CVE-2026-102893 in Zulip
Summary
by MITRE • 10/14/2026
Zulip is an open-source team collaboration tool. In Zulip Server versions through 12.1, GenericOpenIdConnectBackend matches the identity provider email claim to an active account's delivery_email without rejecting an explicitly false email_verified claim. If a configured identity provider returns an email address that the authenticating user does not control together with email_verified=false, Zulip can log the user into the existing account for that address. Exploitation requires a misconfigured trusted identity provider, and deployments whose authoritative identity provider only emits email addresses controlled by the authenticating user are not affected. This issue is fixed in version 12.2.
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Analysis
by VulDB Data Team • 10/14/2026
The vulnerability identified in Zulip Server versions through 12.1 represents a critical authentication logic flaw within the GenericOpenIdConnectBackend component, specifically concerning how identity provider claims are processed during the account linking and login process. This issue stems from an insufficient validation of the email_verified attribute returned by OpenID Connect providers. In standard OAuth 2.0 and OIDC flows, the email_verified claim serves as a crucial indicator of whether the end-user has successfully proven ownership of the provided email address to the identity provider. By failing to reject authentication attempts where this claim is explicitly set to false, Zulip inadvertently allows users to authenticate using an email address they do not control, provided that such an address already exists in the local Zulip database as a delivery_email for another account.
From a technical perspective, the flaw lies in the conditional logic used during the user lookup phase of the authentication handshake. When a user attempts to log in via OpenID Connect, the backend retrieves the email claim from the identity provider's token and searches for an existing local account associated with that address. The security defect occurs because the system does not check the validity status of that email before proceeding with the login. Consequently, if a misconfigured or compromised trusted identity provider returns an unverified email address alongside valid authentication credentials, Zulip will associate those credentials with any active user account holding that specific delivery_email. This bypasses the fundamental security assumption that possession of session tokens from a trusted IdP equates to authorized access for the corresponding local user profile.
The operational impact of this vulnerability is significant in environments where multiple users might share or have historically used similar email addresses, or more critically, when an attacker can manipulate their identity provider's response. An adversary could potentially gain unauthorized access to another user's Zulip account by configuring a malicious or misconfigured OpenID Connect provider to return the victim's email address with an email_verified status of false. This scenario is particularly dangerous in enterprise deployments where single sign-on (SSO) integration is prevalent, as it undermines the integrity of the identity federation trust model. It allows for unauthorized access without requiring password theft or traditional credential stuffing attacks, relying instead on the misconfiguration of external authentication services and the lack of strict verification checks within the application logic.
It is important to note that this vulnerability requires specific conditions to be exploitable. The primary prerequisite is a misconfigured trusted identity provider that fails to properly assert email ownership via the email_verified claim or explicitly sets it to false while still issuing valid tokens for an uncontrolled address. Deployments where the authoritative identity provider strictly emits only verified, user-controlled email addresses are not affected by this flaw. This distinction highlights that the risk is largely dependent on the configuration and trustworthiness of the external IdP rather than a universal weakness in Zulip's core architecture.
To mitigate this vulnerability, organizations must upgrade to Zulip Server version 12.2 or later, where the GenericOpenIdConnectBackend has been patched to properly reject authentication attempts associated with unverified email addresses. In addition to upgrading, administrators should audit their identity provider configurations to ensure that the email_verified claim is accurately reflecting the verification status of users and that only verified identities are permitted for SSO login. Implementing strict validation rules on incoming OIDC claims can further harden the system against similar logic flaws in other authentication modules.
This vulnerability aligns with CWE-287, which describes Improper Authentication, as it involves a failure to correctly verify user identity before granting access. Furthermore, from an adversary perspective, this flaw facilitates Account Manipulation and potentially Impersonation within the ATT&CK framework, specifically under techniques related to exploiting trust relationships between systems. By leveraging misconfigured IdPs, attackers can bypass standard authentication controls, highlighting the importance of rigorous validation of all security attributes provided by external identity sources in federated identity architectures.