diff --git a/README.md b/README.md
index 44bae60c..9b18a683 100644
--- a/README.md
+++ b/README.md
@@ -373,7 +373,6 @@ The following tests are not yet implemented and therefore missing:
 - Recommended Test 6.2.50.3
 - Recommended Test 6.2.51
 - Recommended Test 6.2.54.1
-- Recommended Test 6.2.54.2
 - Recommended Test 6.2.54.4
 
 **Informative Tests**
@@ -526,6 +525,7 @@ export const recommendedTest_6_2_48: DocumentTest
 export const recommendedTest_6_2_49: DocumentTest
 export const recommendedTest_6_2_52: DocumentTest
 export const recommendedTest_6_2_53: DocumentTest
+export const recommendedTest_6_2_54_2: DocumentTest
 export const recommendedTest_6_2_54_3: DocumentTest
 ```
 
diff --git a/csaf_2_1/csafAjv.js b/csaf_2_1/csafAjv.js
index 429f5d87..a2b83b50 100644
--- a/csaf_2_1/csafAjv.js
+++ b/csaf_2_1/csafAjv.js
@@ -11,6 +11,7 @@ import cvss_meta from './csafAjv/cvss_meta.js'
 import meta_format_assertion from './csafAjv/meta-format-assertion.js'
 import draft_07_schema from './csafAjv/draft-07-schema.js'
 import selectionList_2_0_0Schema from './csafAjv/SelectionList_2_0_0.schema.js'
+import { registerExtensionSchemas } from './csafAjv/extensionSchemas/index.js'
 
 import { validateTimestamp } from './dateHelper.js'
 
@@ -45,6 +46,7 @@ csafAjv.addSchema(
   selectionList_2_0_0Schema,
   'https://certcc.github.io/SSVC/data/schema/v2/SelectionList_2_0_0.schema.json'
 )
+registerExtensionSchemas(csafAjv)
 
 csafAjv.addFormat('date-time', {
   type: 'string',
diff --git a/csaf_2_1/csafAjv/extensionSchemas/documentation-11.js b/csaf_2_1/csafAjv/extensionSchemas/documentation-11.js
new file mode 100644
index 00000000..6f5d2538
--- /dev/null
+++ b/csaf_2_1/csafAjv/extensionSchemas/documentation-11.js
@@ -0,0 +1,54 @@
+// https://raw.githubusercontent.com/oasis-tcs/csaf/refs/heads/master/csaf_2.1/extension/data/valid/documentation-11/documentation-11-content_1.0.0.json
+// Copied from ../../../csaf/csaf_2.1/test/extension/data/valid/documentation-11/documentation-11-content_1.0.0.json
+export default {
+  $schema:
+    'https://docs.oasis-open.org/csaf/csaf/v2.1/schema/extension-metaschema.json',
+  $id: 'https://raw.githubusercontent.com/oasis-tcs/csaf/refs/heads/master/csaf_2.1/extension/data/valid/documentation-11/documentation-11-content_1.0.0.json',
+  title: 'CSAF Extension: Documentation 11',
+  description:
+    'Representation of a valid extension used for documentation purposes.',
+  type: 'object',
+  required: ['$schema', 'category', 'critical', 'content'],
+  properties: {
+    $schema: {
+      title: 'CSAF Extension Schema',
+      description:
+        'Contains the URL of the CSAF Extension JSON schema which the JSON object promises to be valid for.',
+      type: 'string',
+      format: 'uri',
+      const:
+        'https://raw.githubusercontent.com/oasis-tcs/csaf/refs/heads/master/csaf_2.1/extension/data/valid/documentation-11/documentation-11-content_1.0.0.json',
+    },
+    category: {
+      title: 'Extension Category',
+      description: 'Holds the category of the extension content.',
+      type: 'string',
+      enum: ['essential', 'significant', 'supplementary'],
+    },
+    content: {
+      title: 'Content',
+      description: 'Contains the additional information in its properties.',
+      type: 'object',
+      minProperties: 1,
+      properties: {
+        documentation: {
+          title: 'Documentation content',
+          description:
+            'Contains a constant string to clarify that this is for documentation purposes only and not to be used in production.',
+          type: 'string',
+          const:
+            'This extension is for documentation and test purposed only. It is valid. It is not allowed to be used in a production CSAF.',
+        },
+      },
+      unevaluatedProperties: false,
+    },
+    critical: {
+      title: 'Critical',
+      description:
+        'Determines whether using the extension would fail a mandatory test.',
+      type: 'boolean',
+      const: false,
+    },
+  },
+  additionalProperties: false,
+}
diff --git a/csaf_2_1/csafAjv/extensionSchemas/documentation-12.js b/csaf_2_1/csafAjv/extensionSchemas/documentation-12.js
new file mode 100644
index 00000000..33f8eb88
--- /dev/null
+++ b/csaf_2_1/csafAjv/extensionSchemas/documentation-12.js
@@ -0,0 +1,68 @@
+// https://raw.githubusercontent.com/oasis-tcs/csaf/refs/heads/master/csaf_2.1/extension/data/valid/documentation-12/documentation-12-content_1.0.0.json
