GHSA-524m-q5m7-79mm Mailpit is vulnerable to Cross-Site WebSocket Hijacking (CSWSH) allowing unauthenticated access to emails

• Go github.com/axllent/mailpit: >= 1.2.6, < 1.28.2 → fixed in 1.28.2 • Go github.com/axllent/mailpit: < 0.0.0-20260110031614 → fixed in 0.0.0-20260110031614

Summary The Mailpit WebSocket server is configured to accept connections from any origin. This lack of Origin header validation introduces a Cross-Site WebSocket Hijacking (CSWSH) vulnerability. An attacker can host a malicious website that, when visited by a developer running Mailpit locally, establishes a WebSocket connection to the victim's Mailpit instance (default ws://localhost:8025). This allows the attacker to intercept sensitive data such as email contents, headers, and server statistics in real-time. Vulnerable Code The vulnerability exists in server/websockets/client.go where the CheckOrigin function is explicitly set to return true for all requests, bypassing standard Same-Origin Policy (SOP) protections provided by the gorilla/websocket library. https://github.com/axllent/mailpit/blob/877a9159ceeaf380d5bb0e1d84017b24d2e7b361/server/websockets/client.go#L34-L39 Impact This vulnerability impacts the Confidentiality of the data stored in or processed by Mailpit. Although Mailpit is often used as a local development tool, this vulnerability allows remote exploitation via a web browser. • Scenario: A developer has Mailpit running at localhost:8025. • Trigger: The developer visits a malicious website (or a compromised legitimate site) in the same browser. • Exploitation: The malicious site's JavaScript initiates a WebSocket connection to ws://localhost:8025/api/events. Since the origin check is disabled, the browser allows this cross-origin connection. • Data Leak: The attacker receives all broadcasted events, including full email details (subjects, sender/receiver info) and server metrics. Attack Impact • Real-time notification of new emails • Email metadata (sender, subject, recipients) • Mailbox statistics • All WebSocket broadcast data Recommended Fix The `CheckOrigin` function should be removed to allow gorilla/websocket to enforce its default safe behavior (checking that the Origin matches the Host). Alternatively, strict validation logic should be implemented. Proposed Change (Remove unsafe check): ```go var upgrader = websocket.Upgrader{ ReadBufferSize: 1024, WriteBufferSize: 1024, // CheckOrigin: func(r *http.Request) bool { return true }, // REMOVED EnableCompression: true, } ``` Proof of Concept (PoC): To reproduce this vulnerability: • Start Mailpit (default settings). • Save the following HTML code as poc.html and serve it from a different origin (e.g., using python http.server on port 8000 or opening it directly as a file). • Open the poc_websocket_hijack.html file in your browser. • Send a test email to Mailpit or perform any action in the Mailpit UI. • Observe that the "malicious" page successfully receives the event data.