diff --git a/rosdoc2/verbs/build/builders/sphinx_builder.py b/rosdoc2/verbs/build/builders/sphinx_builder.py
index b6282ea8..e72d6b94 100644
--- a/rosdoc2/verbs/build/builders/sphinx_builder.py
+++ b/rosdoc2/verbs/build/builders/sphinx_builder.py
@@ -65,6 +65,42 @@ def ensure_global(name, default):
 ensure_global('release', "{package.version}")
 ensure_global('version', "{package_version_short}")
 
+# Remove any unsupported extensions
+allowed_extensions = set((
+    # Shipped with sphinx
+    'sphinx.ext.autodoc',
+    'sphinx.ext.autosectionlabel',
+    'sphinx.ext.autosummary',
+    'sphinx.ext.coverage',
+    'sphinx.ext.doctest',
+    'sphinx.ext.duration',
+    'sphinx.ext.extlinks',
+    'sphinx.ext.githubpages',
+    'sphinx.ext.graphviz',
+    'sphinx.ext.ifconfig',
+    'sphinx.ext.imgconverter',
+    'sphinx.ext.inheritance_diagram',
+    'sphinx.ext.intersphinx',
+    'sphinx.ext.linkcode',
+    'sphinx.ext.napoleon',
+    'sphinx.ext.todo',
+    'sphinx.ext.viewcode',
+    # Sphinx-included math extensions
+    'sphinx.ext.imgmath',
+    'sphinx.ext.mathjax',
+    # Installed by us
+    'breathe',
+    'exhale',
+    'myst_parser',
+    'sphinx_rtd_theme',
+))
+for extension in extensions[:]:
+    if extension not in allowed_extensions:
+        print(f'[rosdoc2] *** Warning *** removing extension "{{extension}}", not supported')
+        extensions.remove(extension)
+if extensions:
+    print(f'[rosdoc2] user conf.py specified allowed extensions: {{extensions}}')
+
 if rosdoc2_settings.get('enable_autodoc', True):
     print('[rosdoc2] enabling autodoc', file=sys.stderr)
     extensions.append('sphinx.ext.autodoc')
diff --git a/test/packages/basic_cpp/doc/conf.py b/test/packages/basic_cpp/doc/conf.py
index a6c7e47b..d5b595b9 100644
--- a/test/packages/basic_cpp/doc/conf.py
+++ b/test/packages/basic_cpp/doc/conf.py
@@ -41,6 +41,29 @@
 ## See the rosdoc2_settings below for some options on avoiding that.
 extensions = [
     'sphinx_rtd_theme',
+    'i_do_not_exist',
+    # Shipped with sphinx
+    'sphinx.ext.autodoc',
+    'sphinx.ext.autosectionlabel',
+    'sphinx.ext.autosummary',
+    'sphinx.ext.coverage',
+    'sphinx.ext.doctest',
+    'sphinx.ext.duration',
+    'sphinx.ext.extlinks',
+    'sphinx.ext.githubpages',
+    'sphinx.ext.graphviz',
+    'sphinx.ext.ifconfig',
+    'sphinx.ext.imgconverter',
+    'sphinx.ext.inheritance_diagram',
+    'sphinx.ext.intersphinx',
+    'sphinx.ext.linkcode',
+    'sphinx.ext.napoleon',
+    'sphinx.ext.todo',
+    'sphinx.ext.viewcode',
+    # Sphinx-included math extensions
+    'sphinx.ext.imgmath',
+    'sphinx.ext.mathjax',
+
 ]
 
 # Add any paths that contain templates here, relative to this directory.
@@ -126,3 +149,8 @@
     ## Support markdown
     # 'support_markdown': True,
 }
+
+## This function is required by the linkcode extension.
+## See https://www.sphinx-doc.org/en/master/usage/extensions/linkcode.html
+def linkcode_resolve(domain, info):
+    return None