CVE-2026-22689 is a vulnerability identified in the Mailpit email testing tool, specifically in versions prior to 1.28.2. The root cause is the absence of Origin header validation in the WebSocket server component. WebSockets are used by Mailpit to provide real-time communication between the client and server, typically running locally on ws://localhost:8025. Without validating the Origin header, the WebSocket server accepts connections from any origin, including malicious websites. This flaw enables Cross-Site WebSocket Hijacking (CSWSH), where an attacker hosts a malicious webpage that, when visited by a developer running Mailpit locally, initiates a WebSocket connection to the local Mailpit instance. Through this connection, the attacker can intercept sensitive data such as email contents, headers, and server statistics in real time. This exposure compromises confidentiality but does not affect integrity or availability. Exploitation requires the victim to visit a malicious site (user interaction) but no authentication is needed on the Mailpit instance. The vulnerability has a CVSS 3.1 score of 6.5 (medium severity), reflecting the network attack vector, low attack complexity, no privileges required, and user interaction needed. 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, this vulnerability poses a significant risk to development environments where Mailpit is used for email testing. Sensitive email data, including contents and headers, could be exposed to attackers if developers visit malicious websites while running vulnerable Mailpit versions locally. This can lead to leakage of confidential information, potentially including internal communications or test data that may contain sensitive or proprietary information. Although the vulnerability does not directly impact production systems or server availability, the confidentiality breach could facilitate further attacks or data leaks. Organizations with remote or hybrid workforces may face increased risk due to varied browsing habits. The impact is particularly relevant for software development companies, IT service providers, and any organization relying on Mailpit for email testing and development workflows. Failure to patch could also undermine compliance with data protection regulations such as GDPR, given the potential exposure of personal data in emails.

The primary mitigation is to upgrade Mailpit to version 1.28.2 or later, where the Origin header validation is properly implemented. Until upgrading, organizations should restrict developer access to untrusted websites to reduce the risk of CSWSH attacks. Developers should avoid visiting unknown or suspicious websites while running vulnerable Mailpit versions locally. Network-level controls can be implemented to block outbound WebSocket connections from localhost to external origins. Additionally, configuring local firewalls or endpoint protection solutions to monitor and restrict WebSocket traffic can help mitigate exploitation. Security awareness training should emphasize the risks of visiting untrusted sites during development activities. Organizations should also audit their development environments to identify any instances of vulnerable Mailpit versions and ensure timely patching. Finally, consider isolating development environments or using containerized setups to limit exposure.