diff --git a/components/mbedtls/Kconfig b/components/mbedtls/Kconfig
index 069b15c4672..bb132145484 100644
--- a/components/mbedtls/Kconfig
+++ b/components/mbedtls/Kconfig
@@ -516,6 +516,11 @@ menu "mbedTLS"
                     Name of the custom certificate directory or file. This path is evaluated
                     relative to the project root directory.
 
+                    Certificates must be either PEM encoded with a .pem extension, or DER
+                    encoded with a .der extension. Files with any other extension are not
+                    parsed: pointing this option directly at such a file fails the build,
+                    while such files inside a directory are skipped with a warning.
+
             config MBEDTLS_CERTIFICATE_BUNDLE_DEPRECATED_LIST
                 bool "Add deprecated root certificates"
                 depends on MBEDTLS_CERTIFICATE_BUNDLE && !MBEDTLS_CERTIFICATE_BUNDLE_DEFAULT_NONE
diff --git a/components/mbedtls/esp_crt_bundle/gen_crt_bundle.py b/components/mbedtls/esp_crt_bundle/gen_crt_bundle.py
index 62727057a57..c5e737ceaf2 100755
--- a/components/mbedtls/esp_crt_bundle/gen_crt_bundle.py
+++ b/components/mbedtls/esp_crt_bundle/gen_crt_bundle.py
@@ -46,6 +46,10 @@ except ImportError:
 
 ca_bundle_bin_file = 'x509_crt_bundle'
 
+# Only files with these extensions are parsed, the extension has to match the encoding of the file
+PEM_FILE_EXTENSION = '.pem'
+DER_FILE_EXTENSION = '.der'
+
 quiet = False
 
 
@@ -70,27 +74,44 @@ class CertificateBundle:
     def add_from_path(self, crts_path):
         found = False
         for file_path in os.listdir(crts_path):
-            found |= self.add_from_file(os.path.join(crts_path, file_path))
+            found |= self.add_from_file(os.path.join(crts_path, file_path), strict=False)
 
         if found is False:
             raise InputError('No valid x509 certificates found in %s' % crts_path)
 
-    def add_from_file(self, file_path):
+    def add_from_file(self, file_path, strict=True):
+        """Parse a certificate file
+
+        Only files with a .pem or .der extension are parsed. If strict is True (the file was
+        given directly on the command line), any other extension is an error, otherwise
+        (while scanning a directory) the file is skipped with a warning.
+        """
         try:
-            if file_path.endswith('.pem'):
+            if file_path.endswith(PEM_FILE_EXTENSION):
                 status('Parsing certificates from %s' % file_path)
                 with open(file_path, 'r', encoding='utf-8') as f:
                     crt_str = f.read()
                     self.add_from_pem(crt_str)
                     return True
 
-            elif file_path.endswith('.der'):
+            elif file_path.endswith(DER_FILE_EXTENSION):
                 status('Parsing certificates from %s' % file_path)
                 with open(file_path, 'rb') as f:
                     crt_str = f.read()
                     self.add_from_der(crt_str)
                     return True
 
+            elif os.path.isfile(file_path):
+                msg = (
+                    'Unsupported file extension in %s, certificates are only parsed from files with a '
+                    '%s (PEM encoded) or %s (DER encoded) extension. '
+                    'Please rename the file to match its encoding.'
+                    % (file_path, PEM_FILE_EXTENSION, DER_FILE_EXTENSION)
+                )
+                if strict:
+                    raise InputError(msg)
+                critical('Warning: skipping file. %s' % msg)
+
         except ValueError:
             critical('Invalid certificate in %s' % file_path)
             raise InputError('Invalid certificate')
diff --git a/components/mbedtls/esp_crt_bundle/test_gen_crt_bundle/test_gen_crt_bundle.py b/components/mbedtls/esp_crt_bundle/test_gen_crt_bundle/test_gen_crt_bundle.py
index 759aa656ca2..7cf9faa9402 100755
--- a/components/mbedtls/esp_crt_bundle/test_gen_crt_bundle/test_gen_crt_bundle.py
+++ b/components/mbedtls/esp_crt_bundle/test_gen_crt_bundle/test_gen_crt_bundle.py
@@ -1,6 +1,8 @@
 #!/usr/bin/env python
 import os
+import shutil
 import sys
+import tempfile
 import unittest
 
 try:
@@ -22,6 +24,7 @@ verified_der_bundle = 'baltimore_crt_bundle'
 verified_pem_bundle = 'entrust_crt_bundle'
 invalid_test_file   = 'invalid_crt.pem'
 non_ascii_file      = 'non_ascii_crt.pem'
+unsupported_file    = 'unsupported_ext_crt.crt'
 ca_crts_all_file    = 'cacrt_all.pem'
 
