Cereal (Breach)fast: When Your Vendor Leaks Your Data

Not all spills start in your own kitchen.

On December 7, 2024, WK Kellogg Co.—one of North America’s most iconic cereal manufacturers—experienced a breach. But not through its own servers or IT missteps. Instead, the breach came from the outside: a trusted third-party vendor named Cleo, which handles secure file transfers between Kellogg and its HR service providers.

Behind the breach was CL0P, a ransomware group well-known for targeting zero-day vulnerabilities in third-party software. By exploiting a flaw in Cleo’s platform, CL0P accessed and exfiltrated employee data: names, Social Security numbers, and other personal identifiers.

And like in many of CL0P’s campaigns, they didn’t encrypt anything. They didn’t need to.

They stole the data quietly—and leaked it publicly.

Even worse, WK Kellogg Co. didn’t discover the breach until February 27, 2025—more than two months after it had already happened. By that point, CL0P had already posted about the attack on their leak site.

The incident became public. Headlines were made. And once again, a company’s name ended up in a breach report because a vendor’s security failed.

SR – Stolen Resources

The most chilling part? The victims never saw it coming.

Kellogg employees didn’t fall for phishing links. They didn’t open malicious attachments. They didn’t hand over credentials. They simply had their data—PII meant for internal HR use—transferred through Cleo’s systems.

And that’s where CL0P struck.

The breach compromised HR file transfers containing personally identifiable information (PII). While only a handful of individuals from Maine and New Hampshire have been officially identified, the scope is likely much broader. HR data doesn’t respect state lines—and neither does CL0P.

This is a classic case of supply chain exposure: no user action, just inherited risk.

Trouble in the Supply Chain Paradise

CL0P is not a newcomer to third-party targeting. Their playbook is precise:

1. Find a widely used file-transfer or collaboration platform.

2. Wait for a zero-day.

3. Exploit it quietly.

4. Exfiltrate sensitive data.

5. Post the breach to pressure victims.

In this case, they exploited Cleo’s software, which WK Kellogg Co. used to transfer HR files to external service vendors. Once inside, they siphoned data directly from the vendor’s servers. No malware was installed on Kellogg’s network. No employee accounts were compromised. The weakness was entirely external—but the damage was entirely internal.

It’s a reminder that trusting a platform isn’t the same as securing one.

Third-Party Breach, Own Leak

Too often, organizations focus on securing their own perimeter while leaving vendor exposure out of scope. But incidents like this show that a breach in the supply chain is still your breach—your brand, your liability, your responsibility.

The vendor may be at fault. But your users, your regulators, and your customers still expect you to answer for it.

So what now?

Slam the Door Shut

If your third party touches sensitive data, treat them like they’re inside your walls.

Here’s how to close the gaps:

• Audit vendor platforms regularly. Know how they store, transfer, and protect your data.

• Apply patches quickly—especially for systems that exchange files or credentials.

• Monitor data flows to and from external services.

• Enforce multi-factor authentication for all vendor-facing portals.

• Use zero-trust segmentation to limit what vendors can access if compromised.

• Log everything and alert on anomalies—especially silent exfiltration attempts.

• Simulate breach scenarios that include third-party compromise in your incident response plans.

Because even if the leak doesn’t happen inside your perimeter, it’s still your cereal that ends up spilled.

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