CODEWITHSUNDEEP
pythonsslnginxhttpsclient-certificates
Elsa Culler
Elsa Culler

Reputation: 33

SSL client certificate is not sent by python 'requests' library or s_client, but works fine in web browsers

I am trying to write a client script in Python that accesses a web application and uses SSL client certificates for authentication. Though I am able to access the application from both Firefox and Chrome as long as I have the client certificate loaded, I get the following response whenever I send the request via Python 'requests:

400 No required SSL certificate was sent
nginx/1.4.6 (Ubuntu)

I have also tried Python httplib, s_client, and curl, and get the same error message. I am using the same client certificate for all the testing, in pkcs12 format for the web browsers and pulled out into certificate and key PEM files for the command line tools. My python code looks like:

import requests

CERT = r'/path/to/cert.crt' #Client certificate
KEY = r'/path/to/key.key' #Client private key
CACERT = r'/path/to/ca.crt' #Server certificate chain

session = requests.Session()
session.cert = (CERT, KEY)
resp = session.get('https://my.webapp.com/',
                   verify=CACERT)

print resp.content
session.close()

s_client gives more information about what is happening. Here's an abbreviated version of the output:

$ openssl s_client -cert cert.crt -key key.key -CAfile ca.crt -connect <host>:<port>
CONNECTED(00000003)
depth=2 /C=US/O=GeoTrust Inc./CN=GeoTrust Global CA
verify return:1
depth=1 /C=US/O=GeoTrust, Inc./CN=RapidSSL CA
verify return:1
depth=0 /serialNumber=tgBIwyM-p18O/aDyvyWNKHDnOezzDJag/OU=GT89519184/OU=See www.rapidssl.com/resources/cps (c)13/OU=Domain Control Validated - RapidSSL(R)/CN=*.rexdb.us
verify return:1
---
Certificate chain
...
---
Server certificate 
-----BEGIN CERTIFICATE-----
...
-----END CERTIFICATE-----
subject=...
---
No client certificate CA names sent
---
SSL handshake has read 3519 bytes and written 328 bytes
---
New, TLSv1/SSLv3, Cipher is DHE-RSA-AES128-SHA
Server public key is 4096 bit
Secure Renegotiation IS supported
Compression: NONE
Expansion: NONE
SSL-Session:
    Protocol  : TLSv1
    Cipher    : DHE-RSA-AES128-SHA
    Session-ID: ...
    Session-ID-ctx:
    Master-Key: ...
    Key-Arg   : None
    Start Time: 1409269239
    Timeout   : 300 (sec)
    Verify return code: 0 (ok)
---
GET / HTTP/1.1
Host: my.webapp.com

HTTP/1.1 400 Bad Request
Server: nginx/1.4.6 (Ubuntu)
Date: Thu, 28 Aug 2014 23:41:04 GMT
Content-Type: text/html
Transfer-Encoding: chunked
Connection: close
...
<head><title>400 No required SSL certificate was sent</title></head>
...
closed

I'm reasonably sure this is not an issue with the server because authentication works in a browser. However, the s_client output says 'No client certificate CA names sent', which sounds like a server problem (e.g. the client certificate is not being sent on to the server because the server isn't asking for it). Here is the relevant part of the nginx configuration:

ssl                  on;
ssl_certificate_key  /path/to/server/key.pem;
ssl_certificate      /path/to/server/certificate.pem;
ssl_protocols        TLSv1 TLSv1.1 TLSv1.2;
ssl_ciphers ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:DHE-DSS-AES128-GCM-SHA256:kEDH+AESGCM:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA:ECDHE-ECDSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA:DHE-DSS-AES128-SHA256:DHE-RSA-AES256-SHA256:DHE-DSS-AES256-SHA:DHE-RSA-AES256-SHA:AES128-GCM-SHA256:AES256-GCM-SHA384:ECDHE-RSA-RC4-SHA:ECDHE-ECDSA-RC4-SHA:AES128:AES256:RC4-SHA:HIGH:!aNULL:!eNULL:!EXPORT:!DES:!3DES:!MD5:!PSK;
ssl_prefer_server_ciphers   on;
ssl_session_timeout  5m;

ssl_client_certificate /path/to/client/ca/certificate.crt;
ssl_verify_depth 10;
ssl_verify_client on;

The application uses uwsgi through nginx. There are multiple virtual hosts on the vm, and only this one uses certificate authentication.

The only other potentially relevant difference I can find is that in Firefox the connection is keep-alive and in s_client it is close. I've tried setting Connection: keep-alive in the header, with the same result.

Upvotes: 2

Views: 6416

Answers (1)

Steffen Ullrich
Steffen Ullrich

Reputation: 123531

There are multiple virtual hosts on the vm, and only this one uses certificate authentication.

I assume that means you have multiple certificates behind the same IP address and that the client has to use SNI (Server Name Indication) to send the server the expected hostname inside the SSL handshake. openssl s_client does not use SNI by default and I don't know if python does - it might depend on the version of python you use.

Because the client only sends a certificate if the server tells it do to it might be, that because of missing SNI you run into the wrong configuration part, that is the default part with another certificate and without requirement for client certificates.

I would recommend to try with openssl s_client again, but use the command line option -servername (not documented in man page but shown if called with -h) to explicitly set the expected server name. If this works you need to find a way to use SNI in python too. If this does not work please make a packet dump and make sure with wireshark, that the server really requires the client to send a certificate and the client really does not send a certificate.

Upvotes: 4

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