diff --git a/install/Android.bp b/install/Android.bp
index 8c88bd0..e239ddc 100644
--- a/install/Android.bp
+++ b/install/Android.bp
@@ -26,6 +26,7 @@
 
     defaults: [
         "recovery_defaults",
+        "libspl_check_defaults",
     ],
 
     shared_libs: [
@@ -49,12 +50,51 @@
         "librecovery_utils",
         "libotautil",
         "libsnapshot_nobinder",
+        "ota_metadata_proto_cc",
 
         // external dependencies
         "libvintf",
     ],
 }
 
+cc_test_host {
+    name: "libinstall_host_unittests",
+    defaults: [
+        "libspl_check_defaults"
+    ],
+    srcs: [
+        "spl_check_unittests.cpp",
+    ],
+    static_libs: [
+        "libspl_check",
+    ],
+}
+
+cc_defaults {
+    name: "libspl_check_defaults",
+    static_libs: [
+        "libbase",
+        "ota_metadata_proto_cc",
+        "liblog",
+        "libziparchive",
+        "libz",
+        "libprotobuf-cpp-lite",
+    ],
+}
+
+cc_library_static {
+    name: "libspl_check",
+    recovery_available: true,
+    host_supported: true,
+    defaults: [
+        "libspl_check_defaults",
+    ],
+    srcs: ["spl_check.cpp"],
+    export_include_dirs: [
+        "include",
+    ],
+}
+
 cc_library_static {
     name: "libinstall",
     recovery_available: true,
@@ -73,6 +113,7 @@
         "verifier.cpp",
         "wipe_data.cpp",
         "wipe_device.cpp",
+        "spl_check.cpp",
     ],
 
     header_libs: [
diff --git a/install/include/install/spl_check.h b/install/include/install/spl_check.h
new file mode 100644
index 0000000..e0bfc62
--- /dev/null
+++ b/install/include/install/spl_check.h
@@ -0,0 +1,26 @@
+/*
+ * Copyright (C) 2021 The Android Open Source Project
+ *
+ * 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
+ *
+ *      http://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.
+ */
+
+#include <string_view>
+
+#include <android-base/logging.h>
+#include <ota_metadata.pb.h>
+#include <ziparchive/zip_archive.h>
+
+bool ViolatesSPLDowngrade(const build::tools::releasetools::OtaMetadata& metadata,
+                          std::string_view current_spl);
+
+bool ViolatesSPLDowngrade(ZipArchiveHandle zip, std::string_view current_spl);
diff --git a/install/install.cpp b/install/install.cpp
index 1b220cb..6e74f80 100644
--- a/install/install.cpp
+++ b/install/install.cpp
@@ -47,6 +47,7 @@
 #include <android-base/unique_fd.h>
 
 #include "install/package.h"
+#include "install/spl_check.h"
 #include "install/verifier.h"
 #include "install/wipe_data.h"
 #include "otautil/error_code.h"
@@ -348,6 +349,12 @@
       android::base::GetBoolProperty("ro.virtual_ab.allow_non_ab", false);
   bool device_only_supports_ab = device_supports_ab && !ab_device_supports_nonab;
 
