Since the release of v1.5.5e in March of 2017, the following changes have been applied to become v1.5.5f according to the official Zen Cart website:

• Changes in v1.5.5f:
  - Checkout - As a help to people getting PayPal TLS notifications, the CURL Testing tool now reports compatibility with tlstest.paypal.com
  - Checkout - Square Payments support is now built-in. Start taking credit card payments in just 5 minutes! See www.zen-cart.com/partners/square_setup
  - Checkout - FirstData soft-descriptors now optional via module settings
  - Checkout - PayPal - Australia settings explained better in configuration screen
  - Checkout - PayPal IPN-handler posts back over ipnpb URL and now gracefully ignores unrecognized new notification types
  - Checkout - Payeezy module now also accommodates 2-series MC BINs
  - Checkout - Authorize.net AIM - add option to display reason for CVV or date failure
  - Checkout - Authorize.net AIM - improvements to detect always-SSL site configuration
  - Checkout - Credit Card Slamming threshold now customizable via Observer code or plugin
  - Checkout - Internal fix to payment modules to avoid thousands-rounding errors
  - Fix upload-class bug related to filtering desired file-extensions
  - Fix multi-language error (since v155a) in functions_email for PHPMailer
  - Fix possible bug in subdomain handling of secure cookie session handling
  - Fix attribute sort-order rules
  - Fixed - if an invalid or non-existent tax rate was encountered, would get: PHP Warning: A non-numeric value encountered in /includes/functions/functions_taxes.php on line 172
  - Admin: Restored percent calculators on Products Price Manager
  - Admin: Fix a few sanitizer fields to avoid over-cleaning
  - Admin: Update currency-exchange calculators since BOC change
  - Template - Support ISO-8601 style date-of-birth date
  - Template - Compatibility updates to accommodate OnePageCheckout plugin integration
  - Template - Improvements for jQuery conflict prevention, and support older browsers
  - Template - Fix CSS typo in colors
  - Template - hreflang URL correction
  - Template - Fix incorrect use of logo image height
  - Security: CVE-2017-11675 - Low-Risk vulnerability (required your Admin login to be hijacked first). Now prevents exporting to invalid filenames.
  - Optimization - Abort ajax requests from spiders
  - Enhancement - Cache-bypass added for certain db queries, so retrieved data is always fresh
  - Utility - Plugin version-check utility optimized to allow specific version-comparisons, and adapt to timeouts more quickly.
• NOTE: There were NO Database Changes