What API Developers Need to Know About PCI DSS and Financial API Security

How much money do you think the hackers made?

Not from one transaction or from one card.

From scale.

Back in 2014, I worked in hospitality (don’t you check my LinkedIn now).
Restaurants. Bars. Retail kiosks. And every night, hundreds of card transactions flowed through POS machines like clockwork.

Tap.
Insert.
Approved.
Print receipt.
Smile.

All of the transactions went through Oracle MICROS POS systems. “Industry standard”. “Trusted”.

Until 2016, when attackers planted malicious code inside the MICROS support portal.
Not the register itself — the support plumbing behind it.

Credentials could be intercepted. Passwords reset. Trust boundaries quietly bent.

Get this: if even a fraction of transactions across thousands of locations were compromised then… what’s the number?

You don’t need to steal $1,000 from each card.

You steal a few cents.
Across millions of transactions.
For months.

That’s not hacking for drama.
That’s hacking for math.

It was like yesterday. I remember taking calls as a helpdesk support agent — and I only heard about it through hushed conversations between the IT directors.
Low voices. Tight faces. Damage control mode. tut tut tut.

Now I wonder: was it responsibly disclosed to every affected customer and stakeholder?
I don’t remember seeing anything proportionate to the risk.

So it begs the question:

How much did the hackers actually make?

Because here’s the part people forget:

PCI DSS already existed back then. It’s been around since 2004. By 2016, version 3.x was active.

The rules were not new. The risk was not theoretical.

So what went wrong?

Read: Micros POS systems exposed by malware attack

Looking at it through PCI DSS

If your API stores, processes, or transmits cardholder data, it is part of the Cardholder Data Environment (CDE). No loopholes. No creative interpretation.

Let’s take three PCI DSS requirements that hit financial APIs directly.

Requirement 3 — Protect Stored Cardholder Data

API risk: exposing PAN or CVV in responses, logs, or databases.

In POS malware cases, attackers scrape memory to capture raw card data before encryption is applied. In APIs, the modern equivalent is returning more data than necessary or storing raw card details “just in case.”

Security control:

• Tokenization instead of storing PAN
• Encryption at rest
• Never storing CVV
• Masking card numbers in logs and responses

If your API returns full card data for convenience, you’re not building features. You’re building evidence.

Requirement 7 — Restrict Access by Need-to-Know

API risk: Broken Object Level Authorization (BOLA).

User A modifies an ID parameter and suddenly sees User B’s transaction history. That’s not a bug. That’s a breach.

Security control:

• Strict role-based access control
• Explicit ownership validation on every request
• Backend authorization checks (never trust the frontend)

If your API trusts what the client says, congratulations — so will the attacker.

Requirement 10 — Logging and Monitoring

API risk: undetected abuse of financial endpoints.

Slow scraping. Refund manipulation. Transaction enumeration. Quiet abuse that blends into normal traffic.

Security control:

• Structured audit logs
• Identity embedded in every log entry
• Alerting on abnormal patterns
• Tamper-resistant logging pipelines

Logging isn’t there to satisfy auditors.

It’s there so you know when you’re being drained in slow motion.

Why PCI DSS matters for APIs

PCI DSS forces developers to design with paranoia.

It assumes:

• Tokens may leak
• Endpoints will be probed
• Sensitive data must be minimized
• Abuse is not hypothetical

Compliance changes API design:

• Minimal response schemas
• Explicit authorization checks
• TLS only
• Encryption and key management discipline
• API gateways to centralize enforcement

Financial APIs are not CRUD services.

They are money pipelines.

And money attracts math-minded criminals.

The part I wish I’d done differently

If I knew then what I know now, I would have walked straight into that IT conversation.

I would have asked why we were whispering instead of escalating.

I would have reminded them that PCI DSS non-compliance isn’t just embarrassing — it can mean fines from card brands, thousands per month.

Forensic audits. Increased transaction fees. Mandatory remediation.

In extreme cases, losing the ability to process card payments altogether.

Which, in hospitality, is a polite way of saying: game over.

I would have told them that protecting cardholder data isn’t about optics.

It’s about responsibility.

And yes, I probably would have lost my job right there and then.

But I’d have a much more interesting version of this story now, wouldn’t I?

Until we shift security far left — designing retail and payment systems assuming abuse from day one — we will keep reacting instead of preventing.

And in financial systems, reacting is always more expensive.

Always.