diff --git a/block/src/lib.rs b/block/src/lib.rs
index 912cf4401..d02ab5469 100644
--- a/block/src/lib.rs
+++ b/block/src/lib.rs
@@ -691,7 +691,13 @@ impl Request {
 
                 let wz_sector = u64::from_le_bytes(wz_sector);
                 let wz_num_sectors = u32::from_le_bytes(wz_num_sectors);
+
                 let wz_flags = u32::from_le_bytes(wz_flags);
+                // Per virtio spec v1.2 reject write zeroes if any unknown flag is set.
+                if (wz_flags & !VIRTIO_BLK_WRITE_ZEROES_FLAG_UNMAP) != 0 {
+                    warn!("Unsupported flags {wz_flags:#x} in write zeroes request");
+                    return Err(ExecuteError::Unsupported(VIRTIO_BLK_T_WRITE_ZEROES));
+                }
 
                 let wz_offset = wz_sector * SECTOR_SIZE;
                 if wz_offset == 0 && disable_sector0_writes {