CVE-2017-0910 in Server
Summary
by MITRE
In Zulip Server before 1.7.1, on a server with multiple realms, a vulnerability in the invitation system lets an authorized user of one realm on the server create a user account on any other realm.
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Analysis
by VulDB Data Team • 01/16/2023
The vulnerability identified as CVE-2017-0910 affects Zulip Server versions prior to 1.7.1 and represents a critical authorization bypass flaw within the multi-tenant environment. This issue specifically targets servers configured with multiple realms, where each realm operates as an isolated tenant within the same Zulip instance. The vulnerability stems from inadequate cross-realm access controls in the invitation system, allowing malicious actors to exploit the permission model and create user accounts in unauthorized realms.
The technical flaw resides in the invitation processing mechanism where the system fails to properly validate realm boundaries during user account creation. When an authorized user from one realm initiates an invitation process, the system does not adequately verify that the invitation target realm matches the initiating user's realm permissions. This design oversight creates a path where legitimate users can manipulate the invitation flow to extend account creation privileges beyond their authorized realm boundaries. The vulnerability operates at the application logic level and specifically impacts the server-side validation routines that should enforce realm isolation.
The operational impact of this vulnerability is significant for organizations relying on Zulip Server's multi-tenant architecture. An attacker with access to any legitimate realm can potentially escalate privileges by creating accounts in other realms without proper authorization, leading to unauthorized access to sensitive communications, data exposure, and potential lateral movement within the multi-tenant environment. This flaw undermines the fundamental security principle of tenant isolation that Zulip Server relies upon to maintain separate communication spaces for different organizations or departments. The vulnerability can be exploited by users with basic user permissions, making it particularly dangerous in environments where multiple organizations share the same server infrastructure.
This vulnerability aligns with CWE-639 which addresses authorization bypass through multiple paths, and relates to ATT&CK technique T1078 which covers valid accounts and privilege escalation. Organizations should implement immediate mitigations including upgrading to Zulip Server version 1.7.1 or later, where proper realm boundary checks have been implemented. Additional defensive measures include monitoring invitation system logs for unauthorized cross-realm account creations, implementing stricter access controls for invitation privileges, and conducting regular security audits of multi-tenant configurations. The fix addresses the core issue by introducing proper realm validation checks in the invitation processing pipeline, ensuring that user accounts can only be created within the realm of the initiating user's authorization scope. Organizations should also consider implementing network-level restrictions and additional authentication layers to further protect against potential exploitation attempts.