CVE-2026-22689 is a vulnerability in the Mailpit email testing tool, specifically in versions prior to 1.28.2, caused by the WebSocket server's failure to validate the Origin header. WebSockets are used for real-time communication between client and server, and the Origin header is a critical security control that restricts which web origins can establish connections. The absence of this validation allows an attacker to perform Cross-Site WebSocket Hijacking (CSWSH). In this attack, a malicious website can initiate a WebSocket connection to a Mailpit instance running locally on the developer's machine (default ws://localhost:8025). Since Mailpit accepts connections from any origin, the attacker’s site can connect and receive sensitive data streamed over the WebSocket, including email contents, headers, and server statistics. This data leakage occurs without requiring authentication, but the attack depends on the developer visiting the malicious site while Mailpit is running locally. The vulnerability does not impact Mailpit servers exposed to the internet directly but targets local development environments. The flaw is classified under CWE-1385 (Missing Origin Validation in WebSockets). The CVSS 3.1 base score is 6.5 (medium severity), reflecting network attack vector, low attack complexity, no privileges required, user interaction required, and high confidentiality impact but no integrity or availability impact. No known exploits are reported in the wild. The issue was patched in Mailpit version 1.28.2 by implementing proper Origin header validation to restrict WebSocket connections to trusted origins only.

For European organizations, the primary impact is the potential leakage of sensitive email data during development and testing phases where Mailpit is used locally. Developers running vulnerable Mailpit versions and browsing malicious websites could inadvertently expose confidential email contents, headers, and server statistics to attackers. This can lead to information disclosure, intellectual property loss, and potential exposure of sensitive communication data. While the vulnerability does not directly affect production environments or externally facing servers, it poses a risk to the confidentiality of data handled during software development and testing. Organizations with strict data protection regulations such as GDPR must consider the risk of unauthorized data exposure. Additionally, if attackers gain insights into email testing data, it could facilitate further targeted attacks or social engineering campaigns. The requirement for user interaction (visiting a malicious site) somewhat limits the attack scope but does not eliminate the risk, especially in environments where developers frequently access external web content. The vulnerability does not affect system integrity or availability, but the confidentiality breach can have compliance and reputational consequences.

To mitigate this vulnerability, European organizations should immediately upgrade all Mailpit instances to version 1.28.2 or later, where the Origin header validation is properly implemented. Development teams should audit their environments to identify any use of vulnerable Mailpit versions and enforce patch management policies. Additionally, developers should be educated about the risks of visiting untrusted websites while running local development tools that expose WebSocket services. Network segmentation or firewall rules can be applied to restrict local WebSocket connections to trusted origins or block outbound connections from browsers to localhost WebSocket ports. Implementing endpoint security solutions that monitor and alert on suspicious WebSocket connections can provide additional detection capabilities. Organizations should also review their internal policies regarding the use of local testing tools and consider sandboxing or containerizing such tools to limit exposure. Finally, regular security training emphasizing safe browsing habits and awareness of CSWSH attacks will reduce the likelihood of successful exploitation.