Hi @ll,

the executable installer clamwin-0.99-setup.exe (available from
<https://www.clamwin.com/download>) loads and executes DWMAPI.dll
or UXTheme.dll from its "application directory".


For software downloaded with a web browser the application
directory is typically the user's "Downloads" directory: see
<https://insights.sei.cmu.edu/cert/2008/09/carpet-bombing-and-directory-poisoning.html>,
<https://blog.acrossecurity.com/2012/02/downloads-folder-binary-planting.html>
and <https://seclists.org/fulldisclosure/2012/Aug/134> for
"prior art" about this well-known and well-documented vulnerability.


If an attacker places one of the above named DLL in the user's
"Downloads" directory (for example per "drive-by download"
or "social engineering") this vulnerability becomes a remote
code execution.


Proof of concept/demonstration:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

1. visit <https://home.arcor.de/skanthak/sentinel.html>, download
   <https://home.arcor.de/skanthak/download/SENTINEL.DLL>, save it
   as UXTheme.dll in your "Downloads" directory, then copy it as
   DWMAPI.dll;

2. download clamwin-0.99-setup.exe and save it in your "Downloads"
   directory;

3. execute clamwin-0.99-setup.exe from your "Downloads" directory;

4. notice the message boxes displayed from the DLLs placed in
   step 1.

PWNED!


See <https://seclists.org/fulldisclosure/2015/Nov/101>,
<https://seclists.org/fulldisclosure/2015/Dec/86> and
<https://seclists.org/fulldisclosure/2015/Dec/32> plus
<https://home.arcor.de/skanthak/!execute.html> and
<https://home.arcor.de/skanthak/sentinel.html> for details about
this well-known and well-documented BEGINNER'S error!  


stay tuned
Stefan Kanthak


PS: I really LOVE (security) software with such trivial beginner's
    errors. It's a tell-tale sign to stay away from this snakeoil!


Timeline:
~~~~~~~~~

2016-03-06    sent vulnerability report to authors

<security@clamwin.com>: host aspmx.l.google.com[64.233.184.26] said: 550-5.1.1
    The email account that you tried to reach does not exist. Please try
    550-5.1.1 double-checking the recipient's email address for typos or
    550-5.1.1 unnecessary spaces. Learn more at 550 5.1.1
    https://support.google.com/mail/answer/6596 y186si9894139wmy.43 - gsmtp (in
    reply to RCPT TO command)

<clamwin@sf.net>: host mx.sourceforge.net[216.34.181.68] said: 550 unknown user
    (in reply to RCPT TO command)

2016-03-06    report published