There is an unsupported solution here. I've never tried it so I don't know if it actually works. Might be worth a try.

https://www.concrete5.org/community/forums/customizing_c5/packaged-l...

Well, it's LDAP. Not exactly SSO, but probably as close as you will get.

I've just been involved in a proof-of-concept evaluation of concrete5 for use as a client's intranet site. One of the core requirements was that transparent single-signon (ie Active Directory authentication via kerberos) worked with any solution that was evaluated.

As I loved so many other aspects of the concrete5 CMS, I decided to rework the packaged LDAP single-authentication solution into something that would allow for single-signon. Once configured, it worked well for our proof-of-concept although we haven't yet gone live with it in production.

Feel free to give it a go/modify it as you see fit.

A few important caveats :-)
- I'm relatively new to concrete5
- I'm not an experienced PHP programmer (although I do program in Perl quite often)
- Our requirement was to implement a quick yet non-intrusive solution this problem; as such, its not an installable concrete5 add-on nor does it integrate with the CMS as extensively as it possibly could

If you're still keen to give it a try, the installation instructions are contained within the readme.txt

I realise this discussion is a couple of years old, but I'm currently researching concrete5 SSO options and thought I'd give your code a bit more visibility and possibly develop it further. Thanks for the excellent start and detailed README. Love it!

I hope you don't mind that I put your code on Github. It's now at https://github.com/SimonEast/concrete5-windows-single-signon-sso...

For those who need a robust Active Directory/LDAP solution for concrete5. This add-on was recently added to the marketplace.

ExchangeCore LDAP Authentication
https://www.concrete5.org/marketplace/addons/exchangecore-ldap-authe...

Thanks for the plug @mrKDilkington, but I don't want to lead anyone astray into thinking we have an SSO package. The package linked to is an LDAP authentication package. Due to server side configuration that's usually required for SSO we chose not to implement it as part of our package (at least not at this time).

The package does support user syncing though, so it might be feasible for someone to implement their own on_start() event which leveraged the package users in order to implement SSO with relative ease. On the other hand, the code the ntlsteve & simoneast seem to be working with might be a decent free alternative.