SSLContext.hostname_checks_common_name appears to have no effect

[pquentin]

BPO	43522
Nosy	@tiran, @pquentin
PRs	bpo-43522: Fix SSLContext.hostname_checks_common_name (GH-24899) #24899 [3.9] bpo-43522: Fix SSLContext.hostname_checks_common_name (GH-24899) (GH-25451) #25451 [3.8] bpo-43522: Fix SSLContext.hostname_checks_common_name (GH-24899) (GH-25452) #25452
Files	no_san_ignored.py: Reproducer app.py: Sample Flask app client.pem: client.pem server.pem: server.pem server.key: server.key

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/tiran'
closed_at = <Date 2021-04-17.10:47:03.885>
created_at = <Date 2021-03-16.20:07:50.463>
labels = ['expert-SSL', 'type-bug', '3.8', '3.9', '3.10']
title = 'SSLContext.hostname_checks_common_name appears to have no effect'
updated_at = <Date 2021-05-04.05:50:13.339>
user = 'https://github.com/pquentin'

bugs.python.org fields:

activity = <Date 2021-05-04.05:50:13.339>
actor = 'christian.heimes'
assignee = 'christian.heimes'
closed = True
closed_date = <Date 2021-04-17.10:47:03.885>
closer = 'christian.heimes'
components = ['SSL']
creation = <Date 2021-03-16.20:07:50.463>
creator = 'Quentin.Pradet'
dependencies = []
files = ['49879', '49880', '49881', '49882', '49883']
hgrepos = []
issue_num = 43522
keywords = ['patch']
message_count = 9.0
messages = ['388875', '388887', '388888', '388907', '391275', '391277', '391278', '391283', '392853']
nosy_count = 2.0
nosy_names = ['christian.heimes', 'Quentin.Pradet']
pr_nums = ['24899', '25451', '25452']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue43522'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']

Linked PRs

• gh-87688: Amend SSLContext.hostname_checks_common_name docs #100517
• [3.12] gh-87688: Amend SSLContext.hostname_checks_common_name docs (GH-100517) #115594
• [3.11] gh-87688: Amend SSLContext.hostname_checks_common_name docs (GH-100517) #115595

[pquentin]

urllib3 is preparing a v2 with various SSL improvements, such as leaning on the ssl module to match hostnames when possible and reject certificates without a SAN. See https://urllib3.readthedocs.io/en/latest/v2-roadmap.html#modern-security-by-default for more details.

For this reason, we want to set hostname_checks_common_name to False on Python 3.7+ and OpenSSL 1.1.0+. (In other cases, we use a modified version of ssl.match_hostname that does not consider common names.)

I would expect that setting hostname_checks_common_name to False would rejects certificates without SANs, but that does not appear to be the case. I used the following Python code:

    import socket
    import ssl
    
    print(ssl.OPENSSL_VERSION)
    hostname = 'localhost'
    context = ssl.create_default_context()
    context.load_verify_locations("client.pem")
    context.hostname_checks_common_name = False
    
    with socket.create_connection((hostname, 8000)) as sock:
        with context.wrap_socket(sock, server_hostname=hostname) as ssock:
                assert "subjectAltName" not in ssock.getpeercert()

which prints OpenSSL 1.1.1i 8 Dec 2020 and does not fail as expected. I'm testing this on macOS 11.2.2 but this currently breaks our test suite on Ubuntu, Windows and macOS, including on Python 3.10, see https://github.com/urllib3/urllib3/runs/2122811894?check_suite_focus=true.

To reproduce this, I used trustme (https://trustme.readthedocs.io/en/latest/). I modified the code to not include a SAN at all and ran gunicorn --keyfile server.key --certfile server.pem app:app, with app being the Flask quickstart application. I'll try to attach all those files if I manage to do it.

What am I missing?

[tiran]

Oh heck, this is a genuine bug. I'm not yet sure if it's an undocumented API quirk in OpenSSL, a design bug in OpenSSL, or a bug in my code.

Python sets the host flags on the X509_VERIFY_PARAM of the *SSL_CTX. All flags get copied to *SSL struct and later to *X509_STORE_CTX struct. At least I thought that all flags get copied. Apparently hostflags aren't copied from *SSL_CTX to *SSL because the *SSL_CTX doesn't have any verify hosts configured. They are only ever configured on *SSL struct.

https://github.com/openssl/openssl/blob/081a7061f3da07318c4b0f5de67b82285630bf6b/crypto/x509/x509_vpm.c#L202-L213

[tiran]

PS: I don't see any remark or warning about the behavior on the man pages https://www.openssl.org/docs/man1.1.1/man3/X509_VERIFY_PARAM_set_flags.html and https://www.openssl.org/docs/man1.1.1/man3/X509_check_host.html

[pquentin]

Thank you for the quick fix! 🙏 Both the reproducer and the urllib3 test suite run fine with this change.

However, we can't trust HAS_NEVER_CHECK_COMMON_NAME anymore, because it will be True in Python versions where hostname_checks_common_name does not work. Is it possible to have, uh, REALLY_HAS_NEVER_CHECK_COMMON_NAME_I_PROMISE or something like that? :D It could even be private. Otherwise, we will only be able to use hostname_checks_common_name in Python 3.10.0a7+.

[tiran]

New changeset b467d9a by Christian Heimes in branch 'master':
bpo-43522: Fix SSLContext.hostname_checks_common_name (GH-24899)
b467d9a

[tiran]

New changeset cdf0287 by Christian Heimes in branch '3.9':
[3.9] bpo-43522: Fix SSLContext.hostname_checks_common_name (GH-24899) (GH-25451)
cdf0287

[tiran]

New changeset f77ca86 by Christian Heimes in branch '3.8':
[3.8] bpo-43522: Fix SSLContext.hostname_checks_common_name (GH-24899) (GH-25452)
f77ca86

[tiran]

Workaround has been added to upcoming 3.8 to 3.10 releases. Older versions will get fixed by next OpenSSL update.

[tiran]

Seth's urllib3 newsletter reminded me that I forgot to link to OpenSSL issues here.

The problem was caused by a bug in OpenSSL. The issue is fixed in OpenSSL default branch and is scheduled to land in next 1.1.1 release. My changes to Python's ssl module are backports of my OpenSSL fixes for older 1.1.1 releases.

openssl/openssl#14579
openssl/openssl#14856
openssl/openssl@dfccfde