Hi @ll,

the executable installers python-3.5.1-webinstall.exe and
python-3.5.1.exe available on
<https://www.python.org/downloads/windows/> load and execute
multiple DLLs from their "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>,
<http://blog.acrossecurity.com/2012/02/downloads-folder-binary-planting.html>
and <http://seclists.org/fulldisclosure/2012/Aug/134> for "prior art"
about this well-known and well-documented vulnerability.


If an attacker places one of these DLLs in the users "Downloads"
directory (for example per drive-by download or social engineering)
this vulnerability becomes a remote code execution.


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

(verified on Windows XP, Windows Vista, Windows 7, Windows Server
2008 [R2]; should work on newer versions too)

1. visit <http://home.arcor.de/skanthak/sentinel.html>, download
   <http://home.arcor.de/skanthak/download/SENTINEL.DLL> and store
   it as FEClient.dll in your "Downloads" directory, then copy it
   as ClbCatQ.dll (Windows NT 5.x) or ProfAPI.dll (Windows NT 6.x);

2. download python-3.5.1-webinstall.exe and python-3.5.1.exe and
   store them in your "Downloads" directory;

3. run python-3.5.1-webinstall.exe and python-3.5.1.exe from your
   "Downloads" directory;

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

PWNED!


5. copy FEClient.dll as MSI.dll and Version.dll;

6. rerun python-3.5.1-webinstall.exe and python-3.5.1.exe from your
   "Downloads" directory.

DOSSED!


The denial of service from step 6. can easily be turned into an
arbitrary code execution: just create an MSI.dll or Version.dll
with the exports referenced from the executable installers.


For this well-known (trivial, easy to avoid, easy to detect and
easy to fix) beginner's error see
<https://capec.mitre.org/data/definitions/471.html>,
<https://technet.microsoft.com/en-us/library/2269637.aspx>,
<https://msdn.microsoft.com/en-us/library/ff919712.aspx> and
<https://msdn.microsoft.com/en-us/library/ms682586.aspx> plus
<http://blogs.technet.com/b/srd/archive/2014/05/13/load-library-safely.aspx>


Additionally python-3.5.1-webinstall.exe and python-3.5.1.exe
create the UNSAFE temporary directories
%TEMP%\{a75b6a1c-5ef0-42f0-ae73-516b23a1d753}\.b<letter><number>\
and
%TEMP%\{c39d559b-aa83-4476-ba20-988a35a1199a}\.b<letter><number>\
respectively where they unpack some files and a DLL for execution.
An unprivileged user can overwrite/modify these files and the DLL
between their extraction and use/execution.

PWNED once more!


For this well-known (trivial, easy to avoid, easy to detect and
easy to fix) beginner's error see
<https://cwe.mitre.org/data/definitions/377.html>,
<https://cwe.mitre.org/data/definitions/379.html>,
<https://capec.mitre.org/data/definitions/27.html>,
<https://capec.mitre.org/data/definitions/29.html> ...


See <http://seclists.org/fulldisclosure/2015/Nov/101>,
<http://seclists.org/fulldisclosure/2015/Dec/86> and
<http://seclists.org/fulldisclosure/2015/Dec/121> plus
<http://home.arcor.de/skanthak/sentinel.html> and the still unfinished
<http://home.arcor.de/skanthak/!execute.html> for more details and why
executable installers (and self-extractors too) are bad and should be
dumped.


stay tuned
Stefan Kanthak


Timeline:
~~~~~~~~~

2015-11-13    report sent to python.org

2015-11-13    auto-response from python.org
              "will investigate and reply ASAP"

2015-12-23    requested status from vendor
              "How do you define ASAP?"

              NO ANSWER, not even an acknowledgement of receipt

2016-01-15    report published