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Vulnerabilities with Swish

From: Job de Haas <job(at)not-real.itsx.com>
Date: Tue Nov 10 1998 - 21:13:07 GMT
Hello,

While installing the Swish search engine (http://sunsite.berkeley.edu/SWISH-E)
at our site (http://www.itsx.com) we discovered several (potential) 
vulnerabilities. Swish-e and the accompanying configuration package AutoSwish
contain vulnerabilities in the source code of the indexer, in an example perl
script and in the perl scripts generated by AutoSwish for setting up an entry
form. Although the major problem is in the example script we found that 
several sites use this. Also the well known nature of these issues doesn't
seem to make it less desirable to point them out (again).

Impact
------

The vulnerabilities could allow remote access to the web-server as the user
that the server is running as.

Description
-----------

1) Perl script problems

Perl scripts to interface to the indexing and search program are provided
in two fashions: as plain example scripts and auto generated by the 
AutoSwish configuration tool.  The example scripts are provided on the web
site for Swish ( http://sunsite.berkeley.edu/SWISH-E/Manual/webscripts.html).
The scripts call the search program with parameters in the following manner:

	open(SWISH,"$swish -w $query -m $results -f $index|");

The example scripts do this without stripping the user supplied arguments of 
shell meta-characters, AutoSwish generated scripts do some stripping.
Still, subversion might be possible by providing command line arguments as
search strings. This is a problem due to the way the arguments are processed
by the indexing program.

This behavior can be prevented by using exec (which enforces the query to
be a single argument) and by removing any leading dashes from the user 
supplied strings.

This should possibly be something like:

	$query =~ s/^-+(.*)/$1/;
	$results =~ s/^-+(.*)/$1/;
        open(SWISH,"-|") || exec $swish,"-w",$query,"-m",$results,"-f",$index;


2) Buffer overflows

The code of the actual index and search program contains numerous buffer
overflows. These are too superfluous to mention. For the arguments these can
be circumvented by doing some preliminary limitation on the size of these 
user supplied arguments. The following will allow you to keep using the 
binaries you have:

	$query =~ s/(.{256}).*/$1/;
	$results =~ s/(.{256}).*/$1/;

Of course limiting the allowable characters in the query also severely limits
the possibilities for exploiting an overflow. We have not fully evaluated what
the impact could be when a user has control over the files being indexed.

Solution
--------

Make sure that the program executing the index program 'swish' does not
perform argument expansion and meta-character interpretation in a shell,
disallows user supplied arguments starting with a dash and limits the 
arguments to safe lengths (no larger than 1000 bytes). A proposed patch is 
attached below.

Relevant information concerning security issues while programming for web 
sites can be found at http://www.w3.org/Security/Faq/www-security-faq.html


Job

--------------------------
Job de Haas | job@itsx.com
ITSX | http://www.itsx.com


Patch for samplescript:
===========================

--- samplescript        Tue Sep 29 14:01:35 1998
+++ samplescript.new    Mon Nov  2 22:27:46 1998
@@ -72,7 +72,12 @@
 
 $count=0;
 
-open(SWISH, "$swish -w $query -m $results -f $index|");
+# Remove leading dashes and limit to 256 characters
+$query =~ s/^-+(.*)/$1/;
+$results =~ s/^-+(.*)/$1/;
+$query =~ s/(.{256}).*/$1/;
+$results =~ s/(.{256}).*/$1/;
+open(SWISH,"-|") || exec $swish,"-w",$query,"-m",$results,"-f",$index;
 
 #Check for errors

============================
Received on Tue Nov 10 13:22:34 1998