diff --git a/botocore/useragent.py b/botocore/useragent.py
index 8cfc731ee6..f837fc8699 100644
--- a/botocore/useragent.py
+++ b/botocore/useragent.py
@@ -116,6 +116,16 @@ def to_string(self):
         return self._value
 
 
+# This is not a public interface and is subject to abrupt breaking changes.
+# Any usage is not advised or supported in external code bases.
+try:
+    from botocore.customizations.useragent import modify_components
+except ImportError:
+    # Default implementation that returns unmodified User-Agent components.
+    def modify_components(components):
+        return components
+
+
 class UserAgentString:
     """
     Generator for AWS SDK User-Agent header strings.
@@ -271,6 +281,9 @@ def to_string(self):
             *self._build_app_id(),
             *self._build_extra(),
         ]
+
+        components = modify_components(components)
+
         return ' '.join([comp.to_string() for comp in components])
 
     def _build_sdk_metadata(self):
diff --git a/tests/unit/test_useragent.py b/tests/unit/test_useragent.py
index 640a2f7469..a649dd9350 100644
--- a/tests/unit/test_useragent.py
+++ b/tests/unit/test_useragent.py
@@ -14,16 +14,23 @@
 
 import pytest
 
+import botocore.useragent
 from botocore import __version__ as botocore_version
 from botocore.config import Config
 from botocore.useragent import (
     UserAgentString,
     sanitize_user_agent_string_component,
 )
+from tests import mock
 
 from .. import requires_crt
 
 
+# Returns a list of unmodified User-Agent components.
+def unmodified_components(components):
+    return components
+
+
 @pytest.mark.parametrize(
     'raw_str, allow_hash, expected_str',
     [
@@ -52,6 +59,9 @@ def test_sanitize_ua_string_component(raw_str, allow_hash, expected_str):
     assert actual_str == expected_str
 
 
+@mock.patch.object(
+    botocore.useragent, 'modify_components', unmodified_components
+)
 def test_basic_user_agent_string():
     ua = UserAgentString(
         platform_name='linux',
@@ -113,6 +123,9 @@ def test_shared_test_case():
     assert indices == list(sorted(indices)), 'Elements were found out of order'
 
 
+@mock.patch.object(
+    botocore.useragent, 'modify_components', unmodified_components
+)
 def test_user_agent_string_with_missing_information():
     # Even when collecting information from the environment fails completely,
     # some minimal string should be generated.