diff --git a/README.md b/README.md
index 2f16af0f..dd186e2e 100644
--- a/README.md
+++ b/README.md
@@ -369,7 +369,6 @@ The following tests are not yet implemented and therefore missing:
 - Informative Test 6.3.19.5
 - Informative Test 6.3.20
 - Informative Test 6.3.21.2
-- Informative Test 6.3.21.3
 - Informative Test 6.3.21.4
 - Informative Test 6.3.21.5
 - Informative Test 6.3.21.6
@@ -523,6 +522,7 @@ 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_1: DocumentTest
+export const informativeTest_6_3_21_3: DocumentTest
 ```
 
 [(back to top)](#bsi-csaf-validator-lib)
diff --git a/csaf_2_1/informativeTests.js b/csaf_2_1/informativeTests.js
index 22d3a07d..2fef9c62 100644
--- a/csaf_2_1/informativeTests.js
+++ b/csaf_2_1/informativeTests.js
@@ -14,3 +14,4 @@ export { informativeTest_6_3_5 } from './informativeTests/informativeTest_6_3_5.
 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_1 } from './informativeTests/informativeTest_6_3_21_1.js'
+export { informativeTest_6_3_21_3 } from './informativeTests/informativeTest_6_3_21_3.js'
diff --git a/csaf_2_1/informativeTests/informativeTest_6_3_21_3.js b/csaf_2_1/informativeTests/informativeTest_6_3_21_3.js
new file mode 100644
index 00000000..7183b351
--- /dev/null
+++ b/csaf_2_1/informativeTests/informativeTest_6_3_21_3.js
@@ -0,0 +1,21 @@
+import { walkPath } from '../../lib/walkPaths.js'
+
+/**
+ * This implements the informative test 6.3.21.3 of the CSAF 2.1 standard.
+ * @param {any} doc
+ * @returns
+ */
+export async function informativeTest_6_3_21_3(doc) {
+  const ctx = {
+    infos: /** @type {Array<{ message: string; instancePath: string }>} */ ([]),
+  }
+
+  await walkPath(doc, '/document/x_extensions[]', async (instancePath) => {
+    ctx.infos.push({
+      instancePath,
+      message: `The element "$.document.x_extensions" exist.`,
+    })
+  })
+
+  return ctx
+}
diff --git a/tests/csaf_2_1/oasis.js b/tests/csaf_2_1/oasis.js
index 0f97584b..ba95ca5f 100644
--- a/tests/csaf_2_1/oasis.js
+++ b/tests/csaf_2_1/oasis.js
@@ -62,7 +62,6 @@ const excluded = [
   '6.3.19.5',
   '6.3.20',
   '6.3.21.2',
-  '6.3.21.3',
   '6.3.21.4',
   '6.3.21.5',
   '6.3.21.6',