Originally reported at https://github.com/rg3/youtube-dl/issues/7951

Steps to reproduce:
1. Build 99665:dcf9e9ae5393 with Visual Studio 2015
2. Download and extract PsTools [1]
3. PsExec.exe -l python.exe
4. In Python, run:

    import _ssl
    _ssl.enum_certificates("CA")
    _ssl.enum_crls("CA")

Results:
Python 3.6.0a0 (default, Dec 25 2015, 02:42:42) [MSC v.1900 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import _ssl
>>> _ssl.enum_certificates("CA")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
PermissionError: [WinError 5] Access is denied
>>> _ssl.enum_crls("CA")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
PermissionError: [WinError 5] Access is denied
>>>

Windows Vista and above have a security mechanism called "Low Integrity Level". [2] With that, only some specific registry keys are writable. In the original _ssl.c, both enum_certificates() and enum_crls() calls CertOpenSystemStore(). At least on my system CertOpenSystemStore() tries to open HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\SystemCertificates\CA with read/write permissions. (Observed with Process Monitor [3]) The request fails in Low Integrity Level processes as it's not in the range of writable registry keys.

Here I propose a fix: open certificate stores with the read-only flag. There are some points I'm not sure in this patch:
1. CERT_STORE_PROV_SYSTEM_A: I guess strings are bytestrings in C level?
2. CERT_SYSTEM_STORE_LOCAL_MACHINE: In accounts of Administrators, CertOpenSystemStore() tries to open keys under HKLM only, while in restricted (standard) accounts, this function tries to open keys under HKCU with R/W permission and keys under HKLM read-only. I think open system global stores is OK here.
A different perspective: Wine developers always open keys under HKCU in CertOpenSystemStore()

Environment: Windows 7 SP1 (6.1.7601) x86, an account in Administrators group. Tested with python.exe Lib\test\test_ssl.py both in a normal shell and within `PsExec -l`

Ref: issue17134, where these codes appear the first time

[1] https://technet.microsoft.com/en-us/sysinternals/pstools.aspx
[2] https://msdn.microsoft.com/en-us/library/bb625960.aspx
[3] https://technet.microsoft.com/en-us/sysinternals/processmonitor.aspx
[4] https://github.com/wine-mirror/wine/blob/master/dlls/crypt32/store.c

Looks good to me.

Is it worth dropping psexec.exe into the test suite so we can add a test for this (or maybe into tools so we can run it from a build without redistributing the exe)? It'll probably be helpful elsewhere too (symlink tests, for example).

psexec.exe can be run from the the live server.

    >>> subprocess.call(r'\\live.sysinternals.com\tools\psexec.exe -s whoami')

    PsExec v2.11 - Execute processes remotely
    Copyright (C) 2001-2014 Mark Russinovich
    Sysinternals - www.sysinternals.com


    nt authority\system
    whoami exited on THISPC with error code 0.
    0

But the executable could also be cached on the test system.

PsExec.exe seems not redistributable. PAExec is an alternative but I've not tried it. [1] Another option is re-implementing a tiny program for lowering the integrity level based on example codes provided in [2], which I've not tried yet, either. The latter option seems better to me as I didn't find codes for lowering the integrity level in PAExec's source code. [3]

[1] https://www.poweradmin.com/paexec/
[2] https://msdn.microsoft.com/en-us/library/bb625960.aspx
[3] https://github.com/poweradminllc/PAExec

OK I've just succeeded in creating a low integrity level process with my own codes. Now the problem is: how can I integrate this tool into the test system? Seems the integrity level is per-process, while all tests are run in the same process.

Added tests for ssl.enum_certificates() and ssl.enum_crls() within low integrity processes.

Can we use ctypes in the test suite? That would probably be simpler here, as the C code is straightforward. (Question is for the other core devs, not Chi, as I know we're not supposed to use ctypes in the stdlib.)

ctypes in the test suite is fine, just be sure it's guarded properly (with either `ctypes = test.support.import_module("ctypes")` or `if sys.platform == 'win32'): ...`.

Testing based on integrity level doesn't require creating a child process. I'm attaching a ctypes-based example that defines a context manager that temporarily sets the integrity level of the current thread's impersonation token. 

To get the impersonation token, I initially tried using ImpersonateSelf / RevertToSelf, but I was unhappy with how it fails for nested contexts since RevertToSelf always switches back to the process token. I opted to instead call OpenThreadToken / OpenProcessToken, DuplicateTokenEx, and SetThreadToken. 

I chose to use the WELL_KNOWN_SID_TYPE enum values to get the label SIDs via CreateWellKnownSid. Note that I omitted the GetLengthSid call when passing the size of the TOKEN_MANDATORY_LABEL to SetTokenInformation. It only needs the size of the primary buffer. The SID it points at is a sized structure (i.e. SubAuthorityCount). 

Example:

    import winreg

    HKLM = winreg.HKEY_LOCAL_MACHINE
    subkey = r'SOFTWARE\Microsoft\SystemCertificates\CA'
    access = winreg.KEY_ALL_ACCESS

    >>> key = winreg.OpenKey(HKLM, subkey, 0, access)    
    >>> print(key)
    <PyHKEY:0x0000000000000178>
    >>> key.Close()

Repeat with low integrity level:

    >>> with token_integrity_level('low'):
    ...      winreg.OpenKey(HKLM, subkey, 0, access)
    ...
    Traceback (most recent call last):
      File "<stdin>", line 2, in <module>
    PermissionError: [WinError 5] Access is denied 

A context manager like this could be added to the test helper module that was proposed in issue 22080. It could also add the ability to impersonate with a restricted copy of the process token -- like what UAC creates. psexec -l does this by calling CreateRestrictedToken followed by SetInformationToken for the TokenIntegrityLevel and TokenDefaultDacl.

New changeset 8ff4c1827499 by Benjamin Peterson in branch '3.5':
merge 3.4 (closes #25939)
https://hg.python.org/cpython/rev/8ff4c1827499

New changeset d6474257ef38 by Benjamin Peterson in branch 'default':
merge 3.5 (closes #25939)
https://hg.python.org/cpython/rev/d6474257ef38

The patch itself seems fine, so I committed that. It doesn't seem like how best to test this has been figured out, so leaving the issue open.

Thanks for accepting my patch. I'm curious: any reason not applying to 2.7 branch? We're building youtube-dl.exe with py2exe on Python 2.7 as py2exe on 3.x sometimes fails. (https://github.com/rg3/youtube-dl/issues/5094)

It was fixed in 2.7 - https://hg.python.org/cpython/rev/3cddcf471c70

The issue number wasn't in the commit, so it didn't appear here.

Didn't see it. Sorry for bothering.

Benjamin, Steve, can we close this ticket?

Yes, and done.