Add session transfer for SSO cookies on custom domains

After OIDC login, if the redirect targets a trust center custom
domain, the callback now redirects through a session-transfer
endpoint on that domain. The endpoint verifies an HMAC-signed,
time-limited token and sets the session cookie on the custom
domain before redirecting to the final URL.

The continue URL is bound into the signed token payload to
prevent open-redirect attacks via parameter tampering.

Signed-off-by: Bryan Frimin <bryan@getprobo.com>

pkg/server/api/trust/v1/session_transfer_handler.go

@@ -0,0 +1,86 @@
+// Copyright (c) 2026 Probo Inc <hello@getprobo.com>.
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+// REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+// AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+// INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+// LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+// OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+package trust_v1
+
+import (
+	"errors"
+	"net/http"
+
+	"go.gearno.de/kit/httpserver"
+	"go.gearno.de/kit/log"
+	"go.probo.inc/probo/pkg/gid"
+	"go.probo.inc/probo/pkg/iam"
+	"go.probo.inc/probo/pkg/securecookie"
+	"go.probo.inc/probo/pkg/server/api/authn"
+)
+
+type SessionTransferHandler struct {
+	iam           *iam.Service
+	sessionCookie *authn.Cookie
+	cookieSecret  string
+	logger        *log.Logger
+}
+
+func NewSessionTransferHandler(
+	iamSvc *iam.Service,
+	cookieConfig securecookie.Config,
+	logger *log.Logger,
+) *SessionTransferHandler {
+	return &SessionTransferHandler{
+		iam:           iamSvc,
+		sessionCookie: authn.NewCookie(&cookieConfig),
+		cookieSecret:  cookieConfig.Secret,
+		logger:        logger,
+	}
+}
+
+func (h *SessionTransferHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	ctx := r.Context()
+
+	token := r.URL.Query().Get("token")
+	if token == "" {
+		httpserver.RenderError(w, http.StatusBadRequest, errors.New("missing token"))
+		return
+	}
+
+	claims, err := authn.VerifySessionTransfer(token, h.cookieSecret)
+	if err != nil {
+		h.logger.WarnCtx(ctx, "invalid session transfer token", log.Error(err))
+		httpserver.RenderError(w, http.StatusBadRequest, errors.New("invalid or expired token"))
+		return
+	}
+
+	continueURL := claims.ContinueURL
+	if continueURL == "" {
+		continueURL = "/"
+	}
+
+	sessionID, err := gid.ParseGID(claims.SessionID)
+	if err != nil {
+		httpserver.RenderError(w, http.StatusBadRequest, errors.New("invalid token"))
+		return
+	}
+
+	session, err := h.iam.SessionService.GetSession(ctx, sessionID)
+	if err != nil {
+		h.logger.ErrorCtx(ctx, "cannot get session for transfer", log.Error(err))
+		httpserver.RenderError(w, http.StatusBadRequest, errors.New("invalid or expired token"))
+		return
+	}
+
+	h.sessionCookie.Set(w, session)
+
+	http.Redirect(w, r, continueURL, http.StatusFound)
+}