------------------------------------------------------------------------ -=\ BSCW Security Issues - Audit report 02 - 7. Sept. 2001 \=- ------------------------------------------------------------------------ BSCW is a groupware system that runs on a webserver. For more information about BSCW visit the developer website (http://bscw.gmd.de/ and http://www.orbiteam.de). While auditing the BSCW system, i discovered two more vulnerabilities. This document explains the vulnerabilities, how i did notice them and what you can do to fix them. ----------------------------------------------------------- -=\ Vulnerability no. 1: insecure default configuration \=- ----------------------------------------------------------- Type: Insecure default configuration. Effect: Gives unwanted people the possibility to register as user of the BSCW server. Software affected: All 3.x versions of BSCW, version 4 not tested, but probably as well. Severity: Low risk / Medium risk Very high risk, if other security issues exists Solution: Think. -=\ Description \=- Normally the BSCW software is configured to allow self registration of users. This enables the administrator to register himself as the first user, after setting up the server. Self registration can normally be done by accessing: http://your.bscwserver.url/pub/english.cgi?op=rmail Although allowing self registration of users can be a wanted configuration, in most cases this isn't the case. Many BSCW servers have are targeted to a closed user community. In my opinion the BSCW system should register the server admin in the install procedure and should not allow self registration out of the box. If the admins have to enable the self registration of users by changing a configuration file, they might think twice about it. The major danger of self registration is not that you give unwanted users access to your system and allow them to put files there. You give them access to a complex script running on your webserver and the possibility to exploit security holes. I checked for BSCW servers with a popular internet search engine and was able to self register in quite a lot of them, even on those that seemed to be there for a closed user community. You should think twice before setting up self registration, the better choice is to change the configuration, so that only a couple of trusted users are allowed to register new users. Example (line of config.py located in your <bscw-dir>/src/): MAY_REGISTER = ['joedoe','jane'] Allows the users "joedoe" and "jane" to invite new users into the system. Note that users classified as admins are allowed to invite new users also. ------------------- -=\ Fix \=- ------------------- No fix for this, as it isn't a real bug. Maybe a changed installation procedure, without the need to enable self registration, would be a good idea. If you don't need self registration of users, set the MAY_REGISTER directive in your config.py file. -------------------------------------------------------------- -=\ Vulnerability no. 2: shell meta characters not filtered \=- --------------------------------------------------------------- Type: Some shell meta characters are not filtered from user input when calling external programs. Effect: Gives malicious a user the possibility to run any shell script he wants, under the UID of the BSCW software. Software affected: All 3.x versions of BSCW running under Unix like OS. Version 4 not tested (probably vulnerable too. edit: Bug has been fixed in the 21. Dec. Version 4 release). Depending on how external programs are called under Windows, a similar vunerability may exist in BSCW for Windows. Severity: Ouch. Very high risk. Solution: Change the way external tools are called immediately. If you dont need and external conversion tool, diable it. Wait for a patch from GMD/Orbiteam. -=\ Description \=- The BSCW system gives the users the possibility to convert files into other formats (e.g. GIF into JPEG). This is done by calling external tools. The user can enter the filename of the converted file. Since the user input is handed as parameter to the external programs, which are called via a shell, shell meta characters should be filtered out of the user input. Most of them are filtered by BSCW, but there are a few which aren't: &;^()[]{} The dangerous characters are "&",";","^". I'll explain the vulnerability, using the conversion of a JPEG to a GIF as example: After you have set your skill level in your userprofile to "Expert", you have the ability to convert certain file formats into another format. BSCW achieves this by calling external helper tools. Lets say we have a file "test.jpg" in a folder we can access. We click on the "convert" option. In the following dialog we choose our settings for the conversion, we select "GIF" and "no encoding". We can enter the name of the outputfile also, the default is the the name of the file ("test.jpg" in our case). We dont change the name. Hitting the convert button gives you a file named "test.gif". Now we enter some shell meta characters as file name: "'`/\|<>*?&;^()[]{} And get an output similar like this: Some text that the conversion wasnt successfully. ( /bin/X11/djpeg -gif -outfile /BSCW/Tmp/@8279_1/&;^()[]{} /BSCW/Tmp/@8279_1/@8279_2 ) 2>&1 . This is the output of the shell call which the BSCW system did. Looking at the metachars you can see that "'`\|<>*? are filtered, while &;^()[]{} are not. The @8279_1 and @8279_2 are internal object reference codes that BSCW creates. Now we use ;ls; as file name for the conversion (; is the command separator for shell commands), we get something like: /bin/X11/djpeg: can't open /BSCW/Tmp/@8558_1/ @8558_2 sh: /BSCW/Tmp/@8558_1/@8558_2: cannot execute ( /bin/X11/djpeg -gif -outfile /BSCW/Tmp/@8558_1/;ls; /BSCW/Tmp/@8558_1/@8558_2 ) 2>&1 . We executed the "ls" command (output is "/BSCW/@8558_1/@8558_2"). So there is one file in this temporary directory, which is in fact our "test.jpg" file. Then we get the "cannot execute" error, since the shell tries to execute "/BSCW/Tmp/@8558_1/@8558_2" (we separated it in the commandline by ";"). Now we create our exploit shell script: echo code executed on webserver uname -a We use "test.jpg" as name for this script and upload it on the BSCW server, setting the MIME type to "jpeg" manually in the upload dialog. Since the BSCW creates the temp file for conversion without the exec bit set, we have to execute by calling the shell with the file as argument. We do this by giving ";sh" as file name for the converted file: /bin/X11/djpeg: can't open /BSCW/Tmp/@9586_1/ code executed on bscw server: SunOS marin 5.8 Generic_111848-01 sun4u sparc SUNW,Ultra-4 ( /bin/X11/djpeg -gif -outfile /BSCW/Tmp/@9586_1/; sh /BSCW/Tmp/@9586_1/@9586_2 ) 2>&1 . ------------------- -=\ Fix \=- ------------------- The configuration for calling external conversion programs are in the file "config_converters.py", located in the "/src" directory of your BSCW installation. It contains one entry for each conversion possibility (gif->jpeg, jpeg->gif, gif->ps ...). Those Entries look like this: # JPEG -> GIF (0.8) ('image/jpeg', 'image/gif', '0.8', '/usr/bin/X11/djpeg -gif -outfile %(dest)s %(src)s', 'Colors, if more than 256'), Change it to: # JPEG -> GIF (0.8) ('image/jpeg', 'image/gif', '0.8', '/usr/bin/X11/djpeg -gif -outfile "%(dest)s" "%(src)s"', 'Colors, if more than 256'), Do this for all the conversion programs. That way parameters are quoted and not interpreted. Thomas Seliger tom[at]wiretap(dot)de
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