HTML tags are not allowed in SKU attribute

Problem description

HTML tags are not allowed in SKU attribute error is thrown on attempt to edit product in Magento backend at Catalog > Manage products > Product page.

Cause

SKU attribute value contains HTML tags and it is not allowed since SUPEE-10570 or Magento 1.9.3.8 version, SKU attribute requirements were hardened by the following code added to Mage_Adminhtml_Catalog_ProductController controller in app/code/core/Mage/Adminhtml/controllers/Catalog/ProductController.php:

--- app/code/core/Mage/Adminhtml/controllers/Catalog/ProductController.php
+++ app/code/core/Mage/Adminhtml/controllers/Catalog/ProductController.php
@@ -723,6 +723,16 @@ class Mage_Adminhtml_Catalog_ProductController extends Mage_Adminhtml_Controller
             $this->_filterStockData($data['product']['stock_data']);
 
             $product = $this->_initProductSave();
+            // check sku attribute
+            $productSku = $product->getSku();
+            if ($productSku && $productSku != Mage::helper('core')->stripTags($productSku)) {
+                $this->_getSession()->addError($this->__('HTML tags are not allowed in SKU attribute.'));
+                $this->_redirect('*/*/edit', array(
+                    'id' => $productId,
+                    '_current' => true
+                ));
+                return;
+            }
 
             try {
                 $product->save();

Solution

Edit SKU attrubute value and delete all HTML tags (, , etc.) If it is not possible due to some reason, you can rollback modifications in the code listed in Cause section which imposed such restrictions (by copying app/code/core/Mage/Adminhtml/controllers/Catalog/ProductController.php into app/code/local/Mage/Adminhtml/controllers/Catalog/ProductController.php and changing local copy).