This page introduces important policy and procedures associated with InCommon metadata. Other pages describe the availability of multiple metadata aggregates and provide guidance on how to configure specific metadata clients. General configuration issues, including the configuration of outbound firewalls, are discussed below.
Metadata Refresh Policy
InCommon expects participants to refresh metadata daily to ensure that SAML deployments have access to the most up-to-date keys and other registered information. Some software implementations (such as Shibboleth) handle metadata easily, but regardless of your software, please read this entire page to understand the requirements and pitfalls associated with metadata consumption.
It is strongly recommended that InCommon SPs and IdPs refresh and verify metadata at least daily. An optimal configuration would attempt to refresh metadata every hour (assuming your client supports HTTP Conditional GET).
Participants are strongly encouraged to use metadata client software that properly handles metadata; failure to do so can have profound effects on the successful use of the Federation. In addition to maintaining the security of your own deployment, proper metadata use is critical to ensure that other participants can depend on your system behaving correctly when they make changes.
The security implications of metadata refresh!
Regular metadata refresh protects users against spoofing and phishing, and is a necessary precaution in the event of key compromise. Failure to refresh metadata exposes you, your users, and other Federation participants to unnecessary risk.
In addition, if you don't refresh your metadata regularly, it is likely that a software implementation will fail at some point since the XML document carries an expiration date (validUntil) that causes the metadata to expire in two weeks. InCommon strongly recommends that you do not rely on the actual length of this validity interval in any way, and in fact, we reserve the right to shorten the validity interval with little or no notice.
Metadata Refresh Process
The mechanics of metadata refresh:
- Choose the right metadata aggregate for your particular deployment
- Deploy and configure an automated metadata refresh process:
- Install and configure your metadata client software
- Validate the expiration date on downloaded metadata
- Verify the XML signature on downloaded metadata
- Adjust your outbound firewall rules (if necessary)
Signature Verification
Federation metadata is signed for integrity and authenticity. Participants are strongly encouraged to verify the XML signature on the metadata file before use; failure to do so will seriously compromise the security of your SAML deployment.
Verify the XML signature on InCommon metadata!
A trusted metadata process MUST verify the XML signature on InCommon metadata. It is not sufficient to request the metadata via a TLS-protected HTTP connection, which is why the sample process shown below does not rely on TLS.
The InCommon Federation is based on the Explicit Key Trust Model, one of several possible metadata trust models. To bootstrap the trust fabric of the Federation, participants download and configure an authentic copy of the Metadata Signing Certificate into their metadata refresh process. The certificate must be obtained securely since all subsequent operations depend on it.
Once the certificate file is locally installed, you can use it to verify the signature on the metadata file. For example, you could use the XmlSecTool (or some similar 3rd-party tool) to verify the signature:
$ MD_PATH=/path/to/InCommon-metadata.xml
$ /usr/bin/curl --silent http://md.incommon.org/InCommon/InCommon-metadata.xml > $MD_PATH
$ ./xmlsectool.sh --verifySignature --signatureRequired \
--certificate $CERT_PATH --inFile $MD_PATH
You may also want to schema validate the metadata:
$ ./xmlsectool.sh --validateSchema \
--schemaDirectory $SCHEMA_DIR --inFile $MD_PATH
For convenience, we provide a set of (suitably modified) schema files that permit offline schema validation.
Expiry Verification
Federation metadata has an expiration date, much like an X.509 certificate. It is important that expired metadata not be accepted, otherwise an attacker would be able to substitute expired metadata in conjunction with metadata refresh. In particular, a metadata file should not be accepted if any of the following conditions are true:
- If the metadata file does not have a
validUntilattribute on the root element. - If the
validUntilattribute on the root element is expired. - If the
validUntilattribute on the root element is too far into the future.
A metadata reload process should check each of the above conditions before accepting the metadata; alternatively if your SAML implementation is known to ignore/reject expired metadata (a basic correctness requirement), it may be sufficient to ensure that a validUntil attribute exists and is not unexpectedly far into the future.
Verify the expiration date independently!
Verifying the signature on a SAML metadata file does not verify the presence or value of an expiration date. The only way to verify the expiration date is to parse the XML.
Firewall Configuration
Depending on your environment, you may have to poke a hole in an outbound firewall to allow your metadata client to reach the metadata server. In that case, you will actually want to poke two holes in that firewall since there are two metadata servers as described below.
Hostname md.incommon.org resolves to one of two identical servers, either in Michigan (207.75.165.125) or Indiana (140.182.44.53). The actual server used at any given point in time is unspecified and left to the discretion of InCommon Operations. If one of the servers goes down or requires maintenance, the other can be brought up within minutes, with minimal disruption of services.
Therefore, please make sure both your SAML implementation and your metadata refresh processes are configured with hostname md.incommon.org (as opposed to an IP address). On the other hand, make sure your outbound firewall (if any) is configured with both IP addresses (207.75.165.125 and 140.182.44.53).