diff --git a/3rdparty/CMakeLists.txt b/3rdparty/CMakeLists.txt
index 14a4674..fa149bd 100644
--- a/3rdparty/CMakeLists.txt
+++ b/3rdparty/CMakeLists.txt
@@ -1,10 +1,2 @@
-execute_process(COMMAND ${MBEDTLS_PYTHON_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/../scripts/config.py -f ${CMAKE_CURRENT_SOURCE_DIR}/../include/mbedtls/mbedtls_config.h get MBEDTLS_ECDH_VARIANT_EVEREST_ENABLED RESULT_VARIABLE everest_result)
-execute_process(COMMAND ${MBEDTLS_PYTHON_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/../scripts/config.py -f ${CMAKE_CURRENT_SOURCE_DIR}/../include/mbedtls/mbedtls_config.h get MBEDTLS_PSA_P256M_DRIVER_ENABLED RESULT_VARIABLE p256m_result)
-
-if(${everest_result} EQUAL 0)
-    add_subdirectory(everest)
-endif()
-
-if(${p256m_result} EQUAL 0)
-    add_subdirectory(p256-m)
-endif()
+add_subdirectory(everest)
+add_subdirectory(p256-m)
diff --git a/3rdparty/everest/CMakeLists.txt b/3rdparty/everest/CMakeLists.txt
index 4ad367e..ee86f1d 100644
--- a/3rdparty/everest/CMakeLists.txt
+++ b/3rdparty/everest/CMakeLists.txt
@@ -11,6 +11,19 @@
           include/everest/kremlib
           ${MBEDTLS_DIR}/library/)
 
+# Pass-through MBEDTLS_CONFIG_FILE and MBEDTLS_USER_CONFIG_FILE
+# This must be duplicated from library/CMakeLists.txt because
+# everest is not directly linked against any mbedtls targets
+# so does not inherit the compile definitions.
+if(MBEDTLS_CONFIG_FILE)
+    target_compile_definitions(everest
+        PUBLIC MBEDTLS_CONFIG_FILE="${MBEDTLS_CONFIG_FILE}")
+endif()
+if(MBEDTLS_USER_CONFIG_FILE)
+    target_compile_definitions(everest
+        PUBLIC MBEDTLS_USER_CONFIG_FILE="${MBEDTLS_USER_CONFIG_FILE}")
+endif()
+
 if(INSTALL_MBEDTLS_HEADERS)
 
   install(DIRECTORY include/everest
diff --git a/3rdparty/p256-m/CMakeLists.txt b/3rdparty/p256-m/CMakeLists.txt
index 0001dd2..7ff1308 100644
--- a/3rdparty/p256-m/CMakeLists.txt
+++ b/3rdparty/p256-m/CMakeLists.txt
@@ -9,6 +9,19 @@
          $<INSTALL_INTERFACE:include>
   PRIVATE ${MBEDTLS_DIR}/library/)
 
+# Pass-through MBEDTLS_CONFIG_FILE and MBEDTLS_USER_CONFIG_FILE
+# This must be duplicated from library/CMakeLists.txt because
+# p256m is not directly linked against any mbedtls targets
+# so does not inherit the compile definitions.
+if(MBEDTLS_CONFIG_FILE)
+    target_compile_definitions(p256m
+        PUBLIC MBEDTLS_CONFIG_FILE="${MBEDTLS_CONFIG_FILE}")
+endif()
+if(MBEDTLS_USER_CONFIG_FILE)
+    target_compile_definitions(p256m
+        PUBLIC MBEDTLS_USER_CONFIG_FILE="${MBEDTLS_USER_CONFIG_FILE}")
+endif()
+
 if(INSTALL_MBEDTLS_HEADERS)
 
   install(DIRECTORY :${CMAKE_CURRENT_SOURCE_DIR}
diff --git a/ChangeLog.d/fix-cmake-3rdparty-custom-config.txt b/ChangeLog.d/fix-cmake-3rdparty-custom-config.txt
new file mode 100644
index 0000000..ec543aa
--- /dev/null
+++ b/ChangeLog.d/fix-cmake-3rdparty-custom-config.txt
@@ -0,0 +1,3 @@
+Bugfix
+   * Fix the build with CMake when Everest or P256-m is enabled through
+     a user configuration file or the compiler command line. Fixes #8165.