sign and/or encrypt SAML assetions, hack MITM

[Peter Schober]

Morgan, Andrew J via users <users at shibboleth.net> [2024-02-21 16:20 CET]:
> If you are able to modify the assertion without the SP rejecting it,
> then that SP is not validating the signature.  Personally, I would
> not use SAML with an SP that does not validate the signature.  As
> you have found, anyone can modify the assertion to impersonate
> another user - critical security bug.  Have you reported this issue
> to the vendor's security contact?

There are 3 cases here, I think: (Not) Rejecting a message without any
(i.e., possibly removed) signatures, messages with an incorrect
signature, and messages with a valid signature but from an untrusted
(not known out-of-band) key.

Testing for this on a larger scale isn't trivial (and might include
legal aspects) but an activity within GÉANT has recently started to
look into this:
https://wiki.geant.org/display/GWP5/Scalable+testing+for+insecure+SAML+signature+validation

If you're interested in this but can't find any contact info in the
wiki page above (and its child pages) let me know.

-peter