maximum length of 65,565 [5] which theoretically allows 524,280 bit passwords. On the positive side (from an attackers point of view) brute-force attacks against PostScript passwords can be performed extremely fast because the PostScript interpreter can be programmed to literally crack itself. A simple PostScript password cracker testing for numerical values as passwords is given below:

/min 0 def /max 1000000 def
statusdict begin {
min 1 max
{dup checkpassword {== flush stop} {pop} ifelse} for
} stopped pop


Tested printers were capable of performing between 5,000 and 100,000 password verifications per second. Such enormous cracking rates can be achieved because a printer's RIP is highly optimized for fast processing of PostScript code. Brother based devices are exceptions as BR-Script only accepts one password per second but also checks for the very first character of the password only which effectively limits the key size to 256 characters or 8 bit. As it seems, Kyocera's KPDL does not support permanent PostScript passwords at all.

How to test this attack?

The lock and unlock commands of PRET can be used to test brute-force attacks against numeric (integer) PostScript passwords:

./pret.py -q printer ps
Connection to printer established

Welcome to the pret shell. Type help or ? to list commands.
printer:/> lock 999
printer:/> unlock


Who can perform this attack?

Anyone who can print, for example through USB drive or cable, Port 9100 printing or Cross-site printing. Feedback from the printer is not required because attackers can blindly remove the password protection by cracking it in a single print job.

SNMP

Ancient HP printers (manufactured 2003 and earlier) had a bug which allowed an attacker to retrieve the password for the embedded web server through SNMP requests. The vulnerable OID to be requested (.iso.org.dod.internet.private.enterprises.hp.nm.system.net-peripheral.net-printer.generalDeviceStatus.gdPasswords) was even documented by HP. Other vendors may have similar SNMP based issues. Penetration testers may find flaws by studying the various publicly available MIBs released by printer manufacturers.

How to test this attack?

To test this attack against ancient HP printers, the snmpset tool can be used as shown below:

snmpget -v1 -c public printer iso.3.6.1.4.1.11.2.3.9.1.1.13.0
iso.3.6.1.4.1.11.2.3.9.1.1.13.0 = Hex-STRING: 41 41 41 00 …


Vulnerable devices will return the password in hexadecimal (here: AAA), while newer devices do only respond with zerobytes.

Who can perform this attack?

Anyone who can send network packets to port 161/udp of the printer device.

Pass-Back

Another interesting class of attacks is pass-back attacks were ‘an MFP device is directed into authenticating [...] against a rogue system rather than the expected server’ [6]. This works in setups where a printer/MFP authenticates users via an external LDAP server. Note that the credentials to access the LDAP server are stored on the MFP itself. If the MFP allows an attacker to change the address of the LDAP server while keeping the stored credentials, whenever someone (for example, the attacker itself) tries to authenticate with the MFP, the MFP leaks the original LDAP credentials to the attacker-controlled server. This example shows that passwords resident on printers may not only harm the device itself if integrated into a company's network. Printers and MFPs – which may offer insufficient protection – are therefore a good starting point in network penetration tests.

How to test this attack?

Check if you can change printer settings like the LDAP hostname while keeping the old LDAP password.

Who can perform this attack?

Usually anyone who can access the printer's embedded web server. This may include CRSF attacker, if the web application running on the printer has no CSRF protection.

1. Security Bulletin HPSBPI02575 SSRT090255 Rev. 1, HP Inc., 2010
2. Attacking Networked Embedded Devices, Black Hat USA, FX and FtR of Phenoelit, 2002
3. Printer Job Language Technical Reference Manual, HP Inc., 1997, ch. 6-21
4. PostScript Language Reference Manual Supplement for Version 2016, Adobe Systems Inc., 1995, p. 194
5. PostScript Language Reference Manual, 3rd Edition, Adobe Systems Inc., 1999, p. 739
6. Anatomy of a Pass-Back-Attack: Intercepting Authentication Credentials Stored in Multifunction Printers, D. Heiland and M. Belton, 2011