CVE-2022-31017 in Zulip
Summary
by MITRE • 06/25/2022
Zulip is an open-source team collaboration tool. Versions 2.1.0 through and including 5.2 are vulnerable to a logic error. A stream configured as private with protected history, where new subscribers should not be allowed to see messages sent before they were subscribed, when edited causes the server to incorrectly send an API event that includes the edited message to all of the stream’s current subscribers. This API event is ignored by official clients, but can be observed by using a modified client or the browser’s developer tools. This bug will be fixed in Zulip Server 5.3. There are no known workarounds.
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Analysis
by VulDB Data Team • 07/15/2022
The vulnerability CVE-2022-31017 affects Zulip server versions 2.1.0 through 5.2, representing a critical logic error in the platform's access control mechanisms. This flaw specifically impacts private streams with protected history configuration where the intended security model prevents new subscribers from accessing messages sent prior to their subscription. The vulnerability stems from an improper handling of API events when stream configurations are modified, creating a scenario where the server fails to properly enforce access restrictions during the message delivery process.
The technical implementation of this vulnerability involves a breakdown in the server's event processing system where edited messages are broadcast to all current stream subscribers regardless of their subscription timing. When a private stream with protected history is modified, the server incorrectly generates an API event that includes the edited message content and sends it to every existing subscriber in the stream. This behavior violates the fundamental principle of message access control that should prevent new subscribers from viewing pre-existing messages, creating an information disclosure risk that undermines the security model of private communications.
The operational impact of this vulnerability extends beyond simple information leakage, as it enables unauthorized access to historical messages within private streams. While official Zulip clients properly ignore these erroneous API events, the vulnerability becomes exploitable through modified client implementations or browser developer tools, allowing malicious actors to observe message content that should remain restricted to authorized subscribers only. This creates a potential vector for data exfiltration and violates the confidentiality expectations of users relying on private stream functionality for sensitive communications. The vulnerability affects the core security model of Zulip's access control system and represents a failure in maintaining proper access boundaries between different subscription states.
This vulnerability aligns with CWE-284 Access Control Issues, specifically addressing improper access control mechanisms in the server's event distribution system. The flaw demonstrates a weakness in the principle of least privilege enforcement, where the system fails to properly validate access permissions during message delivery operations. From an ATT&CK perspective, this vulnerability maps to T1071.004 Application Layer Protocol: DNS and T1566 Credential Access, as it enables unauthorized access to protected communications that could potentially contain sensitive information. The vulnerability also relates to T1567 Credential Access through indirect means, as it allows access to information that should be restricted to authorized users only. The lack of known workarounds means that organizations must rely on upgrading to Zulip Server 5.3 or later versions to achieve proper protection against this logic error. This vulnerability highlights the critical importance of proper access control validation during server-side event processing and demonstrates how seemingly minor implementation flaws can create significant security gaps in collaborative platforms. The fix in version 5.3 addresses the core logic error by implementing proper access control checks during API event generation and delivery, ensuring that edited messages are only distributed to subscribers with appropriate access rights based on their subscription timing.