Hi @ll,

a looong time ago Microsoft "addressed" a so called "blended"
threat: Internet Explorer loaded and executed DLLs placed on
the user's desktop.

See <https://technet.microsoft.com/en-us/library/953818>
(titled "Blended Threat from Combined Attack Using Apple's
Safari on the Windows Platform") plus
<https://blogs.technet.com/b/srd/archive/2009/04/14/ms09-014-addressing-the-safari-carpet-bomb-vulnerability.aspx>
for this vulnerability and the (obviously shortsighted)
attempt to fix it.


What about EXEs placed on the user's desktop?

Besides LoadLibrary*() there exist also CreateProcess*() and
ShellExecute*() in the Win32 API to load and execute files.


What about programs beside Internet Explorer?

There exist quite some programs besides Internet Explorer in
every version of Windows whose developers exercise neither all
due diligence nor defense in depth and whose QA is sound asleep
... the Windows Explorer for example, or the Control Panel
executable.


On Windows Embedded POSReady 2009 alias Windows XP SP3,
    Start->Run explorer [ENTER]
runs a rogue explorer.exe (and of course explorer.com, explorer.bat,
explorer.cmd too) which happens to be present
* in the user's profile directory "%USERPROFILE%\",
* on the user's desktop, i.e. the directory "%USERPROFILE%\Desktop\".

The location depends on the policy setting

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\Currentversion\Policies\Explorer]
"StartRunNoHOMEPATH"=dword:00000000 ; -> User Profile
"StartRunNoHOMEPATH"=dword:00000001 ; -> Desktop

documented in <https://support.microsoft.com/en-us/kb/264061>


Proof of concept:
~~~~~~~~~~~~~~~~~

1. visit <http://home.arcor.de/skanthak/sentinel.html>, then
   download <http://home.arcor.de/skanthak/download/SENTINEL.EXE>
   and save it as explorer.exe on your desktop as well as your
   user profile.

2. Start->Run explorer [ENTER]

PWNED!


Also give rundll32.exe a try: rename the explorer.exe saved on
your desktop and the user profile to rundll32.exe, then
    Start->Run control userpasswords [ENTER]
(or use one of the many other "control ..." commands documented
for example in <https://support.microsoft.com/en-us/kb/180025>).

PWNED again!


JFTR: of course "SafeProcessSearchMode" is set: this is supposed
      to alter Windows' NOTORIOUS unsafe search path, but is
      obviously ineffective!
     


On Windows Vista and newer versions of Windows
    Start->Run <filename> [ENTER]
runs a rogue <filename>.exe (and of course <filename>.com,
<filename>.bat, <filename>.cmd too) which happens to be present
* in the user's profile directory "%USERPROFILE%\",
* on the user's desktop, i.e. the directory "%USERPROFILE%\Desktop\",
again depending on the above named policy.


JFTR: defining the environment variable
      NoDefaultCurrentDirectoryInExePath
      (<https://msdn.microsoft.com/en-us/library/ms684269.aspx>)
      doesn't change this behaviour.


Now take a look at
<https://www.microsoft.com/security/portal/definitions/adl.aspx>:

| Antimalware and antispyware updates
|
| For antimalware and antispyware, the latest definitions are
| 1.223.45.0, dated May 28, 2016 10:52 AM UTC.
|
| To download these updates:
| 1. Check whether your version of Windows is 32-bit or 64-bit.
| 2. In the table below, right-click on the link that will work
|    for your version of Windows and choose Save target as... or
|    Save link as...
| 3. Save the file to your Desktop.
| 4. When the file has finished downloading, go to your Desktop
|    and double-click the file (it will be called mpam-fe.exe,
|    mpas-fe.exe, or mpam-feX64.exe).
| 5. Follow the prompts to install the update.

JFTR: other snakeoil vendors have similar instructions on their
      websites too.


Users who follow these instructions set the default location for
saving downloads to their desktop; if they later visit a website
that triggers the download of an executable <filename>.{com,exe,
cmd,bat} which the user confirms (or doesn't notice: see "drive-by
download") and then use Start->Run <filename> [ENTER] they run the
rogue program placed on their Desktop (if "StartRunNoHomePath" is
set).


See <https://cwe.mitre.org/data/definitions/426.html> and
<https://cwe.mitre.org/data/definitions/427.html> plus
<https://insights.sei.cmu.edu/cert/2008/09/carpet-bombing-and-directory-poisoning.html>
for this well-known vulnerability.

For "prior art" see <http://www.securityfocus.com/archive/1/513595>


Microsoft: I recommend to dump your "cloud first, mobile first"
strategy and switch to "safety and security first" instead.

MSRC: your communication habit is GREAT, once again! NOT!


Mitigation(s):
~~~~~~~~~~~~~~

Deny execution in the "%USERPROFILE%" of every user plus
"%ALLUSERSPROFILE%" alias "%ProgramData%"

* via the inheritable NTFS ACE (D;OIIO;WP;;;WD) meaning
  "deny execution of files in this directory and below for
  everyone",

* via SAFER alias software restriction policies (see
  <http://home.arcor.de/skanthak/SAFER.html> or
  <http://www.mechbgon.com/srp/index.html> for instructions, and
  <https://technet.microsoft.com/en-us/library/aa940985.aspx>
  for Microsoft's recommendation).


stay tuned
Stefan Kanthak