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Title: ssl.enum_certificates() will not return all certificates trusted by Windows
Type: behavior Stage: needs patch
Components: Extension Modules, SSL, Windows Versions: Python 3.7, Python 3.6, Python 3.5
Status: open Resolution:
Dependencies: Superseder:
Assigned To: christian.heimes Nosy List: Adam.Goodman, christian.heimes, jteh, loewis, nagle, paul.moore, pitrou, steve.dower, tim.golden, yan12125, zach.ware
Priority: normal Keywords:

Created on 2014-03-13 19:48 by Adam.Goodman, last changed 2022-04-11 14:57 by admin.

File name Uploaded Description Edit Adam.Goodman, 2014-03-13 19:49 simple test script to request
Messages (7)
msg213452 - (view) Author: Adam Goodman (Adam.Goodman) Date: 2014-03-13 19:48
Starting with Vista, Microsoft began shipping only a very minimal set of root CA certificates with Windows. Microsoft does trust many other authorities, but for these, Windows relies on the "Update Root Certificates" feature:

"... if the application is presented with a certificate issued by a certification authority in a PKI that is not directly trusted, the Update Root Certificates feature (if it is not turned off) will contact the Windows Update Web site to see if Microsoft has added the certificate of the root CA to its list of trusted root certificates. If the CA has been added to the Microsoft list of trusted authorities, its certificate will automatically be added to the set of trusted root certificates on the user's computer."

Critically, this update mechanism is only invoked if you're using CryptoAPI functions to validate a specific chain; if you just ask Windows to enumerate the certificates it knows about, it won't pull anything down from Windows Update.

(Some concrete numbers: on a clean installation of Windows 8.1, running certmgr.msc shows 18 certificates listed in the "Trusted Root Certification Authorities"; by contrast, OS X comes with over 200 trusted roots).

To confirm this is an issue, I did the following:

1. Start with a clean Windows 8.1 VM image (I used the one from from It is critical that the image be completely clean - i.e. you have never visited in any web browser, etc.

2. Install Python 3.4.0 RC 3 (32-bit)

3. Run the attached script (which just does a request to with cert validation enabled). It prints out 14 CA certificate subjects, then fails with "ssl.SSLError: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:598)". At the time I'm reporting this issue, uses a certificate that chains back to the "DigiCert High Assurance EV Root CA", which is not included in clean installations of Windows.

4. Browse to in Internet Explorer

5. Run the attached script again. Now it prints out 17 CA certificate subjects, and the HTTPS request completes successfully.

Right now, the only idea I have for resolving this would require significant architectural changes - instead of pulling the certificates from Windows into an OpenSSL context, hook the OpenSSL verify callbacks to retrieve the leaf and intermediate certificates provided by the server, then use CryptoAPI functions (probably CertGetCertificateChain?) to have Windows perform the actual chain validation.
msg213461 - (view) Author: Christian Heimes (christian.heimes) * (Python committer) Date: 2014-03-13 20:33
Thanks for you tests!

Yes, I was aware of the situation in general. Personally I think it is an unfortunate decision of Microsoft to download root CA certs on demand. When I developed the feature I only experimented with a fresh but fully patched VM of Windows 7 Professional. The VM had more root CAs installed so I didn't think it's going to bite the majority users for common sites. In retrospective I *might* have trigger cert downloads accidentally...

I also tried to implement a OpenSSL's verify hook but my code was far from ready for 3.4 beta. I'll have to implement a proper solution for Python 3.5. The situation on OSX and Windows isn't perfect.

KB931125 lists a way to trigger a full download of all known root certs. Do you still have a fresh VM around? I won't have time to test the tool from KB931125 before 3.4.0 is released.
msg213479 - (view) Author: Adam Goodman (Adam.Goodman) Date: 2014-03-13 21:39
I just tried installing the root certificate update from KB931125 on a clean VM. Now I have 369 trusted root CAs, according to certmgr.msc. (I imagine it would be unreasonable to expect all windows python users to do this, though...)

