CVE-2022-31017 is a logic error vulnerability affecting Zulip, an open-source team collaboration platform widely used for messaging and project coordination. The vulnerability exists in versions 2.1.0 through 5.2 inclusive. Specifically, it concerns private streams configured with protected history, where new subscribers are not supposed to see messages sent before their subscription. However, when an existing message in such a stream is edited, the Zulip server erroneously sends an API event containing the edited message to all current subscribers of the stream, regardless of their subscription time. Official Zulip clients ignore this API event, but it can be observed by attackers or unauthorized users using modified clients or browser developer tools. This results in unintended disclosure of historical messages to users who should not have access, violating the confidentiality principle. The root cause is a logic flaw characterized by an expression that is always true (CWE-571) and incorrect control flow implementation (CWE-670). The issue does not require authentication beyond being a subscriber to the stream, but it does require the ability to observe or intercept API events, which may be possible for insiders or users with elevated access. There are no known workarounds, and the vulnerability is fixed starting with Zulip Server version 5.3. No exploits have been reported in the wild to date.

For European organizations using Zulip for internal communications, this vulnerability poses a moderate confidentiality risk. Private streams with protected history are intended to restrict access to sensitive historical messages. The flaw allows current subscribers to potentially access edited historical messages that should remain hidden, leading to unauthorized disclosure of potentially sensitive or confidential information. This could impact sectors with strict data privacy requirements such as finance, healthcare, government, and critical infrastructure. Although the vulnerability does not affect message integrity or availability, the breach of confidentiality could result in compliance violations under GDPR and other data protection regulations, reputational damage, and loss of trust among collaborators. The fact that official clients ignore the leaked API event reduces the risk of widespread exploitation but does not eliminate the threat from malicious insiders or attackers using custom clients or developer tools. Since Zulip is used by various organizations across Europe, the scope of affected systems depends on the adoption of vulnerable versions and the use of private streams with protected history.

1. Upgrade all Zulip server instances to version 5.3 or later immediately to apply the official fix. 2. Audit current Zulip streams configured as private with protected history and restrict subscription permissions to trusted users only. 3. Monitor network traffic and API event logs for unusual access patterns or the use of non-standard clients that might be exploiting this vulnerability. 4. Educate users and administrators about the risk of using modified clients or browser developer tools to access unauthorized messages. 5. Implement network segmentation and strict access controls around Zulip servers to limit exposure to potential attackers or insiders. 6. If upgrading immediately is not feasible, consider temporarily disabling message editing in private streams with protected history to prevent triggering the vulnerability. 7. Review and enhance logging and alerting mechanisms to detect suspicious API event access or message edits in sensitive streams.