diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -67,6 +67,18 @@
    By default, this is turned off. Turning it on results in a different ABI (additional
    symbols but also potentially different layouts of types), and one should not mix code
    built against a dylib that has debug mode and code built against a regular dylib." OFF)
+option(LIBCXX_ENABLE_HARDENED_MODE
+  "Enable the hardened mode inside the compiled library and make it the default
+   when compiling user code. Note that users can override this setting in their
+   own code. Setting this to 'ON' implicitly enables assertions, unless
+   `LIBCXX_ENABLE_ASSERTIONS` has been explicitly set to 'OFF'. This does not
+   affect the ABI." OFF)
+option(LIBCXX_ENABLE_HARDENED_DEBUG_MODE
+  "Enable the debug mode inside the compiled library and make it the default
+   when compiling user code. Note that users can override this setting in their
+   own code. Setting this to 'ON' implicitly enables assertions, unless
+   `LIBCXX_ENABLE_ASSERTIONS` has been explicitly set to 'OFF'. This does not
+   affect the ABI." OFF)
 option(LIBCXX_ENABLE_RANDOM_DEVICE
   "Whether to include support for std::random_device in the library. Disabling
    this can be useful when building the library for platforms that don't have
@@ -797,6 +809,16 @@
 else()
   config_define(0 _LIBCPP_ENABLE_ASSERTIONS_DEFAULT)
 endif()
+if (LIBCXX_ENABLE_HARDENED_MODE)
+  config_define(1 _LIBCPP_ENABLE_HARDENED_MODE_DEFAULT)
+else()
+  config_define(0 _LIBCPP_ENABLE_HARDENED_MODE_DEFAULT)
+endif()
+if (LIBCXX_ENABLE_HARDENED_DEBUG_MODE)
+  config_define(1 _LIBCPP_ENABLE_HARDENED_DEBUG_MODE_DEFAULT)
+else()
+  config_define(0 _LIBCPP_ENABLE_HARDENED_DEBUG_MODE_DEFAULT)
+endif()
 
 if (LIBCXX_PSTL_CPU_BACKEND STREQUAL "serial")
   config_define(1 _LIBCPP_PSTL_CPU_BACKEND_SERIAL)
diff --git a/libcxx/cmake/caches/Generic-hardened-debug-mode.cmake b/libcxx/cmake/caches/Generic-hardened-debug-mode.cmake
new file mode 100644
--- /dev/null
+++ b/libcxx/cmake/caches/Generic-hardened-debug-mode.cmake
@@ -0,0 +1 @@
+set(LIBCXX_ENABLE_HARDENED_DEBUG_MODE ON CACHE BOOL "")
diff --git a/libcxx/cmake/caches/Generic-hardened-mode.cmake b/libcxx/cmake/caches/Generic-hardened-mode.cmake
new file mode 100644
--- /dev/null
+++ b/libcxx/cmake/caches/Generic-hardened-mode.cmake
@@ -0,0 +1 @@
+set(LIBCXX_ENABLE_HARDENED_MODE ON CACHE BOOL "")
diff --git a/libcxx/docs/DesignDocs/HardenedMode.rst b/libcxx/docs/DesignDocs/HardenedMode.rst
new file mode 100644
--- /dev/null
+++ b/libcxx/docs/DesignDocs/HardenedMode.rst
@@ -0,0 +1,37 @@
+==========
+Hardened Mode
+==========
+
+.. contents::
+   :local:
+
+.. _using-hardened-mode:
+
+Using the hardened mode
+====================
+
+The hardened mode enables a set of security-critical assertions that prevent
+undefined behavior caused by violating preconditions of the standard library.
+These assertions can be done with relatively little overhead in constant time
+and are intended to be used in production by security-conscious projects.
+
+In addition to the hardened mode, libc++ also provides the debug mode which
+contains all the checks from the hardened mode and additionally more expensive
+checks that may affect the complexity of algorithms. The debug mode is intended
+to be used for testing, not in production.
+
+Vendors can enable the hardened mode by building the library with the
+``LIBCXX_ENABLE_HARDENED_MODE`` option, and similarly enable the debug mode by
+building with the ``LIBCXX_ENABLE_HARDENED_DEBUG_MODE`` option.
+
+The hardened mode requires ``LIBCXX_ENABLE_ASSERTIONS`` to work. If
+``LIBCXX_ENABLE_ASSERTIONS`` was not set explicitly, enabling
+``LIBCXX_ENABLE_HARDENED_MODE`` will implicitly enable
+``LIBCXX_ENABLE_ASSERTIONS``. If ``LIBCXX_ENABLE_ASSERTIONS`` was explicitly
+disabled, this will effectively disable the hardened mode.
+
+Enabling the hardened mode or the debug mode has no impact on the ABI.
+
+Iterator bounds checking
+------------------------
+TODO(hardening)
diff --git a/libcxx/docs/ReleaseNotes.rst b/libcxx/docs/ReleaseNotes.rst
--- a/libcxx/docs/ReleaseNotes.rst
+++ b/libcxx/docs/ReleaseNotes.rst
@@ -71,6 +71,10 @@
   Anything that does not rely on having an actual filesystem available will now work, such as ``std::filesystem::path``,
   ``std::filesystem::perms`` and similar classes.
 