+// Copied from ../../../csaf/csaf_2.1/test/extension/data/valid/documentation-12/documentation-12-content_1.0.0.json
+export default {
+  $schema:
+    'https://docs.oasis-open.org/csaf/csaf/v2.1/schema/extension-metaschema.json',
+  $id: 'https://raw.githubusercontent.com/oasis-tcs/csaf/refs/heads/master/csaf_2.1/extension/data/valid/documentation-12/documentation-12-content_1.0.0.json',
+  title: 'CSAF Extension: Documentation 12',
+  description:
+    'Representation of a valid extension used for documentation purposes.',
+  type: 'object',
+  required: ['$schema', 'category', 'critical', 'content'],
+  properties: {
+    $schema: {
+      title: 'CSAF Extension Schema',
+      description:
+        'Contains the URL of the CSAF Extension JSON schema which the JSON object promises to be valid for.',
+      type: 'string',
+      format: 'uri',
+      const:
+        'https://raw.githubusercontent.com/oasis-tcs/csaf/refs/heads/master/csaf_2.1/extension/data/valid/documentation-12/documentation-12-content_1.0.0.json',
+    },
+    category: {
+      title: 'Extension Category',
+      description: 'Holds the category of the extension content.',
+      type: 'string',
+      enum: ['essential', 'significant', 'supplementary'],
+    },
+    content: {
+      title: 'Content',
+      description: 'Contains the additional information in its properties.',
+      type: 'object',
+      minProperties: 1,
+      required: ['documentation'],
+      properties: {
+        documentation: {
+          title: 'Documentation content',
+          description:
+            'Contains a constant string to clarify that this is for documentation purposes only and not to be used in production.',
+          type: 'string',
+          const:
+            'This extension is for documentation and test purposed only. It is valid. It is not allowed to be used in a production CSAF.',
+        },
+        notes: {
+          title: 'Notes',
+          description:
+            'Contains a list of notes to convey a more complex structure.',
+          type: 'array',
+          minItems: 1,
+          items: {
+            title: 'Entry',
+            description: 'Contains a single entry.',
+            type: 'string',
+            minLength: 1,
+          },
+        },
+      },
+      unevaluatedProperties: false,
+    },
+    critical: {
+      title: 'Critical',
+      description:
+        'Determines whether using the extension would fail a mandatory test.',
+      type: 'boolean',
+      const: false,
+    },
+  },
+  additionalProperties: false,
+}
diff --git a/csaf_2_1/csafAjv/extensionSchemas/documentation-13.js b/csaf_2_1/csafAjv/extensionSchemas/documentation-13.js
new file mode 100644
index 00000000..343e23f8
--- /dev/null
+++ b/csaf_2_1/csafAjv/extensionSchemas/documentation-13.js
@@ -0,0 +1,129 @@
+// https://raw.githubusercontent.com/oasis-tcs/csaf/refs/heads/master/csaf_2.1/extension/data/valid/documentation-13/documentation-13-content_1.0.0.json
+// Copied from ../../../csaf/csaf_2.1/test/extension/data/valid/documentation-13/documentation-13-content_1.0.0.json
+export default {
+  $schema:
+    'https://docs.oasis-open.org/csaf/csaf/v2.1/schema/extension-metaschema.json',
+  $id: 'https://raw.githubusercontent.com/oasis-tcs/csaf/refs/heads/master/csaf_2.1/extension/data/valid/documentation-13/documentation-13-content_1.0.0.json',
+  title: 'CSAF Extension: Documentation 13',
+  description:
+    'Representation of a valid extension for configurations used for documentation purposes.',
+  type: 'object',
+  $defs: {
+    configuration_id_t: {
+      title: 'Reference token for configuration instance',
+      description:
+        'Token required to identify a configuration so that it can be referred to from other parts in the document. There is no predefined or required format for the configuration_id as long as it uniquely identifies a configuration in the context of the current document.',
+      type: 'string',
+      minLength: 1,
+      examples: ['CSAFCID-0001', 'CSAFCID-0002'],
+    },
+    configuration_t: {
+      title: 'Configuration',
+      description:
+        'Contains the description of a configuration and assigns it a configuration id.',
+      type: 'object',
+      required: ['configuration_id', 'details'],
+      properties: {
