The technique consists of attackers attempting hundreds of $0 transactions with stolen payment cards to check a card's validity.
The transactions are executed against Magento stores that support the PayPal Payflow Pro integration.
The PayPal Payflow Pro integration is a payment option available on Magento shops that allows an online store to process card transactions via a PayPal merchant (business) account.
Many stores use it because it allows them to receive payments via PayPal using a checkout form embedded on their sites and without users leaving the store to enter details on the PayPal portal.
Abuse detected in the wild
According to a security advisory issued by the Magento team and seen by ZDNet, hackers are abusing the PayPal Payflow Pro integration included in Magento 2.1.x and 2.2.x versions to test the validity of stolen cards.
Crooks aren't using the stolen cards to place orders for real products, but merely initiating a $0-sized transaction and see if it returns any errors --and indirectly confirm that the card details are valid.
It is believed that hackers are buying these cards from so-called "carding forums" --underground cybercrime forums where hackers and ATM skimming groups are putting card details up for sale.
Many of these "card dumps" frequently contain details for old and expired payment cards, and buyers often need a way to validate the details of newly purchased card dumps before using them in fraudulent operations at banks or online stores, or for creating card clones.
The Magento team said that both versions of the Magento CMS are vulnerable --the self-hosted open source version, and the on-premise or cloud-based commercial Magento offerings.
Magento 2.3.x versions might also be vulnerable, but the Magento team has not seen any evidence of abuse against these types of sites, as of yet.
Safeguarding stores is a must
The Magento team is now recommending that shop owners look into putting up a web application firewall (WAF) or other anti-brute-force or bot detection systems in place to protect stores against such abuse.
Store owners might think that they're not subject to losing any money, as hackers are merely testing some payment card details, but the reality is not so.
The Magento team warns store owners that PayPal may suspend their accounts after repeated automated operations. They recommend that store owners reach out to PayPal and inquire about additional anti-fraud security measures they can roll out for their PayPal Business accounts.
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