CVE-2022-39356 is a medium-severity improper authorization vulnerability (CWE-285) affecting Discourse, a widely used open-source platform for community discussions. The flaw exists in the invitation mechanism of Discourse versions up to and including 2.8.9 and 2.9.0.beta10. Specifically, when an invitation link is generated without scoping it to a single email address, an attacker who obtains such a link can exploit it to gain unauthorized access to any non-administrative user's account by entering that user's email address during the invitation acceptance process. This bypasses normal authorization checks, allowing the attacker to impersonate legitimate users without needing their credentials. The vulnerability does not affect administrative accounts but compromises the confidentiality and integrity of affected user accounts. Exploitation requires possession of an invitation link that is not properly scoped, but no further authentication or user interaction beyond accepting the invitation is necessary. There are no known exploits in the wild as of the published date (November 2022). The vendor recommends upgrading to the latest Discourse version where this issue is fixed. As a temporary mitigation, administrators can disable invitations entirely by setting the configuration parameter SiteSetting.max_invites_per_day to 0 or restrict invitations to specific email addresses to prevent abuse of the invitation link mechanism.

For European organizations using Discourse as a community or internal collaboration platform, this vulnerability poses a risk of unauthorized account takeover for non-admin users. Attackers who obtain or intercept an unscoped invitation link could impersonate legitimate users, potentially gaining access to sensitive discussions, private messages, or internal resources shared on the platform. This could lead to leakage of confidential information, disruption of community trust, and potential manipulation of discussions or user data. While administrative accounts are not directly compromised, the impact on user confidentiality and integrity is significant, especially for organizations relying on Discourse for sensitive or regulated communications. The vulnerability could be exploited by insiders or external attackers who gain access to invitation links through phishing, social engineering, or network interception. Given the collaborative nature of Discourse, unauthorized access could also facilitate lateral movement within an organization’s digital ecosystem if linked accounts or credentials are reused elsewhere.

European organizations should prioritize upgrading all Discourse instances to the latest patched version beyond 2.8.9 and 2.9.0.beta10 to fully remediate this vulnerability. Until upgrades can be performed, administrators should immediately disable the invitation feature by setting SiteSetting.max_invites_per_day to 0 to prevent new invitations from being issued. Alternatively, invitations should be scoped explicitly to individual email addresses to ensure that links cannot be reused for unauthorized accounts. Organizations should audit existing invitation links and invalidate any that are not properly scoped. Monitoring and logging of invitation acceptance events should be enhanced to detect suspicious activity, such as multiple acceptances from different email addresses using the same link. User education on the risks of sharing invitation links and phishing awareness can reduce the likelihood of attackers obtaining invitation URLs. Network security controls such as TLS enforcement and secure email gateways can help prevent interception of invitation links. Finally, organizations should review user account permissions and consider additional multi-factor authentication for sensitive accounts to mitigate potential lateral impacts.