CVE-2017-0881 in Chat Application Server
Summary
by MITRE
An error in the implementation of an autosubscribe feature in the check_stream_exists route of the Zulip group chat application server before 1.4.3 allowed an authenticated user to subscribe to a private stream that should have required an invitation from an existing member to join. The issue affects all previously released versions of the Zulip server.
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Analysis
by VulDB Data Team • 11/22/2022
The vulnerability described in CVE-2017-0881 represents a critical access control flaw within the Zulip group chat application server that persisted across multiple versions prior to 1.4.3. This security weakness specifically targeted the autosubscribe functionality implemented in the check_stream_exists route, which is responsible for validating user access permissions when attempting to join streams within the messaging platform. The flaw essentially allowed authenticated users to bypass the intended invitation-based access control mechanism that should have required existing members to extend invitations before new users could join private streams.
The technical implementation of this vulnerability stems from improper validation logic within the server-side route handling the stream existence checks. When users attempted to subscribe to streams through the autosubscribe feature, the server failed to properly verify whether the requesting user possessed the necessary authorization to join private streams. This oversight created a path where authenticated users could exploit the system's permission model to gain access to restricted private streams without proper invitation or approval from existing stream members. The flaw existed in the route's validation logic, which should have enforced the stream membership requirements before allowing subscription requests to proceed.
The operational impact of this vulnerability was significant as it undermined the fundamental security model of private stream communications within the Zulip platform. Attackers could potentially access sensitive private conversations, private stream discussions, and confidential information shared within restricted channels that should have been protected from unauthorized access. This breach of access control could lead to data exposure, privacy violations, and potential information leakage that compromised the integrity of private communications within organizations relying on Zulip for secure messaging. The vulnerability affected all previously released versions of the server, meaning that organizations running any version prior to 1.4.3 were exposed to this risk.
Organizations affected by this vulnerability should have immediately upgraded to version 1.4.3 or later to remediate the access control flaw. The fix implemented in the updated version addressed the validation logic in the check_stream_exists route to properly enforce stream membership requirements before allowing user subscriptions. Security practitioners should have conducted immediate vulnerability assessments of their Zulip deployments to identify systems running vulnerable versions and implemented network monitoring to detect potential exploitation attempts. This vulnerability aligns with CWE-284, which addresses improper access control in software implementations, and could be categorized under ATT&CK technique T1078 for valid accounts and privilege escalation through unauthorized access to restricted resources. The incident highlights the critical importance of proper access control validation in messaging applications where unauthorized access to private communications can result in significant privacy and security implications.