Vulnerabilities

SSOScan currently looks for these five types of vulnerabilities.

1. Access token misuse

When a website uses an access_token from Facebook to authenticate users, impersonation attacks can happen. This implementation is dangerous because the access_token is not tied to a user’s identity or your application ID, but only to user’s permissions. This means a malicious application which holds a victim’s access_token (because the victim has been tricked into using the malicious application) could simply reuse that token to login to the vulnerable website as that victim.

Mitigation. To mitigate this vulnerability, we suggest websites to use either code or signed_request provided by Facebook when authenticating users. If an access_token has to be used, be sure to verify its intended audience explained in the Facebook developer guide: https://developers.facebook.com/docs/facebook-login/access-tokens/#debug.

2. Signed_request misuse

When a website uses signed_request from Facebook to authenticate users, impersonation attacks can still happen if the developer doesn’t check its signature and signed content with care. An attacker can simply forge a signed_request with an incorrect signature, or modify the content of a signed_request intended for another application. When the attacker supplies the fake credential to a vulnerable website, it will still authenticate the attacker as the victim. If access_token is used along with signed_request, it is also important to check that the identities of both credentials agree with each other.

Mitigation. To mitigate this vulnerability, we suggest websites to parse the signedrequest and check its signature using application secret correctly. Visit _https://developers.facebook.com/docs/facebook-login/using-login-with-games/ for more details.

3. Credential leakage via referrer header

Referrer header is auto-appended to requests initiated from any webpage to other parties. If a website includes OAuth credentials (such as access_token, etc.) in their URLs, they should make sure the page content does not include any untrusted resources. For example, if a website includes a picture from an untrusted source ‘evil.com’, that maliciouis site will be able to get all OAuth credentials of users logging in to your site.

Mitigation. To mitigate this vulnerability, we suggest a website consume user’s OAuth credentials at server side and do not return such information back to the client side. Such credentials can be stored in web server’s back end DB and can be linked to the user by setting a cookie in user’s browser.

4. Client secret leakage

Including client_secret on client side is dangerous, it enables any user to impersonate the application. It can be used to forge signed_request to log in as anybody to your application

Mitigation. To mitigate this vulnerability, we suggest the web application never include client_secret on the client side.

5. Credential leakage via page content

Including Facebook’s SSO OAuth credentials (e.g. access_token) in the landing page content (e.g. registration page) after the user has logged in can be dangerous because there might be other third-party scripts embedded on the webpage, and they have full access to the host information. Once the OAuth credentials are exfiltrated, potential risks include leaking sensitive victim information and powerful permissions such as posting to victim’s timeline as the vulnerable application, and even impersonation attacks (reuse the OAuth credentials to login as the victim to the vulnerable website).

Mitigation. To mitigate this vulnerability, we suggest the web application never include OAuth credentials in the page content. If for some reason client-side JavaScript needs access to them, we suggest this information be fetched from your app server using AJAX requests and stored as private variables (in a closure).