How API Security Works

APIs are now the top attack vector, facing 68% more threats per host and 16 times more DDoS traffic than traditional web apps. These findings come from the State of Application Security 2024 report, based on analysis of over 2 billion API attacks blocked by AppTrana WAAP.

As APIs become critical to digital transformation, understanding how to secure them—beyond just authentication—is no longer optional.

This blog breaks down how API security works, how WAAPs protect APIs in real time, and what makes protection effective at scale.

What Is API Security?

API Security is the practice of protecting Application Programming Interfaces (APIs) from misuse, abuse, and attacks. In a digital landscape where APIs power everything from mobile apps to enterprise integrations, unsecured APIs can expose sensitive data and business logic to attackers.

API security ensures that APIs only process valid, authenticated, and authorized requests while preventing data breaches, downtime, and unauthorized access.

How Does API Security Work?

API security works by combining proactive controls across the API lifecycle:

• Discovery: Identifies all APIs, including undocumented and shadow APIs
• Validation: Ensures inputs and access patterns conform to expected behavior
• Authentication & Authorization: Enforces identity and access policies
• Threat Detection: Monitors traffic for abuse, anomalies, and known attacks
• Protection: Blocks malicious requests in real-time with minimal false positives

These controls are implemented through gateways, proxies, or fully managed WAAP (Web Application & API Protection) platforms.

API Protection Methods

Effective API security relies on layered detection strategies that combine rule-based enforcement, behavioral learning, and threat intelligence. Each model addresses different attack surfaces and use cases:

1. Positive Security Model (Allow Known Good)

This model only allows requests that strictly adhere to a predefined API schema, such as OpenAPI or Swagger documentation. If a request includes unexpected fields, uses unsupported HTTP methods (e.g., PUT instead of POST), or exceeds defined data limits, it is immediately blocked.

Example:
An API endpoint /api/user/create expects { “name”: “John”, “email”: “john@example.com” }. If a client sends an extra field like “isAdmin”: true, the request is dropped to prevent privilege escalation through mass assignment.

Use Case:
Best for modern applications with documented APIs where security can be automated through schema validation.

2. Negative Security Model (Block Known Bad)

This model scans traffic for known malicious signatures such as SQL injection (‘ OR 1=1–), cross-site scripting (<script>), or directory traversal patterns(../../etc/passwd). It leverages rule sets and threat intel feeds to block traffic matching these patterns.

Example:
If an attacker attempts to exploit an API search endpoint with a payload like “?query=<script>alert(‘XSS’)</script>”, the WAAP recognizes it and blocks the request instantly.

Use Case:
Ideal for APIs where documentation is lacking or legacy applications where you can’t easily enforce strict schema validation.

3. Behavioral & AI-Based Detection

Behavioral models continuously monitor API usage patterns—learning how clients, mobile apps, or third-party integrations normally interact with each endpoint. They then detect anomalies such as sudden traffic spikes, misuse of low-volume endpoints, or automated access patterns indicative of credential stuffing or scraping.

Example:
A login API typically receives 5 requests per user per day. If a user suddenly makes 500 login attempts within 3 minutes from different IPs, behavioral detection kicks in to block the attack despite the requests being technically valid.

Use Case:
Crucial for detecting business logic abuse, bot-driven attacks, and zero-day exploits that do not match known signatures.

Deployment Options

API security can be implemented at multiple layers within the API delivery stack. The right deployment model depends on your architecture, scale, and security priorities. Below are the most common deployment options:

1. API Gateways

Security policies can be enforced directly within gateways like Kong, AWS API Gateway, Apigee, or Azure API Management. These gateways inspect traffic, apply rate limits, validate tokens, and perform basic anomaly detection.

• Best for: Centralized control in monolithic or lightly distributed architectures
• Limitation: Often lacks advanced detection (e.g., behavioral learning, zero-day mitigation)

A detailed breakdown is available in Is an API Gateway Enough for API Security?

2. WAAP Platforms (Web Application & API Protection)

WAAPs like AppTrana provide full-spectrum protection by combining WAF, API security, DDoS protection, bot mitigation, and vulnerability scanning in a single platform. They can be deployed in-line (reverse proxy) or in front of API gateways.

• Best for: Unified protection across web and API layers
• Limitation: May require configuration or tuning for custom APIs, unless fully managed

3. Sidecar Proxies & Service Meshes

With architectures like Istio, Linkerd, or Envoy, API traffic is intercepted within each microservice using a sidecar proxy. This allows fine-grained access control and local traffic inspection per service.

