Mailpit: WebSocket origin check bypass via percent-encoded path (regression of CVE-2026-22689)

Summary

The cross-site WebSocket hijacking fix was reimplemented as an origin check gated on a raw-URI prefix test, but Go's ServeMux routes on the percent-decoded path, so requesting /%61pi/events reaches the WebSocket handler while skipping the only origin control, and the upgrader itself accepts every origin. Confirmed at HEAD 408b30d. Affects 1.29.0 through 1.30.5.

The defect

Two halves that were each correct in isolation. server/websockets/client.go accepts any origin and delegates the check elsewhere:

var upgrader = websocket.Upgrader{                       // line 33
    ...
    CheckOrigin: func(_ *http.Request) bool {            // line 37
        // origin is checked via server.go's CORS settings
        return true                                      // line 39
    },
}

server/server.go performs that check but keys it on the RAW request target:

if strings.HasPrefix(r.RequestURI, config.Webroot+"api/") || htmlPreviewRouteRe.MatchString(r.RequestURI) {   // line 320
    if allowed := corsOriginAccessControl(r); !allowed {
        http.Error(w, "Blocked due to CORS violation", http.StatusForbidden)
        return
    }

r.RequestURI is the untouched wire target; Go's ServeMux routes on the percent-DECODED path. So for /%61pi/events: strings.HasPrefix("/%61pi/events", "/api/") is FALSE (origin check skipped), ServeMux decodes %61 to "a" and routes to /api/events, and the upgrader's CheckOrigin returns true.

Measured, default config, no auth: /api/events with Origin: https://evil.example returns 403; /%61pi/events with the same Origin returns 101 Switching Protocols and begins streaming. With a message delivered over SMTP while the cross-origin socket was open, the attacker origin received the ID, Message-Id, From, To, Cc, Bcc, Subject ("SECRET password reset token abc123"), size, tags, and body Snippet, live. WebSockets are not subject to CORS response-header enforcement, so the absent Access-Control-Allow-Origin header provides no protection once the upgrade succeeds.

Regression provenance: commit 6f1f4f3 (2026-01-10, v1.28.2) fixed CVE-2026-22689 by DELETING CheckOrigin; commit a63bcd9 (2026-01-31, first in v1.29.0) reintroduced CheckOrigin returning true and replaced the protection with the bypassable raw-prefix test.

Attacker model and verification

Any website the developer visits while Mailpit is running. No credentials, no ability to send mail, no interaction beyond visiting a page. Requires Mailpit without --ui-auth-file (the default, and the same precondition as the original CVE). The bypass was measured live against a real Mailpit instance on loopback, including the 403-versus-101 control pair; authentication still holds (the encoded path returns 401 when --ui-auth-file is set); browser reachability was confirmed against the WHATWG URL parser, which preserves %61.

Suggested fix

Do not make security decisions on r.RequestURI. Key the check on r.URL.Path, the decoded value the router uses, so the gate and the route agree. Better, restore a real CheckOrigin on the upgrader so the WebSocket carries its own origin enforcement rather than depending on a middleware prefix match. Secondary (Low, not claimed as XSS): server/apiv1/message.go lines 154-155 echo an attacker-chosen Content-Type with Content-Disposition: inline; this is blocked today by the nonce CSP.

Tooling

I used AI assistance while investigating. The bypass was measured live against a real Mailpit instance on loopback, including the 403-versus-101 control pair and the authenticated 401 case, and I separately confirmed at HEAD the CheckOrigin returning true with its delegating comment and the RequestURI-keyed prefix gate.