diff --git a/sdk/lib/_http/http_impl.dart b/sdk/lib/_http/http_impl.dart
index 804bacc2222..e0c1e2f6a12 100644
--- a/sdk/lib/_http/http_impl.dart
+++ b/sdk/lib/_http/http_impl.dart
@@ -3059,11 +3059,8 @@ class _HttpClient implements HttpClient {
     );
   }
 
-  static bool _isSubdomain(Uri subdomain, Uri domain) {
-    return (subdomain.isScheme(domain.scheme) &&
-        subdomain.port == domain.port &&
-        (subdomain.host == domain.host ||
-            subdomain.host.endsWith("." + domain.host)));
+  static bool _isSameOrigin(Uri a, Uri b) {
+    return a.isScheme(b.scheme) && a.host == b.host && a.port == b.port;
   }
 
   // Only visible for testing.
@@ -3072,17 +3069,16 @@ class _HttpClient implements HttpClient {
     required Uri originalUrl,
     required Uri redirectUrl,
   }) {
-    if (_isSubdomain(redirectUrl, originalUrl)) {
-      return true;
-    }
-
-    const nonRedirectHeaders = [
+    // It is only safe to copy sensitive headers when redirecting to the
+    // same origin (RFC 6454 section 4).
+    const sensitiveHeaders = [
       "authorization",
       "www-authenticate",
       "cookie",
       "cookie2",
     ];
-    return !nonRedirectHeaders.contains(headerKey.toLowerCase());
+    return !sensitiveHeaders.contains(headerKey.toLowerCase()) ||
+        _isSameOrigin(redirectUrl, originalUrl);
   }
 
   Future<_HttpClientRequest> _openUrlFromRequest(
diff --git a/tests/standalone/io/http_redirect_test.dart b/tests/standalone/io/http_redirect_test.dart
index 4672157bcec..50451afbb72 100644
--- a/tests/standalone/io/http_redirect_test.dart
+++ b/tests/standalone/io/http_redirect_test.dart
@@ -582,15 +582,70 @@ void testShouldCopyHeadersOnRedirect() {
   );
   checkShouldCopyHeader("cat", "http://foo.com", "http://foo.com:80/foo", true);
 
-  // Redirect to subdomain.
+  // Redirect to subdomain. Credential-carrying headers MUST NOT be forwarded
+  // to a subdomain (the subdomain can be attacker-controlled on shared apex
+  // hosts, and RFC 6265 host-only cookies must not be sent to subdomains).
+  // Non-sensitive headers continue to follow the redirect.
   checkShouldCopyHeader(
     "authorization",
     "https://foo.com",
     "https://www.foo.com",
-    true,
+    false,
+  );
+  checkShouldCopyHeader(
+    "cookie",
+    "https://foo.com",
+    "https://www.foo.com",
+    false,
+  );
+  checkShouldCopyHeader(
+    "cookie2",
+    "https://foo.com",
+    "https://www.foo.com",
+    false,
+  );
+  checkShouldCopyHeader(
+    "www-authenticate",
+    "https://foo.com",
+    "https://www.foo.com",
+    false,
   );
   checkShouldCopyHeader("cat", "https://foo.com", "https://www.foo.com", true);
 
+  // Redirect to same host: all sensitive headers are still forwarded.
+  checkShouldCopyHeader(
+    "cookie",
+    "https://foo.com",
+    "https://foo.com/path",
+    true,
+  );
+  checkShouldCopyHeader(
+    "cookie2",
+    "https://foo.com",
+    "https://foo.com/path",
+    true,
+  );
+  checkShouldCopyHeader(
+    "www-authenticate",
+    "https://foo.com",
+    "https://foo.com/path",
+    true,
+  );
+
+  // Redirect to a deeper subdomain (e.g. attacker-uploads.example.com).
+  checkShouldCopyHeader(
+    "authorization",
+    "https://example.com",
+    "https://attacker-uploads.example.com",
+    false,
+  );
+  checkShouldCopyHeader(
+    "cookie",
+    "https://example.com",
+    "https://attacker-uploads.example.com",
+    false,
+  );
+
   // Redirect to different domain.
   checkShouldCopyHeader(
     "authorization",