CVE-2022-24751 is a medium-severity race condition vulnerability affecting Zulip, an open-source group chat application widely used for team collaboration. The flaw exists in Zulip versions starting from 4.0 up to, but not including, 4.11. The vulnerability arises during the account deactivation process. Specifically, a race condition occurs due to improper synchronization when a user account is being deactivated concurrently with access attempts by the same user. This concurrency issue can, in rare cases, allow the deactivated user to maintain access to the system despite their account being marked as deactivated. The root cause is a lack of proper locking or atomic operations around shared resources that manage session state and account status, categorized under CWE-362 (Concurrent Execution using Shared Resource with Improper Synchronization). The vulnerability does not require elevated privileges or complex exploitation techniques but depends on timing conditions during concurrent operations. There are no known exploits in the wild, and no workarounds exist other than upgrading. The issue is resolved in Zulip version 4.11 on the 4.x branch and version 5.0-rc1 on the 5.x branch. Upgrading to these versions not only fixes the race condition but also invalidates any cached sessions that might have persisted due to the bug, effectively closing the window for unauthorized continued access by deactivated users.

For European organizations using vulnerable versions of Zulip, this vulnerability could lead to unauthorized access persistence by users who should have been deactivated, potentially violating internal security policies and regulatory requirements such as GDPR. This unauthorized access could allow former employees or compromised accounts to continue accessing sensitive communications, intellectual property, or confidential project data. Although the vulnerability does not directly allow privilege escalation or remote code execution, the persistence of access undermines the integrity and confidentiality of organizational data. The impact is particularly significant for sectors with strict compliance mandates, such as finance, healthcare, and government agencies, where timely and effective user deactivation is critical. Additionally, the vulnerability could complicate incident response and user access audits, as deactivated accounts may appear inactive but remain functionally active. The absence of known exploits reduces immediate risk, but the potential for exploitation in targeted attacks or insider threat scenarios remains. The availability of a patch mitigates long-term risk if promptly applied.

The primary and only effective mitigation is to upgrade Zulip installations to version 4.11 or later on the 4.x branch, or 5.0-rc1 or later on the 5.x branch. Organizations should prioritize patching in their change management cycles and verify that all Zulip instances are updated. Post-upgrade, administrators should review active sessions and user access logs to identify any anomalies or sessions that persisted prior to patching. Implementing strict session management policies, such as enforcing session expiration and multi-factor authentication, can reduce the risk of session hijacking or unauthorized persistence. Additionally, organizations should audit their user deactivation workflows to ensure no other concurrency issues exist and consider adding application-level monitoring for race conditions or synchronization failures. For environments where immediate patching is not feasible, restricting access to Zulip servers via network segmentation or VPNs can reduce exposure. Finally, educating administrators and users about the importance of timely deactivation and session management will help mitigate risks associated with this vulnerability.