diff --git a/third_party/angle/src/libANGLE/renderer/vulkan/vk_utils.cpp b/third_party/angle/src/libANGLE/renderer/vulkan/vk_utils.cpp
--- a/third_party/angle/src/libANGLE/renderer/vulkan/vk_utils.cpp
+++ b/third_party/angle/src/libANGLE/renderer/vulkan/vk_utils.cpp
@@ -1968,7 +1968,16 @@ void GetExtentsAndLayerCount(gl::TextureType textureType,

 vk::LevelIndex GetLevelIndex(gl::LevelIndex levelGL, gl::LevelIndex baseLevel)
 {
-    ASSERT(baseLevel <= levelGL);
+    // SECURITY: glCopy[Sub]TextureCHROMIUM permits a sourceLevel that is below
+    // the source VkImage's mFirstAllocatedLevel. The GLint subtraction below
+    // would underflow to ~0xFFFFFFFE and reach vkCmdCopyImage as
+    // srcSubresource.mipLevel, violating VUID-vkCmdCopyImage-srcSubresource-07967
+    // and yielding a driver-side OOB read. Crash safely instead.
+    if (ANGLE_UNLIKELY(!(baseLevel <= levelGL)))
+    {
+        FATAL() << "GetLevelIndex underflow: levelGL=" << levelGL.get()
+                << " < baseLevel=" << baseLevel.get();
+    }
     return vk::LevelIndex(levelGL.get() - baseLevel.get());
 }