The https request to does succeed - but the code I'd thrown together to print out the SSLContext state breaks somehow:

> Traceback (most recent call last):
>   File "C:/Users/IEUser/Desktop/", line 8, in <module>
>     ca_certs = context.get_ca_certs()
> ssl.SSLError: unknown error (_ssl.c:636)

Peeking at _ssl.c, it seems like it might be failing to decode some attributes on one (or more) of the certs...?
msg213522 - (view) Author: Martin v. Löwis (loewis) * (Python committer) Date: 2014-03-14 07:43
I notice that this issue doesn't contain actual problem statement; Adam only reported what he did and what happened, but not what should have happened instead.

I personally don't think that the problem stated in the title ("ssl.enum_certificates() will not return all certificates trusted by Windows") is a bug - this is correct behavior. It would be unreasonable to expect that enum_certificates() triggers a download of the entire MS root list, when Microsoft has established as a policy that download should be on demand, triggered by verification.

What I would agree *is* a bug is that the certificate verification fails; it should trigger the root download, as is platform convention (hopefully then also conforming to the group policy setting where you can disable root certificate download).

Please leave out unrelated bugs (e.g. a failure to fetch certain certificate attributes) from this bug report. Report them separately instead.
msg213532 - (view) Author: Antoine Pitrou (pitrou) * (Python committer) Date: 2014-03-14 10:32
If this is a Microsoft decision, perhaps it should be documented, then.
msg213888 - (view) Author: Adam Goodman (Adam.Goodman) Date: 2014-03-17 19:53
What Martin said is correct, IMO.

The actual problem I'd like to correct is: If I - for example - create an HTTPSConnection with cert validation enabled, and set to use the default OS trust mechanism, then the validation process should trigger Windows' root CA download mechanism if necessary (i.e. rather than just rejecting the CA cert if it hasn't already been locally cached).

I don't expect that ssl.enum_certificates() ever will return all the certificates that are (implicitly, via the update mechanism) trusted by Windows; that's probably not feasible. I chose that as the title of the issue because it seemed to be the most concrete root-cause, but maybe that wasn't ideal.

(I'll file a separate issue for that traceback I ran into, if I get a chance to dig into it more)
msg235802 - (view) Author: John Nagle (nagle) Date: 2015-02-12 07:09
Amusingly, I'm getting this failure on "" on Windows 7 with Python 2.7.9:

"HTTP error - [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:581)..)"  The current Verisign root cert (Class 3 public) is, indeed, not in the Windows 7 cert store. Verisign has a newer root cert.

That error message ought to be improved. Tell the user which cert was rejected.

"", with a DigiCert certificate, works fine.

I'm going to use the Mozilla certificate store explicitly.
Date User Action Args
2022-04-11 14:57:59adminsetgithub: 65115
2017-02-24 10:05:50yan12125setnosy: + yan12125
2016-09-15 08:09:23christian.heimessetassignee: christian.heimes

nosy: + christian.heimes
components: + SSL
versions: + Python 3.6, Python 3.7
2016-06-12 13:30:57christian.heimessetnosy: - christian.heimes
2016-06-12 13:22:51ppperrysetnosy: + paul.moore, tim.golden, zach.ware
components: + Windows
2016-06-12 11:23:26christian.heimessetassignee: christian.heimes -> (no value)
2015-02-12 07:09:20naglesetnosy: + nagle
messages: + msg235802
2015-01-17 17:59:01pitrousetnosy: + steve.dower
2015-01-16 05:31:03jtehsetnosy: + jteh
2014-03-17 19:53:38Adam.Goodmansetmessages: + msg213888
2014-03-14 10:32:06pitrousetnosy: + pitrou
messages: + msg213532
2014-03-14 07:43:51loewissetnosy: + loewis
messages: + msg213522
2014-03-13 21:39:05Adam.Goodmansetmessages: + msg213479
2014-03-13 20:33:50christian.heimessetassignee: christian.heimes
stage: needs patch
messages: + msg213461
versions: + Python 3.5, - Python 3.4
2014-03-13 19:49:00Adam.Goodmansetfiles: +
2014-03-13 19:48:21Adam.Goodmancreate