diff --git a/services/proxy/pkg/middleware/authentication.go b/services/proxy/pkg/middleware/authentication.go
index 4a4e441f5d..b5dc7d5326 100644
--- a/services/proxy/pkg/middleware/authentication.go
+++ b/services/proxy/pkg/middleware/authentication.go
@@ -337,18 +337,17 @@ func renderTerminalFailure(w http.ResponseWriter, r *http.Request, result Authen
 	}
 
 	// Default: generic 401 without challenges.
-	w.WriteHeader(http.StatusUnauthorized)
-	if webdav.IsWebdavRequest(r) {
-		b, err := webdav.Marshal(webdav.Exception{
-			Code:    webdav.SabredavNotAuthenticated,
-			Message: "Authentication error",
-		})
-		if err != nil {
-			return err
-		}
-		_, _ = w.Write(b)
+	if !webdav.IsWebdavRequest(r) {
+		w.WriteHeader(http.StatusUnauthorized)
+		return nil
 	}
-	return nil
+
+	w.Header().Set("Content-Type", "application/xml; charset=utf-8")
+	w.WriteHeader(http.StatusUnauthorized)
+	return webdav.Encode(w, webdav.Exception{
+		Code:    webdav.SabredavNotAuthenticated,
+		Message: "Authentication error",
+	})
 }
 
 // renderErrorDetails renders a response based on structured error details.
@@ -377,20 +376,16 @@ func renderJSONExpired(w http.ResponseWriter, permissionID string) error {
 		PermissionID: permissionID,
 	}
 
-	body, err := json.Marshal(resp)
-	if err != nil {
-		return err
-	}
-
 	w.Header().Set("Content-Type", "application/json")
 	w.WriteHeader(http.StatusUnauthorized)
-	_, err = w.Write(body)
-	return err
+	return json.NewEncoder(w).Encode(resp)
 }
 
 // renderDAVExpired writes a SabreDAV-compatible XML response for expired guest sessions.
 func renderDAVExpired(w http.ResponseWriter, permissionID string) error {
-	xmlBody, err := xml.Marshal(davExpiredResponse{
+	w.Header().Set("Content-Type", "application/xml; charset=utf-8")
+	w.WriteHeader(http.StatusUnauthorized)
+	return webdav.EncodeXML(w, davExpiredResponse{
 		XmlnsD:    "DAV",
 		XmlnsS:    "http://sabredav.org/ns",
 		Exception: "Sabre\\DAV\\Exception\\NotAuthenticated",
@@ -401,14 +396,6 @@ func renderDAVExpired(w http.ResponseWriter, permissionID string) error {
 			ShareID:        permissionID,
 		},
 	})
-	if err != nil {
-		return err
-	}
-
-	w.Header().Set("Content-Type", "application/xml; charset=utf-8")
-	w.WriteHeader(http.StatusUnauthorized)
-	_, err = w.Write(append([]byte(xml.Header), xmlBody...))
-	return err
 }
 
