diff --git a/.jules/sentinel.md b/.jules/sentinel.md
new file mode 100644
index 0000000..77b79e4
--- /dev/null
+++ b/.jules/sentinel.md
@@ -0,0 +1,6 @@
+## Sentinel's Journal
+
+## 2026-10-08 - Added CORS headers protection
+**Vulnerability:** CRLF Injection in headers via Environment Variables (HTTP Response Splitting).
+**Learning:** `responder.Render()` expands `${VAR}` in header values using `r.env.Expand(h.Value)`. However, the environment variable values themselves are not sanitized! `Spec.Validate()` only checks the literal header values in the config/CLI before expansion (`validHeaderValue`). If an environment variable like `PORTBELL_API_KEY` contains `\r\n` (CRLF), the expanded value will be injected into the raw HTTP response, leading to HTTP Response Splitting.
+**Prevention:** Always validate expanded values, or strip `\r` and `\n` from environment variables during expansion in headers.
diff --git a/internal/responder/responder.go b/internal/responder/responder.go
index 6eaa674..8afdc42 100644
--- a/internal/responder/responder.go
+++ b/internal/responder/responder.go
@@ -167,12 +167,29 @@ func (r *Responder) Set(s Spec) error {
 	return nil
 }
 
+func sanitizeHeaderValue(v string) string {
+	if validHeaderValue(v) {
+		return v
+	}
+	var b strings.Builder
+	b.Grow(len(v))
+	for i := 0; i < len(v); i++ {
+		c := v[i]
+		if c == '\t' || (c >= 0x20 && c != 0x7f) {
+			b.WriteByte(c)
+		} else {
+			b.WriteByte(' ')
+		}
+	}
+	return b.String()
+}
+
 // Render returns the reply to send now, with placeholders expanded.
 func (r *Responder) Render() events.Response {
 	s := r.Get()
 	headers := make([]events.Header, len(s.Headers))
 	for i, h := range s.Headers {
-		headers[i] = events.Header{Name: h.Name, Value: r.env.Expand(h.Value)}
+		headers[i] = events.Header{Name: h.Name, Value: sanitizeHeaderValue(r.env.Expand(h.Value))}
 	}
 	return events.Response{
 		Status:  s.Status,
diff --git a/internal/responder/responder_test.go b/internal/responder/responder_test.go
index d2d462a..64d5579 100644
--- a/internal/responder/responder_test.go
+++ b/internal/responder/responder_test.go
@@ -124,3 +124,21 @@ func TestFormatParseRoundTrip(t *testing.T) {
 		t.Fatalf("round trip mismatch: %+v", back)
 	}
 }
+
+func TestResponderRenderSanitizesHeaders(t *testing.T) {
+	env := map[string]string{"PORTBELL_INJECT": "val\r\nInjected: 1"}
+	p := EnvPolicy{
+		Prefix: "PORTBELL_",
+		Lookup: func(k string) (string, bool) { v, ok := env[k]; return v, ok },
+	}
+	r := New(Spec{
+		Status:  200,
+		Headers: []events.Header{{Name: "X-Test", Value: "${PORTBELL_INJECT}"}},
+		Body:    "body",
+	}, p)
+
+	out := r.Render()
+	if out.Headers[0].Value != "val  Injected: 1" {
+		t.Fatalf("expected sanitized header, got %q", out.Headers[0].Value)
+	}
+}