+- It is now possible to use CMake to enable the hardened mode by setting the ``LIBCXX_ENABLE_HARDENED_MODE`` variable,
+  and similarly to enable the debug mode using the ``LIBCXX_ENABLE_HARDENED_DEBUG_MODE`` variable (note that this is the
+  new debug mode distinct from the legacy debug mode that is being removed in this release).
+
 Deprecations and Removals
 -------------------------
 
diff --git a/libcxx/docs/index.rst b/libcxx/docs/index.rst
--- a/libcxx/docs/index.rst
+++ b/libcxx/docs/index.rst
@@ -185,6 +185,7 @@
    DesignDocs/ExtendedCXX03Support
    DesignDocs/FeatureTestMacros
    DesignDocs/FileTimeType
+   DesignDocs/HardenedMode
    DesignDocs/HeaderRemovalPolicy
    DesignDocs/NoexceptPolicy
    DesignDocs/PSTLIntegration
diff --git a/libcxx/include/__config b/libcxx/include/__config
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -191,6 +191,71 @@
 #    define _LIBCPP_ABI_BAD_FUNCTION_CALL_KEY_FUNCTION
 #  endif
 
+// HARDENING {
+
+// Enables the hardened mode which consists of all checks intended to be used in production. Hardened mode prioritizes
+// security-critical checks that can be done with relatively little overhead in constant time. Mutually exclusive with
+// `_LIBCPP_ENABLE_HARDENED_DEBUG_MODE`.
+//
+// #define _LIBCPP_ENABLE_HARDENED_MODE 1
+
+// Enables the debug mode which contains all the checks from the hardened mode and additionally more expensive checks
+// that may affect the complexity of algorithms. The debug mode is intended to be used for testing, not in production.
+// Mutually exclusive with `_LIBCPP_ENABLE_HARDENED_MODE`.
+//
+// #define _LIBCPP_ENABLE_HARDENED_DEBUG_MODE 1
+
+// Available checks:
+
+// TODO(hardening): add documentation for different checks here.
+
+#  ifndef _LIBCPP_ENABLE_HARDENED_MODE
+#    define _LIBCPP_ENABLE_HARDENED_MODE _LIBCPP_ENABLE_HARDENED_MODE_DEFAULT
+#  endif
+#  if _LIBCPP_ENABLE_HARDENED_MODE != 0 && _LIBCPP_ENABLE_HARDENED_MODE != 1
+#    error "_LIBCPP_ENABLE_HARDENED_MODE must be set to 0 or 1."
+#  endif
+
+#  ifndef _LIBCPP_ENABLE_HARDENED_DEBUG_MODE
