Attackers are using a sneaky method to steal payment card data from websites using Magento, eBay’s widely used e-commerce platform.

Researchers from Sucuri, a company that specializes in securing websites, said the attackers can collect any data submitted by a user to Magento but carefully filters out anything that doesn’t look like credit card data.

The attackers are injecting their malicious code into Magento, but it’s still unclear how that process happens, wrote Peter Gramantik, a senior malware researcher with Sucuri.

“It seems though that the attacker is exploiting a vulnerability in Magento core or some widely used module/extension,” he wrote.

All POST requests are collected, but there are rules within the attack script that only collects payment card information.

“If the structure of the POST parameters match, the attacker stores them all—nothing more, but nothing less,” Gramantik wrote. “They’ve got all the billing details processed by the infected site.”

The stolen data is then encrypted using a public encryption key that is included in the malicious script. It is then saved in a fake image file.

If someone were to try to load the image, it wouldn’t be displayed, he wrote. But the attacker can download and decrypt the fake image file, revealing the payment card details.

“Now they have all the billing information processed by the Magento e-commerce website,” he wrote. “It’s all nicely packed, formatted and collected.”

Ebay could not be immediately reached for comment.

Sucuri also found an example of a less-sophisticated but no less effective way to steal data from Magento.

In that example, attack code is injected to Magento’s Checkout Module. It collects payment card data before a transaction is processed. The data is then emailed in plain text to the attacker’s account.

Those behind the method seem to be intimately familiar with how Magento works, Gramantik wrote. “The attacker knows how the module works and the code it’s built on; all he needed to do was use the module’s own variable in which all the sensitive data is stored unprotected.”

Sucuri has seen variations of this attack before. In April, Sucuri’s Denis Sinegubko outlined where hackers see opportunity within Magneto. Most websites using the platform have a checkout form where customers enter their credit card details.

Magento then encrypts that data and either saves it or sends it to a payment gateway to complete the transaction, but there is “a very short period of time when Magento handles sensitive customer information in an unencrypted format,” Sinegubko wrote in a blog post.

Sinegubko wrote that’s a fine method unless hackers find a way to grab the information before it’s encrypted.