 
@@ -73,6 +76,37 @@ class GenCrtBundleTests(Py23TestCase):
         bundle.add_from_pem('')
         self.assertEqual(len(bundle.certificates), bundle_prev_len)
 
+    # A file given directly is expected to be a certificate, so an unknown extension is an error
+    def test_unsupported_extension_input(self):
+        bundle = gen_crt_bundle.CertificateBundle()
+
+        with self.assertRaisesRegex(gen_crt_bundle.InputError, 'Unsupported file extension'):
+            bundle.add_from_file(test_crts_path + unsupported_file)
+
+        self.assertEqual(len(bundle.certificates), 0)
+
+    # While scanning a directory, files with an unknown extension are skipped instead
+    def test_unsupported_extension_in_dir(self):
+        bundle = gen_crt_bundle.CertificateBundle()
+
+        self.assertFalse(bundle.add_from_file(test_crts_path + unsupported_file, strict=False))
+        self.assertEqual(len(bundle.certificates), 0)
+
+        # A directory holding both valid and unsupported files adds only the valid ones
+        with tempfile.TemporaryDirectory() as crts_dir:
+            shutil.copy(test_crts_path + unsupported_file, crts_dir)
+            shutil.copy(test_crts_path + pem_test_file, crts_dir)
+
+            bundle.add_from_path(crts_dir)
+            self.assertEqual(len(bundle.certificates), 1)
+
+        # A directory without any supported file is an error
+        with tempfile.TemporaryDirectory() as crts_dir:
+            shutil.copy(test_crts_path + unsupported_file, crts_dir)
+
+            with self.assertRaisesRegex(gen_crt_bundle.InputError, 'No valid x509 certificates found'):
+                bundle.add_from_path(crts_dir)
+
     def test_non_ascii_crt_input(self):
         bundle = gen_crt_bundle.CertificateBundle()
 
diff --git a/components/mbedtls/esp_crt_bundle/test_gen_crt_bundle/unsupported_ext_crt.crt b/components/mbedtls/esp_crt_bundle/test_gen_crt_bundle/unsupported_ext_crt.crt
new file mode 100644
index 00000000000..82e3bc0bf11
--- /dev/null
+++ b/components/mbedtls/esp_crt_bundle/test_gen_crt_bundle/unsupported_ext_crt.crt
@@ -0,0 +1,25 @@
+Entrust Root Certification Authority
+====================================
+-----BEGIN CERTIFICATE-----
+MIIEkTCCA3mgAwIBAgIERWtQVDANBgkqhkiG9w0BAQUFADCBsDELMAkGA1UEBhMCVVMxFjAUBgNV
+BAoTDUVudHJ1c3QsIEluYy4xOTA3BgNVBAsTMHd3dy5lbnRydXN0Lm5ldC9DUFMgaXMgaW5jb3Jw
+b3JhdGVkIGJ5IHJlZmVyZW5jZTEfMB0GA1UECxMWKGMpIDIwMDYgRW50cnVzdCwgSW5jLjEtMCsG
+A1UEAxMkRW50cnVzdCBSb290IENlcnRpZmljYXRpb24gQXV0aG9yaXR5MB4XDTA2MTEyNzIwMjM0
+MloXDTI2MTEyNzIwNTM0MlowgbAxCzAJBgNVBAYTAlVTMRYwFAYDVQQKEw1FbnRydXN0LCBJbmMu
+MTkwNwYDVQQLEzB3d3cuZW50cnVzdC5uZXQvQ1BTIGlzIGluY29ycG9yYXRlZCBieSByZWZlcmVu
+Y2UxHzAdBgNVBAsTFihjKSAyMDA2IEVudHJ1c3QsIEluYy4xLTArBgNVBAMTJEVudHJ1c3QgUm9v
+dCBDZXJ0aWZpY2F0aW9uIEF1dGhvcml0eTCCASIwDQYJKoZIhvcNAQEBBQADggEPADCCAQoCggEB
+ALaVtkNC+sZtKm9I35RMOVcF7sN5EUFoNu3s/poBj6E4KPz3EEZmLk0eGrEaTsbRwJWIsMn/MYsz
+A9u3g3s+IIRe7bJWKKf44LlAcTfFy0cOlypowCKVYhXbR9n10Cv/gkvJrT7eTNuQgFA/CYqEAOww
+Cj0Yzfv9KlmaI5UXLEWeH25DeW0MXJj+SKfFI0dcXv1u5x609mhF0YaDW6KKjbHjKYD+JXGIrb68
+j6xSlkuqUY3kEzEZ6E5Nn9uss2rVvDlUccp6en+Q3X0dgNmBu1kmwhH+5pPi94DkZfs0Nw4pgHBN
+rziGLp5/V6+eF67rHMsoIV+2HNjnogQi+dPa2MsCAwEAAaOBsDCBrTAOBgNVHQ8BAf8EBAMCAQYw
+DwYDVR0TAQH/BAUwAwEB/zArBgNVHRAEJDAigA8yMDA2MTEyNzIwMjM0MlqBDzIwMjYxMTI3MjA1
+MzQyWjAfBgNVHSMEGDAWgBRokORnpKZTgMeGZqTx90tD+4S9bTAdBgNVHQ4EFgQUaJDkZ6SmU4DH
+hmak8fdLQ/uEvW0wHQYJKoZIhvZ9B0EABBAwDhsIVjcuMTo0LjADAgSQMA0GCSqGSIb3DQEBBQUA
+A4IBAQCT1DCw1wMgKtD5Y+iRDAUgqV8ZyntyTtSx29CW+1RaGSwMCPeyvIWonX9tO1KzKtvn1ISM
+Y/YPyyYBkVBs9F8U4pN0wBOeMDpQ47RgxRzwIkSNcUesyBrJ6ZuaAGAT/3B+XxFNSRuzFVJ7yVTa
+v52Vr2ua2J7p8eRDjeIRRDq/r72DQnNSi6q7pynP9WQcCk3RvKqsnyrQ/39/2n3qse0wJcGE2jTS
+W3iDVuycNsMm4hH2Z0kdkquM++v/eu6FSqdQgPCnXEqULl8FmTxSQeDNtGPPAUO6nIPcj2A781q0
+tHuu2guQOHXvgR1m0vdXcDazv/wor3ElhVsT/h5/WrQ8
+-----END CERTIFICATE-----
diff --git a/docs/en/api-reference/protocols/esp_crt_bundle.rst b/docs/en/api-reference/protocols/esp_crt_bundle.rst
index ec10d5cb055..87f58700491 100644
--- a/docs/en/api-reference/protocols/esp_crt_bundle.rst
+++ b/docs/en/api-reference/protocols/esp_crt_bundle.rst
@@ -30,6 +30,10 @@ Most configuration is done through menuconfig. CMake generates the bundle accord
  * :menuitem:`CONFIG_MBEDTLS_DEFAULT_CERTIFICATE_BUNDLE`: decide which certificates to include from the complete root certificate list.
  * :menuitem:`CONFIG_MBEDTLS_CUSTOM_CERTIFICATE_BUNDLE_PATH`: specify the path of any additional certificates to embed in the bundle.
 
