diff --git a/Dockerfile b/Dockerfile
index 87155d2f6..1ff333b0a 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -39,6 +39,7 @@ COPY ./pybind_interface/ /qsim/lib/
 COPY ./qsimcirq_tests/ /qsim/qsimcirq_tests/
 COPY ./requirements.txt /qsim/requirements.txt
 COPY ./pyproject.toml /qsim/pyproject.toml
+COPY ./setup.py /qsim/setup.py
 
 WORKDIR /qsim/
 
diff --git a/qsimcirq_tests/setuppy_test.py b/qsimcirq_tests/setuppy_test.py
new file mode 100644
index 000000000..3a49e42c3
--- /dev/null
+++ b/qsimcirq_tests/setuppy_test.py
@@ -0,0 +1,78 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Test setup.py code."""
+
+import os
+import subprocess
+import sys
+
+
+def run_setup(cmake_args, root_dir):
+    env = os.environ.copy()
+    env["CMAKE_ARGS"] = cmake_args
+
+    try:
+        # Using sys.executable to run setup.py with the same Python executable.
+        return subprocess.run(
+            [sys.executable, "setup.py", "build_ext"],
+            env=env,
+            capture_output=True,
+            text=True,
+            cwd=str(root_dir),
+        )
+    except FileNotFoundError as e:
+        # This Dummy class emulates what subprocess.run() returns.
+        class Dummy:
+            stderr = f"Command or file not found: {e}"
+            returncode = 1
+
+        return Dummy()
+    except PermissionError as e:
+
+        class Dummy:
+            stderr = f"Permission denied: {e}"
+            returncode = 1
+
+        return Dummy()
+    except OSError as e:
+
+        class Dummy:
+            stderr = f"OS error occurred: {e}"
+            returncode = 1
+
+        return Dummy()
+    except Exception as e:
+        # Fallback for unexpected errors in subprocess.run itself
+        class Dummy:
+            stderr = f"Unexpected error: {str(e)}"
+            returncode = 1
+
+        return Dummy()
+
+
+def test_valid_cmake_args(pytestconfig):
+    res = run_setup("-DCMAKE_CXX_STANDARD=17", pytestconfig.rootpath)
+    # If it fails, it shouldn't be because of our validation.
+    assert "arguments must begin with a dash" not in res.stderr
+
+
+def test_invalid_cmake_args_no_dash(pytestconfig):
+    res = run_setup("NOT_A_FLAG", pytestconfig.rootpath)
+    assert "arguments must begin with a dash" in res.stderr
+
+
+def test_invalid_cmake_args_malicious(pytestconfig):
+    res = run_setup("-DVAR=VAL ; rm -rf /", pytestconfig.rootpath)
+    assert "arguments must begin with a dash" in res.stderr
diff --git a/setup.py b/setup.py
index b9a9b7b36..34e51fc23 100644
--- a/setup.py
+++ b/setup.py
@@ -85,8 +85,13 @@ def build_extension(self, ext):
         # Append additional CMake arguments from the environment variable.
         # This is e.g. used by cibuildwheel to force a certain C++ standard.
         additional_cmake_args = os.environ.get("CMAKE_ARGS", "")
-        if additional_cmake_args:
-            cmake_args += additional_cmake_args.split()
+        for arg in additional_cmake_args.split():
+            if not arg.startswith("-D"):
+                raise RuntimeError(
+                    f"The value '{arg}' in the environment variable CMAKE_ARGS "
+                    "is invalid; only definition arguments starting with '-D' are allowed."
+                )
+            cmake_args.append(arg)
 
         cfg = "Debug" if self.debug else "Release"
         build_args = ["--config", cfg]