+  const auto current_spl = android::base::GetProperty("ro.build.version.security_patch", "");
+  if (ViolatesSPLDowngrade(zip, current_spl)) {
+    LOG(ERROR) << "Denying OTA because it's SPL downgrade";
+    return INSTALL_ERROR;
+  }
+
   if (package_is_ab) {
     CHECK(package->GetType() == PackageType::kFile);
   }
diff --git a/install/spl_check.cpp b/install/spl_check.cpp
new file mode 100644
index 0000000..c26ab82
--- /dev/null
+++ b/install/spl_check.cpp
@@ -0,0 +1,78 @@
+/*
+ * Copyright (C) 2021 The Android Open Source Project
+ *
+ * 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
+ *
+ *      http://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.
+ */
+
+#include "install/spl_check.h"
+
+bool ViolatesSPLDowngrade(const build::tools::releasetools::OtaMetadata& metadata,
+                          std::string_view current_spl) {
+  const auto& post_spl = metadata.postcondition().security_patch_level();
+  if (current_spl.empty()) {
+    LOG(WARNING) << "Failed to get device's current security patch level. Target SPL is "
+                 << post_spl << " permitting OTA install";
+    return false;
+  }
+  // SPL(security patch level) is expected to be in format yyyy-mm-dd, e.g.  2018-05-29. Given this
+  // specific format, comparing two SPL can be done by just regular string comparison. If the format
+  // must lay out year/month/date in the exact order, and must properly prepend dates with 0(for
+  // example, 05 for May). Otherwise this comparison doesn't work. We don't expect SPL date formats
+  // to change, leave this as is.
+  if (post_spl < current_spl) {
+    LOG(ERROR) << "Current SPL: " << current_spl << " Target SPL: " << post_spl
+               << " this is considered a downgrade";
+    if (metadata.spl_downgrade() || metadata.downgrade()) {
+      LOG(WARNING)
+          << "SPL downgrade detected, but OTA package explicitly permitts this(OtaMetadata has "
+             "spl_downgrade / downgrade bit set).Permitting update anyway.Installing a SPL "
+             "downgrade OTA can cause /data fail to decrypt and device fails to boot.";
+      return false;
+    }
+    return true;
+  } else {
+    LOG(INFO) << "old spl: " << current_spl << " new spl: " << post_spl << " CHECK passes";
+  }
+  return false;
+}
+
+bool ViolatesSPLDowngrade(ZipArchiveHandle zip, std::string_view current_spl) {
+  static constexpr auto&& OTA_OTA_METADATA = "META-INF/com/android/metadata.pb";
+  ZipEntry64 metadata_entry;
+  if (FindEntry(zip, OTA_OTA_METADATA, &metadata_entry) != 0) {
+    LOG(WARNING) << "Failed to find " << OTA_OTA_METADATA
+                 << " treating this as non-spl-downgrade, permit OTA install. If device bricks "
+                    "after installing, check kernel log to see if /data failed to decrypt";
+    return false;
+  }
+  const auto metadata_entry_length = metadata_entry.uncompressed_length;
+  if (metadata_entry_length > std::numeric_limits<size_t>::max()) {
+    LOG(ERROR) << "Failed to extract " << OTA_OTA_METADATA
+               << " because's uncompressed size exceeds size of address space. "
+               << metadata_entry_length;
+    return false;
+  }
+  std::vector<uint8_t> ota_metadata(metadata_entry_length);
+  int32_t err = ExtractToMemory(zip, &metadata_entry, ota_metadata.data(), metadata_entry_length);
+  if (err != 0) {
+    LOG(ERROR) << "Failed to extract " << OTA_OTA_METADATA << ": " << ErrorCodeString(err);
+    return false;
+  }
+  using build::tools::releasetools::OtaMetadata;
+  OtaMetadata metadata;
+  if (!metadata.ParseFromArray(ota_metadata.data(), ota_metadata.size())) {
+    LOG(ERROR) << "Failed to parse ota_medata";
+    return false;
+  }
+  return ViolatesSPLDowngrade(metadata, current_spl);
+}
diff --git a/install/spl_check_unittests.cpp b/install/spl_check_unittests.cpp
new file mode 100644
index 0000000..709b69c
--- /dev/null
+++ b/install/spl_check_unittests.cpp
@@ -0,0 +1,45 @@
+/*
+ * Copyright (C) 2021 The Android Open Source Project
+ *
+ * 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
+ *
+ *      http://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.
+ */
+
+#include <gtest/gtest.h>
+
+#include "install/spl_check.h"
+#include "ota_metadata.pb.h"
+
+using build::tools::releasetools::OtaMetadata;
+class SplCheckUnittest : public ::testing::Test {
+ public:
+  OtaMetadata metadata;
+};
+
+TEST_F(SplCheckUnittest, OlderSPL) {
+  metadata.set_spl_downgrade(false);
+  metadata.mutable_postcondition()->set_security_patch_level("2021-04-25");
+  ASSERT_TRUE(ViolatesSPLDowngrade(metadata, "2021-05-01"));
+}
+
+TEST_F(SplCheckUnittest, NewerSPL) {
+  metadata.set_spl_downgrade(false);
+  metadata.mutable_postcondition()->set_security_patch_level("2021-06-01");
+  ASSERT_FALSE(ViolatesSPLDowngrade(metadata, "2021-05-05"));
+}
+
+TEST_F(SplCheckUnittest, OlderSPLPermit) {
+  // If spl_downgrade is set to true, OTA should be permitted
+  metadata.set_spl_downgrade(true);
+  metadata.mutable_postcondition()->set_security_patch_level("2021-04-11");
+  ASSERT_FALSE(ViolatesSPLDowngrade(metadata, "2021-05-11"));
+}
\ No newline at end of file