+        configuration_id: { $ref: '#/$defs/configuration_id_t' },
+        details: {
+          title: 'Details of the Configuration',
+          description:
+            'Contains all details as human-readable description on how to identify the configuration.',
+          type: 'string',
+          minLength: 1,
+        },
+      },
+      additionalProperties: false,
+    },
+    relationship_t: {
+      title: 'Relationship',
+      description:
+        'Establishes a link between an existing full_product_name_t element and a configuration, allowing the document producer to define a combination that form a new full_product_name entry.',
+      type: 'object',
+      required: [
+        'configuration_reference',
+        'full_product_name',
+        'product_reference',
+      ],
+      properties: {
+        configuration_reference: {
+          title: 'Configuration Reference',
+          description:
+            'Holds a Configuration ID that refers to the Configuration element, which is referenced as the second element of the relationship.',
+          $ref: '#/$defs/configuration_id_t',
+        },
+        full_product_name: {
+          $ref: 'https://docs.oasis-open.org/csaf/csaf/v2.1/schema/csaf.json#/$defs/full_product_name_t',
+        },
+        product_reference: {
+          title: 'Product reference',
+          description:
+            'Holds a Product ID that refers to the Full Product Name element, which is referenced as the first element of the relationship.',
+          $ref: 'https://docs.oasis-open.org/csaf/csaf/v2.1/schema/csaf.json#/$defs/product_id_t',
+        },
+      },
+      additionalProperties: false,
+    },
+  },
+  required: ['$schema', 'category', 'critical', 'content'],
+  properties: {
+    $schema: {
+      title: 'CSAF Extension Schema',
+      description:
+        'Contains the URL of the CSAF Extension JSON schema which the JSON object promises to be valid for.',
+      type: 'string',
+      format: 'uri',
+      const:
+        'https://raw.githubusercontent.com/oasis-tcs/csaf/refs/heads/master/csaf_2.1/extension/data/valid/documentation-13/documentation-13-content_1.0.0.json',
+    },
+    category: {
+      title: 'Extension Category',
+      description: 'Holds the category of the extension content.',
+      type: 'string',
+      enum: ['essential', 'significant', 'supplementary'],
+    },
+    content: {
+      title: 'Content',
+      description: 'Contains the additional information in its properties.',
+      type: 'object',
+      minProperties: 1,
+      properties: {
+        configuration_tree: {
+          title: 'Configuration Tree',
+          description:
+            'Contains configurations and their bindings to products.',
+          type: 'object',
+          required: ['configurations', 'relationships'],
+          properties: {
+            configurations: {
+              title: 'List of Configurations',
+              description: 'Contains a list of configuration elements.',
+              type: 'array',
+              minItems: 1,
+              uniqueItems: true,
+              items: { $ref: '#/$defs/configuration_t' },
+            },
+            relationships: {
+              title: 'List of Relationships',
+              description:
+                'Contains a list of product-configuration combinations.',
+              type: 'array',
+              minItems: 1,
+              uniqueItems: true,
+              items: { $ref: '#/$defs/relationship_t' },
+            },
+          },
+        },
+      },
+      unevaluatedProperties: false,
+    },
+    critical: {
+      title: 'Critical',
+      description:
+        'Determines whether using the extension would fail a mandatory test.',
+      type: 'boolean',
+      const: true,
+    },
+  },
+  additionalProperties: false,
+}
diff --git a/csaf_2_1/csafAjv/extensionSchemas/index.js b/csaf_2_1/csafAjv/extensionSchemas/index.js
new file mode 100644
index 00000000..e28cf5e8
--- /dev/null
+++ b/csaf_2_1/csafAjv/extensionSchemas/index.js
@@ -0,0 +1,78 @@
+import documentation11 from './documentation-11.js'
+import documentation12 from './documentation-12.js'
+import documentation13 from './documentation-13.js'
+
+/**
+ * @typedef {object} ExtensionSchemaEntry
+ * @property {import('ajv').AnySchemaObject} schema
+ * @property {'official' | 'registered' | 'experimental'} class Class per
+ *   spec section 2.4.1 ("Classes").