 type davExpiredResponse struct {
diff --git a/services/proxy/pkg/middleware/terminal_failure_test.go b/services/proxy/pkg/middleware/terminal_failure_test.go
new file mode 100644
index 0000000000..8039d0c119
--- /dev/null
+++ b/services/proxy/pkg/middleware/terminal_failure_test.go
@@ -0,0 +1,87 @@
+package middleware
+
+import (
+	"encoding/json"
+	"encoding/xml"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// hostileShareID would break out of an XML/JSON document if it were not escaped.
+const hostileShareID = `<script>alert("x")</script>&'`
+
+func TestRenderTerminalFailure_Generic(t *testing.T) {
+	t.Run("non-dav request", func(t *testing.T) {
+		rr := httptest.NewRecorder()
+		req := httptest.NewRequest(http.MethodGet, "/graph/v1beta1/me/drive/sharedWithMe", nil)
+
+		require.NoError(t, renderTerminalFailure(rr, req, TerminalFailed()))
+
+		assert.Equal(t, http.StatusUnauthorized, rr.Code)
+		assert.Empty(t, rr.Body.String())
+	})
+
+	t.Run("dav request", func(t *testing.T) {
+		rr := httptest.NewRecorder()
+		req := httptest.NewRequest("PROPFIND", "/dav/spaces/abc", nil)
+
+		require.NoError(t, renderTerminalFailure(rr, req, TerminalFailed()))
+
+		assert.Equal(t, http.StatusUnauthorized, rr.Code)
+		assert.Equal(t, "application/xml; charset=utf-8", rr.Header().Get("Content-Type"))
+		assert.True(t, strings.HasPrefix(rr.Body.String(), xml.Header))
+		assert.Contains(t, rr.Body.String(), "Sabre\\DAV\\Exception\\NotAuthenticated")
+	})
+}
+
+func TestRenderTerminalFailure_GuestSessionExpiredJSON(t *testing.T) {
+	rr := httptest.NewRecorder()
+	req := httptest.NewRequest(http.MethodGet, "/graph/v1beta1/me/drive/sharedWithMe", nil)
+
+	require.NoError(t, renderTerminalFailure(rr, req, AuthenticationResult{
+		State:        AuthenticationFailed,
+		Terminal:     true,
+		ErrorDetails: GuestSessionExpiredDetails{PermissionID: hostileShareID},
+	}))
+
+	assert.Equal(t, http.StatusUnauthorized, rr.Code)
+	assert.Equal(t, "application/json", rr.Header().Get("Content-Type"))
+
+	var body struct {
+		ErrorType    string `json:"error_type"`
+		PermissionID string `json:"permissionId"`
+	}
+	require.NoError(t, json.Unmarshal(rr.Body.Bytes(), &body))
+	assert.Equal(t, "session_expired", body.ErrorType)
+	assert.Equal(t, hostileShareID, body.PermissionID)
+	// json.Encoder escapes <, > and & so the body is safe even if it were rendered as HTML
+	assert.NotContains(t, rr.Body.String(), "<script>")
+}
+
+func TestRenderTerminalFailure_GuestSessionExpiredDAV(t *testing.T) {
+	rr := httptest.NewRecorder()
+	req := httptest.NewRequest("PROPFIND", "/dav/spaces/abc", nil)
+
+	require.NoError(t, renderTerminalFailure(rr, req, AuthenticationResult{
+		State:        AuthenticationFailed,
+		Terminal:     true,
+		ErrorDetails: GuestSessionExpiredDetails{PermissionID: hostileShareID, IsDAV: true},
+	}))
+
+	assert.Equal(t, http.StatusUnauthorized, rr.Code)
+	assert.Equal(t, "application/xml; charset=utf-8", rr.Header().Get("Content-Type"))
+	assert.True(t, strings.HasPrefix(rr.Body.String(), xml.Header))
+
+	// encoding/xml cannot unmarshal prefixed element names such as d:error, so
+	// inspect the document directly.
+	body := rr.Body.String()
+	assert.Contains(t, body, "<s:Exception>Sabre\\DAV\\Exception\\NotAuthenticated</s:Exception>")
+	assert.Contains(t, body, "<opencloud:error_type>session_expired</opencloud:error_type>")
+	assert.Contains(t, body, "<opencloud:share_id>&lt;script&gt;alert(&#34;x&#34;)&lt;/script&gt;&amp;&#39;</opencloud:share_id>")
+	assert.NotContains(t, body, "<script>")
+}
diff --git a/services/proxy/pkg/webdav/response.go b/services/proxy/pkg/webdav/response.go
index 0279042719..6b9e5e0b71 100644
--- a/services/proxy/pkg/webdav/response.go
+++ b/services/proxy/pkg/webdav/response.go
@@ -2,6 +2,7 @@ package webdav
 
 import (
 	"encoding/xml"
+	"io"
 	"net/http"
 )
 
@@ -57,6 +58,26 @@ func Marshal(e Exception) ([]byte, error) {
 	return []byte(xml.Header + string(xmlstring)), err
 }
 
+// Encode writes the given Exception as a SabreDAV error document, including the
+// XML header, to w.
+func Encode(w io.Writer, e Exception) error {
+	return EncodeXML(w, &errorXML{
+		Xmlnsd:    "DAV",
+		Xmlnss:    "http://sabredav.org/ns",
+		Exception: codesEnum[e.Code],
+		Message:   e.Message,
+		Header:    e.Header,
+	})
+}
+
+// EncodeXML writes v as an XML document, including the XML header, to w.
+func EncodeXML(w io.Writer, v any) error {
+	if _, err := io.WriteString(w, xml.Header); err != nil {
+		return err
+	}
+	return xml.NewEncoder(w).Encode(v)
+}
+
 // http://www.webdav.org/specs/rfc4918.html#ELEMENT_error
 type errorXML struct {
 	XMLName   xml.Name `xml:"d:error"`