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Re: Cgisecurity.com Advisory #3.1
From: Gunther Birznieks <gunther () EXTROPIA COM>
Date: Wed, 10 Jan 2001 08:10:21 +0800

Clarification to the remote execution versus remote file reading portion of
the advisory:

1) Very old versions of bbs_forum.cgi suffered from ability to execute
commands through lack of input handling.

This was fixed several years ago two-fold: (1) adding taint mode and (2)
tightening perl's open() commands so that the file operator (eg <) were
included literally instead of relying on the default open operator being
"open for read".

Part of this advisory was written from testing this pre-taintmode version
of the script.

2) However, taintmode does not check user input for reading files. So a
directory traversal/arbitrary file reading bug remained in the read
parameter when used in conjunction with several other input parameters as
stated below.

When we were alerted by CGISecurity.com, we also gave the script another
quick once-over and discovered a different but similar domino effect on the
reply_to_message parameter as well.

Both those problems are fixed with the patch we provided below in
CGISecurity's posting.

In summary, those people running a version of bbs_forum.cgi without
taintmode are running a really old version and should upgrade completely as
they've ignored our prior security alerts.

Those people who do have taintmode enabled in the script can safely assume
the patch below is the only patch they require.

At 04:52 PM 1/7/01 -0500, admin () cgisecurity com wrote:
The staff at cgisecurity.com have found a security issue with a
forum script that is widley used.

Below is the advisory along with the vendor patch.


                                        [Cgi Security Advisory #3.1]
                                          admin () cgisecurity com

January 3rd 2001

Vendor Contacted
January 3rd 2001

Public Release
January 7th 2001

Script Effected: bbs_forum.cgi

Versions Effected:
(Others unknown)



1. Impact

Any file can be read with the permissions of user nobody(or webserver).
Possible root comprimise in bbs_forum.cgi script. Command execution is
allowed and therefore shell spawning is possible. This has been tested on
unix and linux systems only and it is unknown if windows versions exist
and/or are effected.

One thing to be noted about this hole is that perl was in taint mode, and
still allowed files to be read, and commands to be executed. This was
not originally intended. This is proof that perl -t is not always


www.host.com/cgi-bin/bbs_forum.cgi?forum=<valid forum
Will grab the scripts own sourcecode.
Note: In order for this hole to work a valid forum name must be used,
so simply trying to call read= only may not work.

2. Fixes

The vendor has been contacted about this serious security problem.
Please visit the vendor's website for patches and other important

3. Attached Vendor Patch

Note: This is a patch for people who know what they are doing.
Please visit http://www.extropia.com/hacks/bbs_security0.html
for information on upgrading.

********************* Vendor patch snippet **************************

If you have made extensive modifications to bbs_forum.cgi and do not wish
to start over from scratch, search for the line at the start of
bbs_forum.cgi that says


                      And insert afterwards the following:

                      if ($in{'read'} && $in{'read'} !~ /^\d+-\d+\.msg$/i)
                          print "Invalid Message #";
                          die("Invalid Message # provided: " .
                      if ($in{'reply_to_message'} &&
$in{'reply_to_message'} !~ /^\d+-\d+\.msg$/i) {
                          print "Invalid Reply To Message #";
                          die("Invalid Reply To Message # provided: " .

This code assures the script that the message file
form variables can only consist of the strict filename format of digits
followed by a hyphen followed by some digits followed by the literal
string ".msg".

We recommend updating your script as soon as possible.
Special thanks to cgisecurity.com for pointing our the issue.

**************************** End Patch ******************************

Published to the Public January 2001
Copyright January 2001 Cgisecurity.com

Gunther Birznieks (gunther.birznieks () extropia com)
eXtropia - The Web Technology Company

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