bugtraq - Re: phpBB Worm

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Message-ID: <20041222043459.16337.qmail@www.securityfocus.com>
Date: 22 Dec 2004 04:34:59 -0000
From: <ycw1bh302@...akemail.com>
To: bugtraq@...urityfocus.com
Subject: Re: phpBB Worm

In-Reply-To: <Pine.LNX.4.61.0412212325470.1764@...lbox.prolocation.net>

Forgive me if this is a newbie question, but a site I help run was hit by this, and I'm trying to understand it to protect against future worms.

The worm exploits the phpBB highlight vulnerability.  It uses PHP to run Perl to write the Perl script file, then executes it.  The script then proceeds to traverse the entire directory structure, overwriting .php, .htm, .shtm, .phtm, and on our server, .ssi files, and then spreads itself.  Correct?

I have two questions:

1.  Why has the worm been as effective on Windows servers as on *nix servers?  At the very least, shouldn't the difference in file and directory naming cause a problem?  I looked at the decoded Perl script, but I'm not a Perl expert, so I couldn't understand all of it.  And what about the difference in file permissions?

2.  More importantly, why wasn't the worm's destructive ability limited by file permissions, especially on *nix servers?  If, for example, an HTML file on the server was uploaded by user bob, and has permissions of 755, how can the Perl script delete that file?  Shouldn't the Perl script be created with the Perl process's permissions, which was invoked by PHP, which should have the Web server's permissions, which should be, at least on most *nix servers, the nobody user?

This is a big issue on shared servers, or virtual hosts, whatever you want to call them.  Our site is on a shared server, and our site does not even run phpBB, but most of our HTML files were replaced with the worm's content.  Obviously, then, another site on the server must have an old version of phpBB.  But why could the worm, coming in through another site, modify files created by other users?  Even if the worm's script ran as the owner of the vulnerable viewtopic.php file, how could it then modify non-world-writable files created by other users?

I have long been concerned with the security of PHP scripts, especially on shared servers.  Since PHP almost always runs as an Apache module, and Apache usually runs as nobody, one must make files and directories world-writable for PHP scripts to be able to write to them.  But that means that any process on the server, including anyone's PHP script, can modify the files.

Thanks for any insights.

Adam Porter