CVE-2026-100404 in Zulip
Summary
by MITRE • 10/15/2026
Zulip is an open-source team collaboration tool. In Zulip Server versions through 12.1, guest users can receive new message events for public channels outside their subscriptions. A guest can trigger the issue by registering an event queue with parameters that request messages from an unsubscribed public channel, or an existing event queue can remain affected after the guest's access to the channel is removed. The flaw discloses new public-channel messages to the guest. This issue is fixed in version 12.2.
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Analysis
by VulDB Data Team • 10/15/2026
The vulnerability identified in Zulip Server versions through 12.1 represents a significant authorization bypass within an open-source team collaboration platform, allowing unauthorized information disclosure. As a comprehensive communication tool designed for structured discussions and real-time messaging, Zulip relies on strict access controls to ensure that users only receive data pertinent to their specific roles and subscriptions. The flaw specifically impacts guest accounts, which are typically intended to have restricted visibility compared to full members or administrators. By exploiting the event queue mechanism, these unprivileged actors were able to bypass subscription-based filtering logic, thereby gaining access to message streams in public channels they had not explicitly joined or subscribed to. This undermines the fundamental security principle of least privilege and compromises the confidentiality of communications intended for broader but still controlled audiences within an organization.
The technical root cause lies in how the server handles event queue registration parameters when interacting with public channel data structures. When a guest user registers an event queue, they can specify filters that request messages from specific channels. In vulnerable versions, the backend logic failed to adequately validate whether the requesting user actually held a valid subscription or membership status for those specified public channels before delivering events. Furthermore, the issue exhibited state persistence problems; even after an administrator revoked a guest's access rights to a particular channel, existing event queues could continue to receive new message events from that channel. This indicates a failure in real-time permission checks during the streaming of updates, where cached or lingering queue configurations were not properly invalidated upon role changes, allowing continued data leakage long after access was theoretically removed.
The operational impact of this vulnerability is primarily centered on information disclosure and potential privacy violations within organizational workflows. Since Zulip is often used for sensitive business communications, project planning, and internal discussions, the ability of a guest to view messages from unsubscribed public channels could expose proprietary strategies, confidential personnel matters, or other restricted data. While the affected channels are technically classified as public within the application's hierarchy, they may still be intended only for specific subsets of users who have opted in via subscription. The persistence of this issue after access revocation exacerbates the risk, as it creates a window of opportunity where former guests retain visibility into ongoing conversations without active oversight or explicit permission. This could lead to accidental data leaks if sensitive topics are discussed under the assumption that only subscribed participants can view them.
To mitigate such risks and align with industry security standards, organizations must ensure they operate on patched versions of collaboration software, specifically upgrading Zulip Server to version 12.2 or later where this authorization logic has been corrected. From a broader cybersecurity perspective, this vulnerability maps closely to CWE-862, which denotes Missing Authorization, as the system failed to enforce access control policies for specific resources based on user identity and role. Additionally, it relates to CWE-798, Use of Hard-coded Credentials or Default Values if one considers the persistence aspect as a failure in state management that defaults to retaining old permissions rather than resetting them upon revocation. In terms of adversary tactics, this behavior aligns with ATT&CK technique T1530, Data from Information Repositories, where an attacker accesses data stored within organizational systems without proper authorization. Regular audits of user roles and event queue configurations are recommended as part of a defense-in-depth strategy to ensure that access controls remain consistent across all system states and interactions.