+.. note::
+
+    Only PEM encoded certificates with a ``.pem`` extension and DER encoded certificates with a ``.der`` extension are parsed. The extension must match the encoding of the file, for example a PEM encoded certificate saved as ``.crt`` is not accepted. If :ref:`CONFIG_MBEDTLS_CUSTOM_CERTIFICATE_BUNDLE_PATH` points directly at a file with any other extension, the build fails; if such a file is found inside a certificate directory, it is skipped and a warning is printed.
+
 To enable the bundle when using ESP-TLS simply pass the function pointer to the bundle attach function:
 
 .. code-block:: c
diff --git a/docs/zh_CN/api-reference/protocols/esp_crt_bundle.rst b/docs/zh_CN/api-reference/protocols/esp_crt_bundle.rst
index 4e16018de2b..c183c8e2960 100644
--- a/docs/zh_CN/api-reference/protocols/esp_crt_bundle.rst
+++ b/docs/zh_CN/api-reference/protocols/esp_crt_bundle.rst
@@ -30,6 +30,10 @@ ESP x509 证书包 API 提供了一种简便的方法，帮助你安装自定义
  * :menuitem:`CONFIG_MBEDTLS_DEFAULT_CERTIFICATE_BUNDLE`：决定添加证书列表中的哪些证书。
  * :menuitem:`CONFIG_MBEDTLS_CUSTOM_CERTIFICATE_BUNDLE_PATH`：指定要在证书包中嵌入的其他证书的路径。
 
+.. note::
+
+    只有扩展名为 ``.pem`` 的 PEM 编码证书和扩展名为 ``.der`` 的 DER 编码证书会被解析。扩展名必须与文件的编码格式一致，例如保存为 ``.crt`` 的 PEM 编码证书不会被接受。如果 :ref:`CONFIG_MBEDTLS_CUSTOM_CERTIFICATE_BUNDLE_PATH` 直接指向其他扩展名的文件，构建将失败；如果证书目录中存在此类文件，该文件会被跳过并打印警告。
+
 要在使用 ESP-TLS 时启用证书包，将函数指针指向证书包的 attach 函数：
 
 .. code-block:: c