src/java.httpclient/share/classes/java/net/http/HttpHeadersImpl.java

@@ -70,11 +70,11 @@
         return Optional.ofNullable(l == null ? null : l.get(0));
     }
 
     @Override
     public List<String> allValues(String name) {
-        return headers.get(name);
+        return Collections.unmodifiableList(headers.get(name));
     }
 
     @Override
     public Map<String, List<String>> map() {
         return Collections.unmodifiableMap(headers);