Tests API rate limiting for bypass vulnerabilities using Python (requests/aiohttp) and Burp Suite Turbo Intruder to manipulate headers (e.g. X-Forwarded-For spoofing), IPs, HTTP methods, API versions, and encodings, mapping findings to OWASP API4:2023 Unrestricted Resource Consumption. Use when assessing, under written authorization, whether rate limits can be bypassed to enable brute force or resource-exhaustion attacks.
Tests API rate limiting for bypass vulnerabilities using Python (requests/aiohttp) and Burp Suite Turbo Intruder to manipulate headers (e.g. X-Forwarded-For spoofing), IPs, HTTP methods, API versions, and encodings, mapping findings to OWASP API4:2023 Unrestricted Resource Consumption. Use when assessing, under written authorization, whether rate limits can be bypassed to enable brute force or resource-exhaustion attacks.
Controlling the number of requests a client can make to an API within a time window, typically enforced per IP, per user, or per API key
Unrestricted Resource Consumption
OWASP API4:2023 - APIs that do not properly limit the size or number of resources requested, enabling DoS or brute force attacks
X-Forwarded-For Spoofing
Manipulating the X-Forwarded-For header to make the server believe requests originate from different IP addresses, bypassing IP-based rate limits
Credential Stuffing
Automated injection of stolen username/password pairs against login endpoints, requiring rate limit bypass for large-scale attacks
Token Bucket
Rate limiting algorithm that allows bursts of requests up to a bucket size, refilling at a constant rate
Sliding Window
Rate limiting algorithm that tracks requests in a rolling time window, more resistant to burst attacks than fixed windows
Tools & Systems
Burp Suite Turbo Intruder: High-performance request sender for rate limit testing using Python-based scripting engine
ffuf: Fast web fuzzer capable of testing rate limits with configurable request rates and header manipulation
wfuzz: Web fuzzer with support for header injection, parameter fuzzing, and rate limit evasion techniques
Postman Collection Runner: Automated collection execution with variable rotation for rate limit bypass testing
Gatling/k6: Load testing tools that simulate realistic traffic patterns to test rate limiting under production-like conditions
Common Scenarios
Scenario: Login API Rate Limit Bypass Assessment
Context: A financial services API implements rate limiting on the login endpoint to prevent brute force attacks. The security team wants to verify the effectiveness of these controls before a compliance audit.
Approach:
Baseline: Send 100 requests to POST /api/v1/auth/login - rate limited at request 10 per minute per IP
Test X-Forwarded-For rotation: Send 100 requests with unique X-Forwarded-For values - rate limit bypassed (all requests return 401, not 429)
Test path variation: /api/v1/auth/login/ (trailing slash) resets the rate limit counter
Test API versioning: /api/v2/auth/login has no rate limiting configured (shadow API)
Test parameter pollution: Adding ?_=<random> to each request bypasses the rate limit
Test concurrent requests: 50 simultaneous requests from same IP - 45 succeed before rate limit kicks in (race condition in counter)
Determine that rate limiting is implemented at the nginx reverse proxy level using IP-only tracking, trusting X-Forwarded-For header without validation
Pitfalls:
Sending too many requests too fast and causing actual denial of service to the test environment
Not testing rate limits on password reset, MFA verification, and account enumeration endpoints
Assuming the rate limit applies globally when it may be per-endpoint or per-method only
Missing race conditions in rate limit counters that allow burst bypasses
Not testing both authenticated and unauthenticated rate limiting separately
Output Format
## Finding: Rate Limiting Bypass via X-Forwarded-For Header Spoofing
**ID**: API-RATE-001
**Severity**: High (CVSS 7.3)
**OWASP API**: API4:2023 - Unrestricted Resource Consumption
**Affected Endpoints**:
- POST /api/v1/auth/login
- POST /api/v1/auth/forgot-password
- POST /api/v1/auth/verify-mfa
**Description**:
The API rate limiting implementation relies on the X-Forwarded-For header
to identify client IP addresses. Since the application sits behind a load
balancer that does not strip or validate this header, an attacker can set
arbitrary X-Forwarded-For values to bypass the 10 requests/minute rate limit
on authentication endpoints.
**Bypass Methods Confirmed**:
1. X-Forwarded-For rotation: 1000 login attempts in 60 seconds (vs 10 limit)
2. Trailing slash path variation: /auth/login/ treated as separate endpoint
3. API v2 endpoint: No rate limiting configured
4. Race condition: 50 concurrent requests, 45 succeed before counter updates
**Impact**:
An attacker can perform unlimited brute force attacks against any user
account, bypassing the rate limit designed to prevent credential stuffing.
At 1000 attempts per minute, a 6-digit PIN can be brute-forced in under
17 minutes.
**Remediation**:
1. Configure the load balancer to set X-Forwarded-For and strip client-provided values
2. Implement rate limiting at the application layer using authenticated user ID, not just IP
3. Normalize URL paths before applying rate limit rules (strip trailing slashes, enforce lowercase)
4. Apply rate limits consistently across all API versions and content types
5. Use atomic rate limit counters (Redis INCR) to prevent race conditions
6. Implement progressive delays (exponential backoff) in addition to hard limits
