diff --git a/media/gpu/vaapi/vp9_vaapi_video_encoder_delegate.cc b/media/gpu/vaapi/vp9_vaapi_video_encoder_delegate.cc
index 27a524fad9f6f..8fba8e03b327d 100644
--- a/media/gpu/vaapi/vp9_vaapi_video_encoder_delegate.cc
+++ b/media/gpu/vaapi/vp9_vaapi_video_encoder_delegate.cc
@@ -261,6 +261,18 @@ bool VP9VaapiVideoEncoderDelegate::Initialize(
       return false;
     }
     for (const auto& spatial_layer : config.spatial_layers) {
+      // vaCreateContext() and the VAEncCodedBuffer are sized from
+      // |coded_size_| (derived from |input_visible_size|); a spatial layer
+      // larger than that would cause the VA-API driver to encode past the
+      // context/coded-buffer bounds. This is also enforced at the Mojo
+      // boundary in MojoVideoEncodeAcceleratorService::Initialize().
+      if (spatial_layer.width > coded_size_.width() ||
+          spatial_layer.height > coded_size_.height()) {
+        VLOGF(1) << "Spatial layer " << spatial_layer.width << "x"
+                 << spatial_layer.height << " exceeds coded size "
+                 << coded_size_.ToString();
+        return false;
+      }
       spatial_layer_resolutions.emplace_back(
           gfx::Size(spatial_layer.width, spatial_layer.height));
     }
diff --git a/media/mojo/services/mojo_video_encode_accelerator_service.cc b/media/mojo/services/mojo_video_encode_accelerator_service.cc
index 3d603e63940be..fcfc92de5d2d8 100644
--- a/media/mojo/services/mojo_video_encode_accelerator_service.cc
+++ b/media/mojo/services/mojo_video_encode_accelerator_service.cc
@@ -18,6 +18,7 @@
 #include "media/base/media_util.h"
 #include "media/mojo/mojom/video_encoder_info.mojom.h"
 #include "media/mojo/services/mojo_media_log.h"
+#include "mojo/public/cpp/bindings/message.h"
 #include "mojo/public/cpp/bindings/self_owned_receiver.h"
 #include "mojo/public/cpp/system/platform_handle.h"
 #include "third_party/perfetto/include/perfetto/tracing/track.h"
@@ -147,6 +148,19 @@ void MojoVideoEncodeAcceleratorService::Initialize(
           .Run({EncoderStatus::Codes::kEncoderInitializationError});
       return;
     }
+    // Spatial layers larger than |input_visible_size| are never produced by
+    // a well-behaved renderer and cause hardware encoders to size their VA
+    // context / coded buffer from |input_visible_size| while encoding at the
+    // (larger) spatial-layer size, which is a driver-heap OOB write.
+    if (spatial_layer.width > config.input_visible_size.width() ||
+        spatial_layer.height > config.input_visible_size.height()) {
+      mojo::ReportBadMessage(
+          "VideoEncodeAccelerator.Initialize: spatial_layer larger than "
+          "input_visible_size");
+      std::move(success_callback)
+          .Run({EncoderStatus::Codes::kEncoderInitializationError});
+      return;
+    }
   }
 
   encoder_.reset();