+ * @property {boolean} deprecated On the OASIS deprecated list (spec 2.4.2)?
+ * @property {boolean} denyListed On the OASIS deny list (spec 2.4.2)? MUST
+ *   NOT be used if true.
+ */
+
+/**
+ * Allow list of CSAF Extension Schemas this validator implements locally
+ * (per spec ch. 8, schemas are never fetched over the network).
+ *
+ * The OASIS official/registered/deprecated/deny-listed lists (spec 2.4.2)
+ * aren't available as data here, so entries are limited to the
+ * documentation-11/12/13 test fixtures (not for production use).
+ * `documentation-11`'s class is fixed by the spec's own prose (recommended
+ * tests 6.2.54.1/.2/.4): it's the failing example for "neither official nor
+ * registered", i.e. `experimental`. `documentation-12`/`-13` have no such
+ * fixture tie-in and are arbitrarily assigned `registered`/`official` so all
+ * three code paths of recommendedTest_6_2_54_1 are exercised.
+ *
+ * To add a new schema: add a module next to this file (its `$id` must match
+ * the `$schema` value used in documents), import it here, and add an entry.
+ *
+ * @type {ExtensionSchemaEntry[]}
+ */
+export const extensionSchemas = [
+  {
+    schema: documentation11,
+    class: 'experimental',
+    deprecated: false,
+    denyListed: false,
+  },
+  {
+    schema: documentation12,
+    class: 'registered',
+    deprecated: false,
+    denyListed: false,
+  },
+  {
+    schema: documentation13,
+    class: 'official',
+    deprecated: false,
+    denyListed: false,
+  },
+]
+
+/**
+ * Registers all allow-listed extension schemas on the given Ajv instance so
+ * that `ajv.getSchema(schemaUrl)` resolves them without any network access.
+ *
+ * @param {import('ajv').default} ajv
+ */
+export function registerExtensionSchemas(ajv) {
+  for (const { schema } of extensionSchemas) {
+    ajv.addSchema(schema, schema.$id)
+  }
+}
+
+/**
+ * Classifies a CSAF Extension by its declared `$schema` URL. Returns the
+ * class recorded in `extensionSchemas` if allow-listed, otherwise
+ * `'unknown'` (the real OASIS lists aren't available as data here, so
+ * guessing from the URL would be misleading).
+ *
+ * @param {string} schemaUrl
+ * @returns {'official' | 'registered' | 'experimental' | 'unknown'}
+ */
+export function classifyExtensionSchema(schemaUrl) {
+  const entry = extensionSchemas.find((e) => e.schema.$id === schemaUrl)
+  return entry?.class ?? 'unknown'
+}
diff --git a/csaf_2_1/recommendedTests.js b/csaf_2_1/recommendedTests.js
index 714e3612..57e8ef9d 100644
--- a/csaf_2_1/recommendedTests.js
+++ b/csaf_2_1/recommendedTests.js
@@ -50,4 +50,5 @@ export { recommendedTest_6_2_48 } from './recommendedTests/recommendedTest_6_2_4
 export { recommendedTest_6_2_49 } from './recommendedTests/recommendedTest_6_2_49.js'
 export { recommendedTest_6_2_52 } from './recommendedTests/recommendedTest_6_2_52.js'
 export { recommendedTest_6_2_53 } from './recommendedTests/recommendedTest_6_2_53.js'
+export { recommendedTest_6_2_54_2 } from './recommendedTests/recommendedTest_6_2_54_2.js'
 export { recommendedTest_6_2_54_3 } from './recommendedTests/recommendedTest_6_2_54_3.js'
diff --git a/csaf_2_1/recommendedTests/recommendedTest_6_2_54_2.js b/csaf_2_1/recommendedTests/recommendedTest_6_2_54_2.js
new file mode 100644
index 00000000..36bb17e7
--- /dev/null
+++ b/csaf_2_1/recommendedTests/recommendedTest_6_2_54_2.js
@@ -0,0 +1,44 @@
+import { walkPath } from '../../lib/walkPaths.js'
+import { classifyExtensionSchema } from '#csaf_2_1/csafAjv/extensionSchemas/index.js'
+
+const X_EXTENSIONS_PATHS /** @type {string[]} */ = [
+  '/document/x_extensions[]',
+  '/product_tree/branches[*]/product/x_extensions[]',
+  '/product_tree/full_product_names[]/x_extensions[]',
+  '/product_tree/product_paths[]/full_product_name/x_extensions[]',
+  '/vulnerabilities[]/metrics[]/content/x_extensions[]',
+  '/vulnerabilities[]/x_extensions[]',
+  '/x_extensions[]',
+]
+
+/**
+ * This implements the recommended test 6.2.54.2 of the CSAF 2.1 standard.
