CSP FY: A Magecart Attack That Dodges Policy—and Makes a Joke While Doing It

by Source Defense

When attackers are clever enough to name their cookie “csp_f_y,” you know they’re not just exfiltrating data—they’re mocking your defenses.

In a recent attack spotted by the Source Defense Cyber Research team, a compromised first-party script on a payment page stored sensitive data in a cookie named csp_f_y. The exfiltration didn’t happen immediately—it was triggered on the next page load using location.href, slipping past content security policies (CSP) that would have otherwise blocked malicious outbound requests.

Let that sink in. The attacker:

• Used a first-party script—trusted by the site.
• Delayed the exfiltration, waiting for a benign “thank you” page.
• Leveraged standard browser functions that CSP can’t touch.

What This Means for Security Teams

This attack is a textbook example of how clever cybercriminals exploit gaps in traditional client-side protections:

1. First-Party Blind Spots
   Since the attack originates from trusted first-party code, policy-based defenses like CSP or SRI won’t stop it. This is essentially a server-side attack executed on the client side—making it invisible to most detection tools.
2. Data Stored in Cookies
   Storing sensitive data in a cookie might seem crude, but it’s effective. Monitoring cookie signals is increasingly critical—especially as attackers adopt this behavior to sidestep conventional script-based detection methods.
3. Location.href: The Great Escape
   Many security solutions can’t override or monitor window.location.href, which is used to redirect to the next page. That means detection and prevention mechanisms relying on script tagging or proxies are bypassed altogether.

Why It’s Funny—and Why It’s Serious

The name csp_f_y is equal parts brazen and brilliant. It taunts defenders who rely solely on CSP headers while exfiltrating cardholder data under the radar. It’s a meme-worthy jab that also signals a serious failure in traditional defenses.

The Bigger Picture: You Need a Full-Site, Behavior-Based Defense

This attack highlights why static defenses like CSP are insufficient in today’s dynamic, JavaScript-powered web environment. The cookie name “CSP FY” deserves to be emphasized—but the joke’s on anyone still relying on outdated tools alone.

Key Takeaways:

• Full-site protection matters. Threats don’t limit themselves to the payment page.
• Behavior-based detection is essential. Look beyond script origins to how they behave.
• First-party scripts need scrutiny too. Trusted code can be turned into attack vectors.

At Source Defense, we’re actively researching detection strategies to identify attacks like these—before data makes its way out into the wild.

Let’s keep the attackers from having the last laugh.

For more information about protecting your organization against sophisticated client-side attacks, contact Source Defense for a comprehensive security assessment.

The post CSP FY: A Magecart Attack That Dodges Policy—and Makes a Joke While Doing It appeared first on Source Defense.

*** This is a Security Bloggers Network syndicated blog from Blog | Source Defense authored by Scott Fiesel. Read the original post at: https://sourcedefense.com/resources/csp-fy-a-magecart-attack-that-dodges-policy-and-makes-a-joke-while-doing-it/