We operate a native iOS app that authenticates users via the system browser using OIDC against a 3rd party SaaS authentication broker, which then performs authentication against the individual customer IdPs using SAML within the browser session, and then completes the OIDC login between the broker and our application.
Our application initiates the OIDC login using ASWebAuthenticationSession, using the broker’s library, against the broker, and at that point the authentication workflow is handled by the broker. At the beginning of the login session, the broker sets a session-identifying cookie for their domain, before redirecting the user to their company’s identity provider to authenticate, which then redirects the user back to the broker.
Intermittently, Mobile Safari does not include that previously set cookie on the final communication with the broker, when being redirected from the IdP as the final step of the SAML portion of the login workflow. When the cookie is missing, the broker cannot correlate the authentication response to the original request and the login fails, even though the user successfully authenticated at their identity provider. The same user can sometimes find success after retrying on the same device minutes later, without any changes.
When we first started diagnosing this issue, we were seeing about a 20% rate of these errors across all Mobile Safari logins, which we can identify fairly conclusively (from provider logs, based on their guidance) as being caused by the session cookie set in one request not being provided on the subsequent request to the same domain.
Our authentication broker provider has indicated, based on their server logs, and logs from an affected device, that this behavior is caused by Mobile Safari’s Intelligent Tracking Protection (ITP) causing Mobile Safari to not send the login session cookie to the broker when handling the SAML redirect from the IdP back to them.
Our authentication broker provider recommended that we switch the SAML Request binding setting from POST to Redirect in the SAML configurations against our customer IdPs, which reduced the rate of these errors to about 5% for most of our customers. However, we have at least one customer which is still seeing about a 20% rate in these errors for Mobile Safari logins after this change, and even a 5% error rate seems high.
Our authentication broker has not been able to suggest any further remediation options, and has suggested we contact Apple for assistance.
Our questions are:
- Is it reasonable to assume that ITP is causing this issue?
- Is there any way to confirm, conclusively, that ITP has caused a previously-set cookie to not be provided on a subsequent request to the same domain, i.e. via device logs?
- If so, are there any steps which can be taken to reduce or eliminate this error?
- Changes to how ASWebAuthenticationSession is invoked?
- Changes to the Authentication Broker which would reduce the chance of ITP being triggered?
- Changes to the Customer IdP configuration which would reduce the chance of ITP being triggered?