[+] HumHub .htaccess file upload vulnerability and remote code execution
[+] Discovered by: Jos Wetzels
[+] Vendor: HumHub
[+] Product: HumHub
[+] Versions affected: 0.10.0 and earlier.
[+] Advisory URL: https://www.leakfree.nl/advisories/leakfree_2015_003.html

HumHub [1] versions 0.10.0 and prior suffer from a file upload
sanitation vulnerability which allows an attacker to upload arbitrary
.htaccess files with varying consequences [2]. On default
installations this allows an attacker to achieve remote code execution
while on a minority of installations it allows an attacker to execute
an XSS or CSRF attack.

Direct access to the uploads/file/ directory is denied by HumHub
through the usage of .htaccess but an attacker can upload a .htaccess
file starting with:

<Files ~ "^\.ht">
# Uncomment line below for Apache >= 2.4
# Require all granted
   Order allow,deny
   Allow from all
</Files>

To allow direct access to the uploaded .htaccess file (and override
general .htaccess settings within the directory). Depending on the
enabled apache modules an attacker can execute various types of
attacks ranging from information disclosure (when mod_info and
mod_status are enabled), eg.:

SetHandler server-info

to remote code execution, eg.:

AddType application/x-httpd-php .htaccess
# <?php phpinfo(); ?>

HumHub comes with a .htaccess.dist [3] file in the HumHub root
directory which, if enabled by the user, prevents direct access to
dotfiles (such as .htaccess, .svn, .git, etc.) using mod_rewrite.
Since mod_rewrite is unaffected by the override (and using the
RewriteEngine Off directive does not affect the .htaccess file itself)
this prevents the above scenario from being exploitable. Regardless,
it is still possible for an attacker to execute an unrestricted XSS or
CSRF attack by abusing the ErrorDocument directive, eg.:

ErrorDocument 403 <htmlpayload>

Since the included .htaccess.dist is not enabled by default, however,
an attacker can exploit this vulnerability to its full extent on an
out-of-the-box HumHub installation.

[*] References:

1. http://humhub.org
2. https://github.com/wireghoul/htshells
3. https://github.com/humhub/humhub/blob/master/.htaccess.dist