Authenticate requests session with login.microsoftonline.com

I am trying to authenticate to login.microsoftonline.com to reach a resource hidden behind SSO. I need to get the SAML response to post to SSO to allow me to authenticate. The only problem is I can't authenticate past with microsoft because of the flow token and requests doesn't load the flow token because it needs to be loaded with javascript. How can I authenticate?

I would like to say the answer is easy but it takes some work, and I'm not sure how this work in different environments, but here is what I did for mine.

1. Create a class and init for dynamic info. Here is mine

class Microsoft:
    # TODO: Add MFA option
    def __init__(self, username: str, password: str):
        self.sess = requests.Session()
        self.username = username
        self.password = password
        self.base = "https://login.microsoftonline.com/"
        self.tenant = "redacted/"
        self.url = ""

2. use requests_html HTMLSession and bs4 beautifuSoup (as bs here) to get the flow token

# used to return values of input fields rendered
    def get_html_name_value(self, html: str, name: str) -> str:
        return bs(html, "lxml").find("input", {"name": name}).get("value")

# get redirected to obtain flow token with appropriate SAML
    def _get_tokens(self):
        session = HTMLSession()
        # update this
        resp = session.get(
            "https://sso.redacted.com/cas/clientredirect?client_name=SAML2Client45&"
            + "service=https://full/path/to/service_behind_sso"
        )
        self.url = resp.url
        resp.html.render()
        html = resp.html.html
        data = {
            "flowToken": bs(resp.html.html, "lxml")
            .find(id="i0327")
            .get("value"),
            "ctx": self.get_html_name_value(html, "ctx"),
            "canary": self.get_html_name_value(html, "canary"),
            "hpgrequestid": self.get_html_name_value(html, "hpgrequestid"),
        }
        return session, data

3. Post the data to get the saml token. When you post the data given it will return an html form that needs to be posted.

    def _get_saml_tokens(self):
        sess, payload = self._get_tokens()
        payload["login"] = self.username
        payload["loginfmt"] = self.username
        payload["passwd"] = self.password
        headers = {
            "Host": "login.microsoftonline.com",
            "User-Agent": "Mozilla/5.0 (X11; Linux x86_64; rv:88.0) Gecko/20100101"
            + " Firefox/88.0",
            "Referer": self.url,
            "Origin": "https://login.microsoftonline.com",
        }
        html = sess.post(
            self.base + self.tenant + "/login",
            data=payload,
            headers=headers,
        ).text

        data = {
            "SAMLResponse": self.get_html_name_value(html, "SAMLResponse"),
            "RelayState": self.get_html_name_value(html, "RelayState"),
        }
        return sess, data

4. final step is to post the saml response and relay state so that you can access the service

    def _login(self):
        sess, payload = self._get_saml_tokens()

        return sess.post(
            "https://sso.redacted.com/cas/login?client_name=SAML2Client45",
            data=payload,
        )

From here you can extract the cookies and put them in a normal requests session or keep operating in the HTMLSession()

You can do something similar with token auth for other resources, but I haven't needed to access it yet and haven't written the code.

The key would be to change the initial request in step 2 so step 3 will receive a token and then post the correct auth there. Hopefully everyone can adapt this as needed