+#    define _LIBCPP_ENABLE_HARDENED_DEBUG_MODE _LIBCPP_ENABLE_HARDENED_DEBUG_MODE_DEFAULT
+#  endif
+#  if _LIBCPP_ENABLE_HARDENED_DEBUG_MODE != 0 && _LIBCPP_ENABLE_HARDENED_DEBUG_MODE != 1
+#    error "_LIBCPP_ENABLE_HARDENED_DEBUG_MODE must be set to 0 or 1."
+#  endif
+
+#  if _LIBCPP_ENABLE_HARDENED_MODE && _LIBCPP_ENABLE_HARDENED_DEBUG_MODE
+#    error "Only one of _LIBCPP_ENABLE_HARDENED_MODE and _LIBCPP_ENABLE_HARDENED_DEBUG_MODE can be enabled."
+#  endif
+
+// Hardened mode checks.
+#  if _LIBCPP_ENABLE_HARDENED_MODE
+
+// Automatically enable assertions in hardened mode (unless the user explicitly turned them off).
+#    ifndef _LIBCPP_ENABLE_ASSERTIONS
+#      define _LIBCPP_ENABLE_ASSERTIONS 1
+#    endif
+
+// TODO(hardening): more checks to be added here...
+
+// Debug mode checks.
+#  elif _LIBCPP_ENABLE_HARDENED_DEBUG_MODE
+
+// Automatically enable assertions in debug mode (unless the user explicitly turned them off).
+#    ifndef _LIBCPP_ENABLE_ASSERTIONS
+#      define _LIBCPP_ENABLE_ASSERTIONS 1
+#    endif
+
+// TODO(hardening): more checks to be added here...
+
+// Disable all checks if neither the hardened mode nor the debug mode is enabled.
+#  else
+
+// TODO: more checks to be added here...
+
+#  endif // _LIBCPP_ENABLE_HARDENED_MODE
+
+// } HARDENING
+
 #  define _LIBCPP_TOSTRING2(x) #x
 #  define _LIBCPP_TOSTRING(x) _LIBCPP_TOSTRING2(x)
 
diff --git a/libcxx/include/__config_site.in b/libcxx/include/__config_site.in
--- a/libcxx/include/__config_site.in
+++ b/libcxx/include/__config_site.in
@@ -35,6 +35,10 @@
 #cmakedefine _LIBCPP_PSTL_CPU_BACKEND_SERIAL
 #cmakedefine _LIBCPP_PSTL_CPU_BACKEND_THREAD
 
+// Hardening.
+#cmakedefine01 _LIBCPP_ENABLE_HARDENED_MODE_DEFAULT
+#cmakedefine01 _LIBCPP_ENABLE_HARDENED_DEBUG_MODE_DEFAULT
+
 // __USE_MINGW_ANSI_STDIO gets redefined on MinGW
 #ifdef __clang__
 #  pragma clang diagnostic push
diff --git a/libcxx/utils/ci/buildkite-pipeline.yml b/libcxx/utils/ci/buildkite-pipeline.yml
--- a/libcxx/utils/ci/buildkite-pipeline.yml
+++ b/libcxx/utils/ci/buildkite-pipeline.yml
@@ -486,6 +486,58 @@
           limit: 2
     timeout_in_minutes: 120
 
