CVE-2022-23656 is a cross-site scripting (XSS) vulnerability identified in the Zulip open source team chat application, specifically affecting versions from June 3, 2021, up to but not including March 1, 2022. The vulnerability arises due to improper neutralization of user input during web page generation (CWE-79). An attacker can exploit this by crafting a malicious full name for their Zulip account and sending messages within a topic that includes multiple participants. When a victim user views the recent topics page and triggers the overflow tooltip that displays the attacker's malicious full name, the embedded JavaScript code executes in the victim's browser context. This can lead to unauthorized actions such as session hijacking, credential theft, or other malicious activities that leverage the victim's authenticated session within the Zulip server. The vulnerability requires that the victim user interacts with the recent topics page and specifically triggers the tooltip containing the malicious input. The flaw is present in the main development branch of Zulip Server during the specified timeframe and has been addressed in versions released after March 1, 2022. No known exploits have been reported in the wild as of the published date. The vulnerability affects the confidentiality and integrity of user sessions and data within the Zulip environment, but does not directly impact server availability. Exploitation does not require prior authentication by the attacker, as account creation with a malicious full name suffices, but victim interaction is necessary to trigger the payload execution.

For European organizations using Zulip as a team collaboration tool, this vulnerability poses a risk of session hijacking and unauthorized access to sensitive communications. Attackers could leverage this XSS flaw to execute arbitrary scripts in the context of authenticated users, potentially leading to data leakage, impersonation, or lateral movement within the organization’s communication infrastructure. Given that Zulip is often used for internal team communications, the compromise of user sessions could expose confidential project details, strategic plans, or personal employee information. The impact is particularly significant for organizations with high security requirements, such as financial institutions, government agencies, and critical infrastructure operators. However, since exploitation requires user interaction and the vulnerability is limited to a specific UI component (recent topics page tooltip), the attack surface is somewhat constrained. The absence of known exploits in the wild reduces immediate risk but does not eliminate the potential for targeted attacks. Organizations running self-hosted Zulip servers on affected versions should consider the risk of insider threats or external attackers registering accounts to exploit this vulnerability.

Organizations should promptly upgrade their Zulip servers to versions released after March 1, 2022, where this vulnerability is fixed. In addition to patching, administrators can implement the following specific measures: 1) Restrict account creation to verified users to reduce the risk of attacker-controlled accounts with malicious full names. 2) Implement Content Security Policy (CSP) headers to limit the execution of unauthorized scripts within the Zulip web interface. 3) Educate users to be cautious when interacting with UI elements such as tooltips that display user-generated content. 4) Monitor server logs for unusual account creation patterns or message activity that could indicate exploitation attempts. 5) If upgrading immediately is not feasible, consider disabling or restricting access to the recent topics page or the tooltip feature as a temporary workaround. 6) Regularly review and sanitize all user-generated content displayed in the UI to prevent similar vulnerabilities. These targeted mitigations complement the patch and reduce the window of exposure.