diff --git a/firmware_handler.cpp b/firmware_handler.cpp
index f6344d1..6e64d9d 100644
--- a/firmware_handler.cpp
+++ b/firmware_handler.cpp
@@ -482,10 +482,9 @@
         return false;
     }
 
-    /* Prevent writing to the verification blob before they trigger
-     * verification.
-     */
-    if (item->second->activePath == verifyBlobId)
+    /* Prevent writing to the verification or update blobs. */
+    if (item->second->activePath == verifyBlobId ||
+        item->second->activePath == updateBlobId)
     {
         return false;
     }
diff --git a/test/firmware_state_updatepending_unittest.cpp b/test/firmware_state_updatepending_unittest.cpp
index eb0a54e..43f90e3 100644
--- a/test/firmware_state_updatepending_unittest.cpp
+++ b/test/firmware_state_updatepending_unittest.cpp
@@ -152,6 +152,17 @@
 }
 
 /*
+ * write(session)
+ */
+TEST_F(FirmwareHandlerUpdatePendingTest, WriteToUpdateBlobReturnsFailure)
+{
+    getToUpdatePending();
+
+    EXPECT_TRUE(handler->open(session, flags, updateBlobId));
+    EXPECT_FALSE(handler->write(session, 0, {0x01}));
+}
+
+/*
  * TODO: deleteBlob(blob)
  */
 
@@ -164,10 +175,6 @@
  */
 
 /*
- * write(session)
- */
-
-/*
  * read(session)
  */