+  - label: "Hardened mode"
+    command: "libcxx/utils/ci/run-buildbot generic-hardened-mode"
+    artifact_paths:
+      - "**/test-results.xml"
+      - "**/*.abilist"
+    env:
+        CC: "clang-${LLVM_HEAD_VERSION}"
+        CXX: "clang++-${LLVM_HEAD_VERSION}"
+    agents:
+      queue: "libcxx-builders"
+      os: "linux"
+    retry:
+      automatic:
+        - exit_status: -1  # Agent was lost
+          limit: 2
+    timeout_in_minutes: 120
+
+  - label: "Hardened debug mode"
+    command: "libcxx/utils/ci/run-buildbot generic-hardened-debug-mode"
+    artifact_paths:
+      - "**/test-results.xml"
+      - "**/*.abilist"
+    env:
+        CC: "clang-${LLVM_HEAD_VERSION}"
+        CXX: "clang++-${LLVM_HEAD_VERSION}"
+    agents:
+      queue: "libcxx-builders"
+      os: "linux"
+    retry:
+      automatic:
+        - exit_status: -1  # Agent was lost
+          limit: 2
+    timeout_in_minutes: 120
+
+  - label: "Debug mode"
+    command: "libcxx/utils/ci/run-buildbot generic-debug-mode"
+    artifact_paths:
+      - "**/test-results.xml"
+      - "**/*.abilist"
+    env:
+        CC: "clang-${LLVM_HEAD_VERSION}"
+        CXX: "clang++-${LLVM_HEAD_VERSION}"
+        ENABLE_CLANG_TIDY: "On"
+    agents:
+      queue: "libcxx-builders"
+      os: "linux"
+    retry:
+      automatic:
+        - exit_status: -1  # Agent was lost
+          limit: 2
+    timeout_in_minutes: 120
+
   - label: "Debug mode"
     command: "libcxx/utils/ci/run-buildbot generic-debug-mode"
     artifact_paths:
diff --git a/libcxx/utils/ci/run-buildbot b/libcxx/utils/ci/run-buildbot
--- a/libcxx/utils/ci/run-buildbot
+++ b/libcxx/utils/ci/run-buildbot
@@ -387,6 +387,18 @@
     check-runtimes
     check-abi-list
 ;;
+generic-hardened-mode)
+    clean
+    generate-cmake -C "${MONOREPO_ROOT}/libcxx/cmake/caches/Generic-hardened-mode.cmake"
+    check-runtimes
+    check-abi-list
+;;
+generic-hardened-debug-mode)
+    clean
+    generate-cmake -C "${MONOREPO_ROOT}/libcxx/cmake/caches/Generic-hardened-debug-mode.cmake"
+    check-runtimes
+    check-abi-list
+;;
 generic-debug-mode)
     clean
     generate-cmake -C "${MONOREPO_ROOT}/libcxx/cmake/caches/Generic-debug-mode.cmake"
diff --git a/libcxx/utils/libcxx/test/params.py b/libcxx/utils/libcxx/test/params.py
--- a/libcxx/utils/libcxx/test/params.py
+++ b/libcxx/utils/libcxx/test/params.py
@@ -294,6 +294,30 @@
             AddFeature("libcpp-has-assertions"),
         ],
     ),
+    Parameter(
+        name="enable_hardened_mode",
+        choices=[True, False],
+        type=bool,
+        default=False,
+        help="Whether to enable the hardened mode  when compiling the test suite. This is only meaningful when "
+        "running the tests against libc++.",
+        actions=lambda enabled: [] if not enabled else [
+            AddCompileFlag("-D_LIBCPP_ENABLE_HARDENED_MODE=1"),
+            AddFeature("libcpp-has-hardened-mode"),
+        ],
+    ),
+    Parameter(
+        name="enable_hardened_debug_mode",
+        choices=[True, False],
+        type=bool,
+        default=False,
+        help="Whether to enable the debug mode  when compiling the test suite. This is only meaningful when "
+        "running the tests against libc++.",
+        actions=lambda enabled: [] if not enabled else [
+            AddCompileFlag("-D_LIBCPP_ENABLE_HARDENED_DEBUG_MODE=1"),
+            AddFeature("libcpp-has-hardened-debug-mode"),
+        ],
+    ),
     Parameter(
         name="additional_features",
         type=list,