This module is heavily inspired by the pam-http module by Kragen Sitaker. I rewrote it largely because I wanted to MIT license it (instead of GPL) and because there was some profanity in the source. Also, the version I modeled this off of didn't even compile because it used an old version of libcurl.
This works with libcurl v. 7.21.3 (the one in Ubuntu's repositories).
To build, just type make. It will create mypam.so and a test executable.
The .so file should be put in /lib/security and the PAM config files will need to be edited accordingly.
The config files are located in /etc/pam.d/ and the one I changed was /etc/pam.d/common-auth. This is NOT the best place to put it, as sudo uses this file and you could get unexpected results (like an HTTP user could gain root access; cool huh?).
Put something like this in one of the config files (change the URL to whatever you like):
auth sufficient mypam.so url=https://localhost:2000
account sufficient mypam.so
Sufficient basically means that if this authentication method succeeds, the user is given access.
To test, run the test program with a single argument, the username. I have provided a sample HTTPS server (you'll need your own certificate) that will accept all usernames and passwords. This module does not check the validity of certificates, so a custom one will do.