• Best for: Fine-grained security in microservices or containerized environments
• Limitation: Complex to operate at scale, often requires devops and mesh orchestration expertise

4. Cloud-Delivered API Security

These solutions (like AppTrana in SaaS mode) offer agentless deployment by routing API traffic through an external WAAP. Ideal for hybrid environments or where APIs are hosted across multiple clouds and data centers.

• Best for: Quick, scalable protection with minimal dev effort
• Limitation: Requires DNS changes or traffic redirection for inline enforcement

API Security: Comparing API Gateways, WAAPs and Other Methods

Deployment Option	Strengths	Weaknesses	Best Suited For
API Gateway Integration	Native to API stack, supports token validation & rate limiting	Limited protection against zero-day or behavioral threats	Simpler architectures, early-stage APIs
WAAP Platforms	Unified protection, advanced detection, DDoS/bot/Vuln scanning	May need upfront integration unless fully managed	Enterprises with public-facing apps/APIs
Sidecar Proxies / Service Mesh	Fine-grained policy enforcement per service	High complexity, ops overhead	Large-scale microservices / Kubernetes
Cloud-Delivered API Security	Easy to deploy, covers distributed APIs, minimal integration	Needs DNS changes or network rerouting	Multi-cloud / hybrid environments

 

Operational Modes in API Security

API security is not a one-size-fits-all solution. Depending on the maturity of your API ecosystem, traffic patterns, and risk appetite, different enforcement modes can be applied. Leading API security platforms, including WAAPs, typically support the following modes:

1. Monitor Mode or Log Mode

This passive mode observes API traffic without blocking any requests. Use this mode for learning and observation. It logs behavior, identifies anomalies, and helps build a baseline of legitimate usage patterns.

When to use:

• During the initial rollout of API security tools
• While onboarding new APIs or third-party integrations
• To assess undocumented or shadow APIs before applying enforcement

Key advantages:

• No impact on application functionality
• Ideal for tuning detection models and security rules before blocking is enabled

2. Block Mode

In block mode, the system proactively enforces security policies. Use this mode to actively enforce security policies. It filters out malicious requests based on schema validation, behavioral analysis, known signatures, or access violations.

When to use:

• On stable, well-documented APIs
• For critical endpoints such as login, account updates, or payment processing
• After sufficient observation in monitor mode confirms safe patterns

Key advantages:

• Provides real-time protection against threats
• Helps reduce the risk of data breaches and account compromise

3. Rate Limiting and Throttling

This mode enforces limits on how frequently a user, IP, or token can make API requests. It prevents misuse, resource exhaustion, and abuse from bots or automated scripts.

When to use:

• To protect high-risk endpoints like login or search from brute-force attacks.
• For public APIs that serve a broad range of users or third-party clients
• To manage backend load and ensure fair usage among tenants

Key advantages:

• Prevents volumetric abuse without complex payload inspection
• Shields applications from performance degradation caused by abusive clients

Threats Mitigated by API Security

API Security solutions protect against a wide range of threats, including:

• Broken Object Level Authorization (BOLA)
• Mass Assignment & Parameter Tampering
• Excessive Data Exposure
• SQL/Command/JSON Injection Attacks
• Zombie & Shadow APIs
• Credential Stuffing & Account Takeover
• DDoS and Bot Abuse via APIs
• Improper Asset Management

For more details on how these threats are blocked, read this detailed blog on API vulnerabilities and how to fix them.

9 Methods WAAPs Use to Block API Attacks

Modern Web Application and API Protection (WAAP) platforms use a layered defense approach to secure APIs. These methods allow WAAPs to detect and block malicious requests in real time, without interrupting legitimate traffic.

1. Strict Schema Validation

This method enforces that all API requests match a predefined schema. The WAAP checks for required fields, accepted data types, allowed HTTP methods, and field constraints.

Example:
A profile update API expects:

{
"username": "user123",
"email": "user@example.com"
}
{
"username": "user123",
"email": "user@example.com"
}

If a request includes a field such as “isAdmin”: true or sends the email in an invalid format, the WAAP rejects it immediately. This protects against mass assignment and format-based injection attacks.

Strict schema validation is highly effective when APIs are documented and versioned. It provides a strong first line of defense against malformed or manipulated payloads.

2. Rate Limiting and Throttling

WAAPs enforce usage limits per API key, IP address, or user account. This helps contain abuse from bots, credential stuffing tools, or unintentional spikes in traffic.

