Magento version 126.96.36.199 include patch SUPEE-9767 V2 which includes security patching for the checkout process.
For your current front-end templates to work with this patch you will need to include a form key value to a number of forms in the checkout process.
<?php echo $this->getBlockHtml('formkey') ?>
Load the templates below from your
app/design/frontend/<package>/<theme> directory and add the Form Key code above between the
</form> tags on each.
The basic templates that will need upgraded:
If persistant checkout is enabled:
If you allow shippinng to multiple addresses: