diff --git a/README.md b/README.md
index bd3e87bd..04c710e8 100644
--- a/README.md
+++ b/README.md
@@ -373,7 +373,6 @@ The following tests are not yet implemented and therefore missing:
 - Informative Test 6.2.21.3
 - Informative Test 6.2.21.4
 - Informative Test 6.2.21.5
-- Informative Test 6.2.21.6
 - Informative Test 6.2.21.7
 - Informative Test 6.2.21.8
 - Informative Test 6.2.21.9
@@ -523,6 +522,7 @@ export const informativeTest_6_3_10: DocumentTest
 export const informativeTest_6_3_11: DocumentTest
 export const informativeTest_6_3_12: DocumentTest
 export const informativeTest_6_3_18: DocumentTest
+export const informativeTest_6_3_21_6: DocumentTest
 ```
 
 [(back to top)](#bsi-csaf-validator-lib)
diff --git a/csaf_2_1/informativeTests.js b/csaf_2_1/informativeTests.js
index e15078a9..042600b0 100644
--- a/csaf_2_1/informativeTests.js
+++ b/csaf_2_1/informativeTests.js
@@ -13,3 +13,4 @@ export { informativeTest_6_3_4 } from './informativeTests/informativeTest_6_3_4.
 export { informativeTest_6_3_5 } from './informativeTests/informativeTest_6_3_5.js'
 export { informativeTest_6_3_12 } from './informativeTests/informativeTest_6_3_12.js'
 export { informativeTest_6_3_18 } from './informativeTests/informativeTest_6_3_18.js'
+export { informativeTest_6_3_21_6 } from './informativeTests/informativeTest_6_3_21_6.js'
diff --git a/csaf_2_1/informativeTests/informativeTest_6_3_21_6.js b/csaf_2_1/informativeTests/informativeTest_6_3_21_6.js
new file mode 100644
index 00000000..b51557e9
--- /dev/null
+++ b/csaf_2_1/informativeTests/informativeTest_6_3_21_6.js
@@ -0,0 +1,26 @@
+import { walkPath } from '../../lib/walkPaths.js'
+
+/**
+ * This implements the informative test 6.3.21.6 of the CSAF 2.1 standard.
+ * @param {any} doc
+ * @returns
+ */
+export async function informativeTest_6_3_21_6(doc) {
+  const ctx = {
+    infos: /** @type {Array<{ message: string; instancePath: string }>} */ ([]),
+  }
+
+  await walkPath(
+    doc,
+    '/product_tree/product_paths[*]/full_product_name/x_extensions',
+    async (instancePath) => {
+      ctx.infos.push({
+        instancePath,
+        message:
+          'The element "x_extensions" exists in a path that starts with "$.product_tree.product_paths".',
+      })
+    }
+  )
+
+  return ctx
+}
diff --git a/tests/csaf_2_1/oasis.js b/tests/csaf_2_1/oasis.js
index ffe51ad4..dcfa8ceb 100644
--- a/tests/csaf_2_1/oasis.js
+++ b/tests/csaf_2_1/oasis.js
@@ -66,7 +66,6 @@ const excluded = [
   '6.3.21.3',
   '6.3.21.4',
   '6.3.21.5',
-  '6.3.21.6',
   '6.3.21.7',
   '6.3.21.8',
   '6.3.21.9',