To reset the file and directory permissions for Magento back to their default and secure permissions, run the following commands from the directory where Magento is installed:

find . -type f -exec chmod 644 {} \;
find . -type d -exec chmod 755 {} \;
chmod +x mage

Notes / Supplemental Resources

If you're running Magento 1.4.x and earlier, change the word mage in the last command to pear.