diff --git a/.jules/sentinel.md b/.jules/sentinel.md
new file mode 100644
index 0000000..d5e1eaa
--- /dev/null
+++ b/.jules/sentinel.md
@@ -0,0 +1,4 @@
+## 2023-10-12 - Header Injection via Environment Variables
+**Vulnerability:** HTTP Response Splitting / Header Injection
+**Learning:** `responder.Spec.Validate()` correctly prevents control characters in header values designed by the user. However, when these headers contain environment variable placeholders (`${VAR}`), `responder.Render()` expanded them *after* validation. An attacker could potentially inject arbitrary headers or split the response by manipulating an allowed environment variable to contain `\r\n`.
+**Prevention:** Sanitized the expanded environment variable values in `Render()` by stripping `\r` and `\n` characters before adding them to the final response headers.
diff --git a/internal/responder/responder.go b/internal/responder/responder.go
index 6eaa674..376f096 100644
--- a/internal/responder/responder.go
+++ b/internal/responder/responder.go
@@ -172,7 +172,10 @@ 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)}
+		val := r.env.Expand(h.Value)
+		val = strings.ReplaceAll(val, "\r", "")
+		val = strings.ReplaceAll(val, "\n", "")
+		headers[i] = events.Header{Name: h.Name, Value: val}
 	}
 	return events.Response{
 		Status:  s.Status,