4.13. Security considerations

4.13.1. Authentication types

Authentication is the act of establishing the identity of a user on the client side to the server, and possibly also of establishing the identity of a server to the client.

There are two levels of authentication in CIM-XML:

  • TLS/SSL level authentication (only when HTTPS is used):

    This kind of authentication is also known as transport level authentication. It is used during the TLS/SSL handshake protocol, before any HTTP requests flow.

    In almost all cases (unless an anonymous cipher is used), this involves an X.509 certificate that is presented by the server (therefore called server certificate) and that allows the client to establish the identity of the server.

    It optionally involves an X.509 certificate that is presented by the client (therefore called client certificate) and that allows the server to establish the identity of the client or even of the client user, and thus can avoid the use of credentials in the HTTP level authentication.

    If a client certificate is used, the authentication scheme at the TLS/SSL level is called 2-way authentication (also known as client authentication or mutual SSL authentication). If a client certificate is not used, the authentication scheme is called 1-way authentication (also known as SSL authentication).

    Userid/password credentials do not play any role in TLS/SSL level authentication.

  • HTTP level authentication:

    This kind of authentication is used in HTTP/HTTPS requests and responses (in case of HTTPS, after the TLS/SSL handshake protocol has completed).

    In case of Basic Authentication and Digest Authentication (see RFC2617), it involves passing credentials (userid and password) via the Authenticate and WWW-Authenticate HTTP headers. In case of no authentication, credentials are not passed.

    A client can either provide the Authenticate header along with a request, hoping that the server supports the authentication scheme that was used.

    A client can also omit that header in the request, causing the server to send an error response with a WWW-Authenticate header that tells the client which authentication types are supported by the server (also known as a challenge). The client then repeats the first request with one of the supported authentication types.

    HTTP is extensible w.r.t. authentication schemes, and so is CIM-XML. However, pywbem only supports Basic Authentication and no authentication.

    X.509 certificates do not play any role in HTTP level authentication.

HTTP/HTTPS knows a third level of authentication by the use of session cookies. CIM-XML does not define how cookies would be used, and pywbem does not deal with cookies in any way (i.e. it does not pass cookies provided in a response into the next request).

The following table shows the possible combinations of protocol, TLS/SSL level and HTTP level authentication schemes, which information items need to be provided to the WBEM client library, and whether the combination is supported by pywbem:

Protocol SSL auth. HTTP auth. Credentials Client cert. CA cert. Supported
HTTP N/A None No No No Yes (1)
HTTP N/A Basic Yes No No Yes (2)
HTTP N/A Digest Yes No No No
HTTPS 1-way None No No Yes (3) Yes (1)
HTTPS 1-way Basic Yes No Yes (3) Yes
HTTPS 1-way Digest Yes No Yes (3) No
HTTPS 2-way None No Yes Yes (3) Yes (4)
HTTPS 2-way Basic Yes Yes Yes (3) Yes
HTTPS 2-way Digest Yes Yes Yes (3) No


  1. This option does not allow a server to establish the identity of the user. Its use should be limited to environments where network access is secured.
  2. The use of HTTP Basic Authentication is strongly discouraged, because the password is sent unencrypted over the network.
  3. A CA certificate is needed, unless server certificate verification is disabled via the no_verification parameter (not recommended), or unless an anonymous cipher is used for the server certificate (not recommended).
  4. This is the most desirable option from a security perspective, if the WBEM server is able to establish the user identity based on the client certificate.

The protocol and authentication types that can be used on a connection to a WBEM server are set by the user when creating the WBEMConnection object:

  • The scheme of the URL in the url parameter controls whether the HTTP or HTTPS protocol is used.
  • The cred parameter may specify credentials (userid/password). If specified, pywbem uses them for Basic Authentication at the HTTP level. Pywbem provides an Authenticate HTTP header on each request, and also handles server challenges transparently to the user of the WBEM client library, by retrying the original request.
  • The x509 parameter may specify an X.509 client certificate and key. If specified, pywbem uses 2-way authentication; otherwise it uses 1-way authentication at the TLS/SSL level.
  • The ca_certs parameter controls which X.509 CA certificates are used to validate the X.509 server certificate returned by the WBEM server.

It is important to understand which side actually makes decisions about security-related parameters: The client only decides whether HTTP or HTTPS is used, and whether the server certificate is verified. The server decides everything else: Which HTTP authentication scheme is used (None, Basic, Digest), whether an X.509 client certificate is requested from the client and if so, whether it tolerates a client not providing one. In addition, when HTTPS is used, the client proposes cipher suites it supports, and the server picks one of them.

Therefore, the cred and x509 parameters do not control the authentication scheme that is actually used, but merely prepare pywbem to deal with whatever authentication scheme the WBEM server elects to use.

WBEM servers typically support corresponding configuration parameters.

4.13.2. Verification of the X.509 server certificate

When using HTTPS, the TLS/SSL handshake protocol requires that the server always returns an X.509 server certificate to the client (unless anonymous ciphers are used, which is not recommended).

Pywbem uses the requests Python package for communicating with the WBEM server, and thus delegates the validation of the server certificate to OpenSSL.

If the validation of the server certificate fails, the WBEM operation methods of the WBEMConnection object raise pywbem.ConnectionError with an according error message.

Validation of the server certificate can be disabled via the no_verification parameter of WBEMConnection. Disabling certificate validation makes the communication of pywbem with the WBEM server vulnerable to man-in-the-middle attacks, because the identity of the server cannot be verified.

4.13.3. Use of X.509 client certificates

When using HTTPS, the TLS/SSL handshake protocol provides the option for the client to present an X.509 certificate to the server (therefore called client certificate).

This procedure is initiated by the server, by requesting that the client present a client certificate. If the client does not have one (for example, because the x509 parameter was not specified in pywbem), it must send an empty list of certificates to the server. Depending on the server configuration, the server may or may not accept an empty list. If a client certificate is presented, the server must validate it.

The server can support to accept the user identity specified in the client certificate as the user’s identity, and refrain from sending HTTP challenges that request credentials.

4.13.4. Authentication errors

The operation methods of WBEMConnection raise pywbem.AuthError when the WBEM server returns HTTP status 401 “Unauthorized” and the retries in the client are exhausted. The server typically returns that status in any of these situations:

  • invalid credentials provided by client
  • no credentials provided by client but server requires them
  • user is not authorized to access a resource
  • server does not support the HTTP authentication scheme used by the client. Pywbem uses the “Basic” authentication scheme, which is recommended in DSP0200.