AppSec Pitfalls for Two-Sided B2B Marketplaces

A seller-facing API that lets a partner list inventory carries different risk than a buyer-facing checkout flow handling a transaction. Apply tighter scanning and a shorter remediation window to whichever side touches payment or transaction data, and don't assume that securing one side of the marketplace automatically covers the other; they're often built by different teams with different dependency lists.

Splitting your scanning by side of the marketplace looks like this:

• Apply tighter scanning and a shorter remediation window to the side that touches payment or transaction data, such as the buyer checkout flow.
• Treat a seller-facing listing API as its own risk, with a separate review instead of an assumed match to checkout.
• Review partner-built integration code, such as shipping, payment, or ERP connectors, the same way you review your own.
• Check shared base images across your many small services, since one stale image multiplies the blast radius.

Third-party integrations that sellers or buyers connect through (a shipping API, a payment processor, an ERP connector) each bring their own dependency risk into the platform. Review any partner-built integration code the same way you'd review your own, and don't assume a partner's code has been through the same scanning discipline your own team follows just because they're an established company.

A partner's size or reputation says nothing about how carefully their specific integration was built, and the integration is usually built by a different team than the one that earned the reputation in the first place.

A marketplace handling meaningful transaction volume often runs many small services rather than one monolith, and container sprawl grows with every new feature. A stale base image shared across several of those services multiplies the blast radius of a single vulnerability. Scan images already sitting in your registry on a recurring schedule, not just at build time, since a newly disclosed vulnerability in an existing image needs a way to surface without waiting for a rebuild.

A platform that's grown quickly through several feature launches often has more of these shared images than anyone on the current team can list from memory.

For a marketplace, a security incident and an availability incident often look the same to users: the platform stops working and trust erodes on both sides at once. A dependency vulnerability that leads to a service crash is both a security finding and an uptime problem, so route critical findings through the same incident-response urgency you'd apply to an outage, not a slower general engineering backlog.

Federal guidance sets roughly fourteen days as the outer window for fixing a known exploited vulnerability once it's added to the list¹; a marketplace handling real transactions between businesses has a reasonable case for holding transaction-path code to that standard or tighter, given what's at stake if a flaw in that path gets exploited before it's fixed.

It's tempting to assume a large, established buyer or seller on the platform has its own mature security practice simply because of its size, but a large partner's integration code can be just as dependent on an outdated package as a smaller one's. Apply the same review standard to every partner integration regardless of the partner's size or reputation, and don't let an assumption about a big name substitute for an actual check of what their code does inside your environment.

A smaller partner, ironically, is sometimes easier to get a straight answer from about their own practices than a large one whose security team is harder to reach directly.

Dispute resolution and refund flows tend to get less security attention than checkout itself, since they're used less often and feel lower-stakes, but they often touch the same transaction and payment data and can be a quieter path to abuse if a flaw lets one party manipulate a dispute outcome. Include dispute, refund, and any admin-facing transaction override tools in the same scanning and review scope as checkout, rather than treating them as a lower-priority corner of the platform.

Walk through who can trigger a refund or override a dispute outcome, and confirm that action requires the same authorization checks as the transaction it's reversing. A refund path that's easier to abuse than the checkout flow it undoes is a gap worth closing before it gets found by someone testing the boundaries rather than by your own review.