Example:
A login API may allow five requests per minute per user. If a client exceeds that threshold, the WAAP responds with a rate limit error. This reduces the risk of brute force attacks and ensures backend stability.

Rate limits can be fine-tuned per endpoint and user role to allow legitimate traffic while stopping abuse.

3. Access Control Enforcement

WAAPs verify that each request is properly authenticated and authorized. This includes checking session tokens, validating OAuth scopes, and enforcing object-level access control.

Example:
A user calls the endpoint /api/order/12345 to view an order. If they do not own the order, the WAAP denies the request. This prevents Broken Object Level Authorization (BOLA) vulnerabilities, which are among the most common API risks.

Access control enforcement requires context, such as user identity, role, and ownership, and works best when integrated with identity providers or backend authorization logic.

4. Signature-Based Detection

This approach blocks traffic that matches known attack patterns. WAAPs use a library of signatures to detect SQL injection, cross-site scripting, command injection, and other known exploits.

Example:
If a search API receives a query like “‘ OR 1=1 –“, the WAAP detects it as a SQL injection attempt and blocks it before it reaches the application.

Signature-based detection is most effective when used alongside schema validation. It helps protect legacy APIs or undocumented endpoints that cannot be validated strictly.

5. Positive Security Enforcement

In contrast to blocking known bad traffic, positive security only allows traffic that is explicitly permitted. This means only expected fields, formats, and values are accepted.

Example:
An inventory API expects a product type to be one of [“ELECTRONICS”, “CLOTHING”, “HOME”]. A request with “productType”: “UNKNOWN” is blocked, even if the field is present and correctly formatted.

Positive security enforcement is useful for APIs that have well-defined behavior, particularly in regulated or sensitive environments.

6. Behavioral Anomaly Detection

WAAPs use behavioral models to detect deviations from normal usage. These models are trained on historical traffic and continuously updated as patterns evolve.

Example:
An API normally receives 100 requests per hour from a user. If the same user suddenly makes 10,000 requests in five minutes, the WAAP flags this behavior as anomalous and triggers a block or challenge.

This method is effective against scraping, credential stuffing, and business logic abuse, even when the requests look technically valid.

7. Token and Session Validation

WAAPs verify the integrity and legitimacy of API tokens, including OAuth tokens, API keys, and JWTs. They also check for token expiration, misuse, or replay.

Example:
If a user reuses an expired JWT or tampers with the token payload, the WAAP detects the inconsistency and denies access. It may also limit the number of tokens generated per user to prevent abuse.

Token validation is a key control for preventing session hijacking, privilege escalation, and unauthorized access.

8. Bot and DDoS Protection

WAAPs leverage behavioral analysis, client fingerprinting, and threat intelligence to identify automated traffic. They challenge or block requests that originate from botnets, headless browsers, or script-based tools.

Example:
If a bot floods an API with requests to scrape pricing data or overload the search endpoint, the WAAP identifies the non-human interaction and applies rate limits, CAPTCHA challenges, or outright blocks.

This capability helps maintain API performance and availability during both volumetric and low-and-slow DDoS attacks

9. Real-Time Threat Intelligence

WAAPs leverage global IP reputation data and real-time attack feeds to block traffic from known malicious sources.

Example:
If a known malicious IP address or TOR exit node attempts to access a login API, the WAAP blocks the request immediately, even if the payload appears normal.

This method adds an external layer of situational awareness, helping organizations stay ahead of global attack trends.

How AppTrana WAAP Secures Your APIs

Indusface AppTrana goes beyond traditional WAFs by offering complete, fully managed API security that works out of the box without tuning, false positives, or developer effort.

Here’s how AppTrana secures APIs:

• API Discovery and Classification: Automatically identifies all API endpoints, whether documented or shadow, across your environment.
• Continuous API Documentation: Keeps API definitions up to date by monitoring traffic in real-time, helping eliminate zombie and outdated APIs.
• Positive Security Model Enforcement: Automatically applies schema validation based on learned behavior or uploaded OpenAPI specs to prevent abuse and injection attacks.
• API Vulnerability Scanning & Real-Time Remediation: Continuously scans for vulnerabilities in APIs and blocks exploitation instantly, without waiting for code fixes.
• Behavioral DDoS & Bot Mitigation: Combines AI and human verification to protect APIs from volumetric, slow-rate, and automation-based attacks, backed by an SLA-guaranteed 100 percent availability.

With AppTrana WAAP, security teams get complete visibility, continuous protection, and autonomous remediation so they can focus on innovation, not incident response.

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