+ *
+ * @param {unknown} doc
+ */
+export async function recommendedTest_6_2_54_2(doc) {
+  const ctx = {
+    warnings:
+      /** @type {Array<{ instancePath: string; message: string }>} */ ([]),
+  }
+
+  for (const path of X_EXTENSIONS_PATHS) {
+    await walkPath(doc, path, async (instancePath, value) => {
+      const schemaUrl =
+        value && typeof value === 'object' && '$schema' in value
+          ? /** @type {{ $schema: String }} */ (value).$schema
+          : undefined
+
+      if (typeof schemaUrl !== 'string') return
+
+      if (classifyExtensionSchema(schemaUrl) !== 'official') {
+        ctx.warnings.push({
+          instancePath: `${instancePath}`,
+          message: 'the extension is not an official CSAF Extension',
+        })
+      }
+    })
+  }
+
+  return ctx
+}
diff --git a/tests/csaf_2_1/oasis.js b/tests/csaf_2_1/oasis.js
index 3cd9fcaf..bf413c14 100644
--- a/tests/csaf_2_1/oasis.js
+++ b/tests/csaf_2_1/oasis.js
@@ -33,7 +33,6 @@ const excluded = [
   '6.2.50.3',
   '6.2.51',
   '6.2.54.1',
-  '6.2.54.2',
   '6.2.54.4',
   '6.3.12',
   '6.3.13',
diff --git a/tests/csaf_2_1/recommendedTest_6_2_54_2.js b/tests/csaf_2_1/recommendedTest_6_2_54_2.js
new file mode 100644
index 00000000..4709e21e
--- /dev/null
+++ b/tests/csaf_2_1/recommendedTest_6_2_54_2.js
@@ -0,0 +1,45 @@
+import { recommendedTest_6_2_54_2 } from '../../csaf_2_1/recommendedTests.js'
+import documentation11 from '../../csaf_2_1/csafAjv/extensionSchemas/documentation-11.js'
+import documentation13 from '../../csaf_2_1/csafAjv/extensionSchemas/documentation-13.js'
+
+describe('recommendedTest_6_2_54_2', function () {
+  it('only runs on relevant documents', async function () {
+    const result = await recommendedTest_6_2_54_2({})
+    expect(result.warnings.length).to.equal(0)
+  })
+
+  it('skips extensions that have no $schema property', async function () {
+    const result = await recommendedTest_6_2_54_2({
+      x_extensions: [{ category: 'supplementary', content: {} }],
+    })
+    expect(result.warnings.length).to.equal(0)
+  })
+
+  it('warn when the extension schema is not classified as "official"', async function () {
+    const result = await recommendedTest_6_2_54_2({
+      x_extensions: [
+        {
+          $schema: documentation11.$id,
+          category: 'supplementary',
+        },
+      ],
+    })
+    expect(result.warnings.length).to.equal(1)
+    expect(result.warnings[0].instancePath).to.equal('/x_extensions/0')
+    expect(result.warnings[0].message).to.equal(
+      'the extension is not an official CSAF Extension'
+    )
+  })
+
+  it('does not warn when the extension schema is classified as "official"', async function () {
+    const result = await recommendedTest_6_2_54_2({
+      x_extensions: [
+        {
+          $schema: documentation13.$id,
+          category: 'supplementary',
+        },
+      ],
+    })
+    expect(result.warnings.length).to.equal(0)
+  })
+})