Table of Contents The 5 Deadliest REST API Security Flaws and Their Fixes
Intro
APIs run everything today — banking, healthcare, retail, logistics, insurance, SaaS. At Valency Networks, we have tested thousands of APIs during VAPT projects. The scary part? Too many are dangerously exposed.
Reports show that 40% of data breaches involve insecure APIs, and API-related attacks are rising by 700% year over year. We see this daily. Weak APIs are the fastest way for attackers to bypass your security stack.
Here are the top 5 deadly flaws we keep finding. Each comes with an explanation, a real-world example, and the fix we recommend.
1. Broken Authentication
What it is:
Authentication flaws let attackers impersonate valid users. This happens when tokens are poorly managed, passwords are weak, or sessions never expire. APIs that skip proper authentication checks essentially hand over access keys to anyone who tries hard enough.
Example:
In a healthcare API we tested, access tokens had no expiry. Once an attacker captured it, they could impersonate doctors indefinitely and pull patient records on demand. No alerts, no blocks. That’s a compliance and privacy nightmare.
Our Fix (Valency Approach):
We simulate token replay, brute-force, and JWT tampering during VAPT. Our recommendation is always the same: use short-lived JWTs, enforce rotation, add MFA for sensitive roles, and log every authentication failure. If attackers can’t stay authenticated, they can’t stay inside.
2. Excessive Data Exposure
What it is:
APIs often return entire objects instead of only the required fields. This gives attackers insider-level visibility into fields they shouldn’t see — hashed passwords, internal flags, backend IDs, pricing structures. It’s basically data leakage built into the design.
Example:
In a retail API, we saw product details being fetched with all backend metadata included — discount codes, inventory flags, even hashed passwords of linked customer accounts. All an attacker had to do was parse the JSON and pick targets.
Our Fix (Valency Approach):
We fuzz API responses to extract extra fields and prove exposure. We advise devs to adopt “minimum necessary data” policies and whitelist fields per endpoint. At VAPT, when we show management how their APIs are oversharing, it usually results in immediate code changes.
3. No Rate Limiting
What it is:
Without limits, attackers can hammer APIs with brute force, credential stuffing, or even denial-of-service attempts. Think of it as leaving your login screen open to an infinite number of password guesses per second.
Example:
In a banking API, login attempts were unrestricted. We could script thousands of PIN attempts without being slowed down. A motivated attacker could empty accounts just by running brute-force tools overnight.
Our Fix (Valency Approach):
We run stress tests and scripted brute force during API VAPT to prove the risk. The solution is simple but often ignored: enforce request throttling, set quotas per user/IP, and alert on anomalies. Without it, APIs collapse under both hackers and bots.
4. Injection Attacks (SQL / NoSQL / Command)
What it is:
Injection flaws occur when user inputs are passed directly into queries or commands without sanitization. For APIs, this means attackers can tamper with SQL, NoSQL, LDAP, or OS commands. The impact is total compromise — data theft, manipulation, or system takeover.
Example:
In a logistics API, shipment tracking numbers were directly concatenated into SQL queries. By modifying the request, we accessed shipment data for other companies. With another payload, we altered records, proving that attackers could manipulate logistics operations.
Our Fix (Valency Approach):
We craft manual injection payloads — not just rely on scanners. Our fixes involve parameterized queries, ORM security features, strict input validation, and WAF rules tuned for API traffic. When we show a customer how one input can rewrite their database, the urgency becomes clear.
5. Insecure Direct Object References (IDOR)
What it is:
IDOR flaws happen when APIs expose object identifiers (like account IDs, policy IDs, invoice numbers) without proper authorization checks. Attackers can modify IDs in the request and gain access to data that isn’t theirs.
Example:
In an insurance API, changing the policy ID in a request URL gave us access to another customer’s claim documents. No special skills needed — just editing a number in the request. That’s data leakage at scale.
Our Fix (Valency Approach):
We exploit IDORs during VAPT by incrementing or guessing IDs and showing customers the real impact — cross-account access. The fix is enforcing authorization at the object level for every request. Login checks alone don’t cut it.
Bonus: What Most Companies Miss
Beyond the top 5 flaws, we repeatedly see the same blind spot — no monitoring and no logging.
APIs get hit every minute. Attackers probe endpoints, test payloads, and try brute force constantly. Yet most companies have zero visibility into this. No logs, no alerts, no detection. By the time a breach is discovered, it’s weeks later.
Our Fix (Valency Approach):
During VAPT, we not only attack the API but also check whether the attacks trigger any alerts. In many cases, customers realize their SIEM, API gateway, or WAF never noticed a thing.
We recommend central logging, anomaly detection, and active monitoring of API traffic. Security is not just about preventing attacks — it’s also about knowing when you are under attack.
Don’t wait for attackers to find your weak APIs — let Valency Networks break them first, so we can help you fix them.
Summary
These are not theoretical problems. We find them daily across healthcare, banking, retail, logistics, and insurance systems. APIs are direct gateways into business logic, and attackers know it.
At Valency Networks, our API VAPT goes deep into these flaws. We think like attackers, test like hackers, and secure like defenders. That’s how our customers get peace of mind before the criminals strike.
If your business runs APIs — and it does — securing them is not optional. It’s survival.
