[security] Consider upgrading mellon for Bionic to be able to change signature method (sha1 is used by default)

[Dmitrii Shcherbakov]

SHA1 is used as a SAML message signature method by default in lasso shipped with Bionic.

Mellon versions up to 0.13.1 (bionic) do not expose MellonSignatureMethod and so the lasso default of LASSO_SIGNATURE_METHOD_RSA_SHA1 is used.

As of 0.14.0 (Cosmic) MellonSignatureMethod is exposed and is set by default to LASSO_SIGNATURE_METHOD_RSA_SHA256.

https://github.com/Uninett/mod_auth_mellon/commit/9b17e5c1078a9be90de1e9d03079b34ca4056e96

https://github.com/Uninett/mod_auth_mellon/releases/tag/v0.14.0

There is a discussion here https://crypto.stackexchange.com/a/48291 regarding whether usage of SHA1 is important to avoid for SAML use-cases or not, however, I would let the security team to make a decision on this for Bionic as a long-term release (+ considering ESM).

Typically SAML messages are signed to avoid tampering by an intermediary (an HTTP client) while the transport of signed SAML messages is usually done over HTTPS.

Some identity providers started converting to using SHA256 by default, for example, contemporary versions of ADFS use SHA256 and we already ran into this in the field as we have to explicitly tell IdP operators to configure SHA1 to be accepted.

[Marc Deslauriers (mdeslaur)]

Can I make this bug public?

Switching the default algorithm in a stable release may not be an acceptable change. But, adding the code to be able to configure which algorithm to use seems acceptable to me.

[Dmitrii Shcherbakov (dmitriis)]

Marc,

> Can I make this bug public?

Yes, for sure.

> Switching the default algorithm in a stable release may not be an acceptable change

Makes sense to me.

> But, adding the code to be able to configure which algorithm to use seems acceptable to me.

That would work for field deployments.