Funnel Builder Flaw Under Active Exploitation Enables WooCommerce Checkout Skimming

A critical security vulnerability impacting the
Funnel Builder
plugin for WordPress has come under active exploitation in the wild to
inject malicious JavaScript code
into WooCommerce checkout pages with the goal of stealing payment data.

Details of the activity were
published
by Sansec this week. The vulnerability currently does not have an official CVE identifier. It affects all versions of the plugin before 3.15.0.3. It’s used in more than 40,000 WooCommerce stores. 

The flaw lets unauthenticated attackers inject arbitrary JavaScript into every checkout page on the store, the Dutch e-commerce security company said. FunnelKit, which maintains Funnel Builder, has released a patch for the vulnerability in version 3.15.0.3.

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“Attackers are planting fake Google Tag Manager scripts into the plugin’s ‘External Scripts’ setting,” it noted. “The injected code looks like ordinary analytics next to the store’s real tags, but loads a payment skimmer that steals credit card numbers, CVVs, and billing addresses from checkout.”

Per Sansec, Funnel Builder includes a publicly exposed checkout endpoint that allows an incoming request to choose the type of internal method to run. However, older versions were designed such that they never checked the caller’s permissions or limited which methods are allowed to be invoked.

A bad actor could exploit this loophole by issuing an unauthenticated request that can reach an unspecified internal method that writes attacker-controlled data directly into the plugin’s global settings. The added code snippet is then injected into every Funnel Builder checkout page.

As a result, an attacker could plant a malicious <script> tag that’s triggered on every checkout transaction in a susceptible WordPress site.

In at least one case, Sansec said it observed a payload masquerading as a Google Tag Manager (GTM) loader to launch JavaScript hosted on a remote domain. It subsequently opens a WebSocket connection to the attacker’s command-and-control (C2) server (“wss://protect-wss[.]com/ws”) to retrieve a skimmer that’s tailored to the victim’s storefront.

The end goal of the attack is to siphon credit card numbers, CVVs, billing addresses, and other personal information that could be entered by site visitors at checkout. Site owners are advised to update the Funnel Builder plugin to the latest version and review Settings > Checkout > External Scripts for anything that’s unfamiliar and remove it.

“Dressing skimmers up as Google Analytics or Tag Manager code is a
recurring Magecart pattern
, since reviewers tend to skim straight past anything that looks like a familiar tracking tag,” Sansec said.

The disclosure comes weeks after Sucuri detailed a campaign in which Joomla websites are being backdoored with heavily obfuscated PHP code to contact attacker-controlled C2 servers, receive and process instructions sent by the operators, and serve spammy content to visitors and search engines without the site owner’s knowledge. The ultimate aim is to leverage the sites’ reputation for injecting spam.

“The script acts as a remote loader,” security researcher Puja Srivastava
said
. “It contacts an external server, sends information about the infected website, and waits for instructions. The response from the remote server determines what content the infected site should serve.”

“This approach allows attackers to change the behavior of the compromised website at any time without modifying the local files again. The attacker can inject spam product links, redirect visitors, or display malicious pages dynamically.”

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