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):

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.

References

• GHSA-524m-q5m7-79mm
• https://nvd.nist.gov/vuln/detail/CVE-2026-22689
• axllent/mailpit@6f1f4f3