Detecting Shadow API Endpoints
Overview
Shadow APIs are API endpoints operating within an organization's environment that are not tracked, documented, or secured. They emerge from rapid development cycles, forgotten test environments, deprecated API versions left running, third-party integrations, or developer side projects deployed without governance. Shadow APIs bypass authentication and monitoring controls, creating hidden entry points for attackers. Studies show that up to 30% of API endpoints in large organizations are undocumented, making shadow API detection a critical component of API security posture management.
When to Use
- When investigating security incidents that require detecting shadow api endpoints
- When building detection rules or threat hunting queries for this domain
- When SOC analysts need structured procedures for this analysis type
- When validating security monitoring coverage for related attack techniques
Prerequisites
- API gateway or reverse proxy with traffic logging (Kong, AWS API Gateway, Envoy)
- Network traffic capture capability (packet broker, port mirroring)
- Access to source code repositories and CI/CD pipeline configurations
- Cloud provider access for configuration scanning (AWS, GCP, Azure)
- API documentation inventory (OpenAPI specs, Swagger docs)
- Python 3.8+ for custom discovery tooling
Detection Methods
1. Traffic Analysis and Comparison
Compare live API traffic against documented OpenAPI specifications to identify undocumented endpoints:
#!/usr/bin/env python3
"""Shadow API Endpoint Detector
Compares observed API traffic patterns against documented
OpenAPI specifications to identify undocumented (shadow) endpoints.
"""
import json
import re
import yaml
import sys
from collections import defaultdict
from datetime import datetime
from typing import Dict, List, Set, Tuple, Optional
from dataclasses import dataclass, field
@dataclass
class DiscoveredEndpoint:
method: str
path_pattern: str
first_seen: str
last_seen: str
request_count: int
source_ips: Set[str] = field(default_factory=set)
status_codes: Set[int] = field(default_factory=set)
has_auth_header: bool = False
documented: bool = False
class ShadowAPIDetector:
# Common patterns for parameterized path segments
PARAM_PATTERNS = [
(re.compile(r'/\d+'), '/{id}'),
(re.compile(r'/[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}'), '/{uuid}'),
(re.compile(r'/[a-zA-Z0-9]{20,40}'), '/{token}'),
]
def __init__(self):
self.documented_endpoints: Set[Tuple[str, str]] = set()
self.discovered_endpoints: Dict[Tuple[str, str], DiscoveredEndpoint] = {}
def load_openapi_spec(self, spec_path: str):
"""Load documented endpoints from OpenAPI specification."""
with open(spec_path, 'r') as f:
if spec_path.endswith('.json'):
spec = json.load(f)
else:
spec = yaml.safe_load(f)
paths = spec.get('paths', {})
for path, methods in paths.items():
# Normalize OpenAPI path parameters
normalized_path = re.sub(r'\{[^}]+\}', '{id}', path)
for method in methods:
if method.upper() in ('GET', 'POST', 'PUT', 'DELETE', 'PATCH', 'HEAD', 'OPTIONS'):
self.documented_endpoints.add((method.upper(), normalized_path))
print(f"Loaded {len(self.documented_endpoints)} documented endpoints from {spec_path}")
def normalize_path(self, path: str) -> str:
"""Normalize an observed path by replacing dynamic segments with placeholders."""
# Remove query string
path = path.split('?')[0]
for pattern, replacement in self.PARAM_PATTERNS:
path = pattern.sub(replacement, path)
return path
def process_access_log(self, log_file: str, log_format: str = "common"):
"""Process API access logs to discover endpoints."""
patterns = {
"common": re.compile(
r'(?P<ip>[\d.]+)\s+\S+\s+\S+\s+\[(?P<time>[^\]]+)\]\s+'
r'"(?P<method>\w+)\s+(?P<path>\S+)\s+\S+"\s+(?P<status>\d+)'
),
"json": None # Handle JSON logs separately
}
with open(log_file, 'r') as f:
for line in f:
if log_format == "json":
try:
entry = json.loads(line)
method = entry.get('method', entry.get('http_method', ''))
path = entry.get('path', entry.get('uri', ''))
status = int(entry.get('status', entry.get('status_code', 0)))
ip = entry.get('remote_addr', entry.get('client_ip', ''))
timestamp = entry.get('timestamp', entry.get('@timestamp', ''))
has_auth = bool(entry.get('authorization', entry.get('auth_header', '')))
except json.JSONDecodeError:
continue
else:
match = patterns[log_format].match(line)
if not match:
continue
method = match.group('method')
path = match.group('path')
status = int(match.group('status'))
ip = match.group('ip')
timestamp = match.group('time')
has_auth = 'Authorization' in line
# Only process API paths
if not path.startswith('/api') and not path.startswith('/v'):
continue
normalized = self.normalize_path(path)
key = (method.upper(), normalized)
if key not in self.discovered_endpoints:
self.discovered_endpoints[key] = DiscoveredEndpoint(
method=method.upper(),
path_pattern=normalized,
first_seen=timestamp,
last_seen=timestamp,
request_count=0,
documented=(key in self.documented_endpoints)
)
endpoint = self.discovered_endpoints[key]
endpoint.request_count += 1
endpoint.last_seen = timestamp
endpoint.source_ips.add(ip)
endpoint.status_codes.add(status)
if has_auth:
endpoint.has_auth_header = True
def identify_shadow_apis(self) -> List[DiscoveredEndpoint]:
"""Identify endpoints that are not in the documented specification."""
shadows = []
for key, endpoint in self.discovered_endpoints.items():
if not endpoint.documented:
shadows.append(endpoint)
# Sort by request count descending (most active shadows first)
shadows.sort(key=lambda e: e.request_count, reverse=True)
return shadows
def classify_risk(self, endpoint: DiscoveredEndpoint) -> str:
"""Classify the risk level of a shadow endpoint."""
risk_score = 0
# No authentication observed
if not endpoint.has_auth_header:
risk_score += 3
# High traffic volume
if endpoint.request_count > 1000:
risk_score += 2
elif endpoint.request_count > 100:
risk_score += 1
# Multiple source IPs (wider exposure)
if len(endpoint.source_ips) > 10:
risk_score += 2
# Successful responses (endpoint is functional)
if 200 in endpoint.status_codes or 201 in endpoint.status_codes:
risk_score += 1
# Write operations are higher risk
if endpoint.method in ('POST', 'PUT', 'DELETE', 'PATCH'):
risk_score += 2
# Sensitive path patterns
sensitive_patterns = ['admin', 'internal', 'debug', 'test', 'backup',
'config', 'health', 'metrics', 'graphql', 'console']
for pattern in sensitive_patterns:
if pattern in endpoint.path_pattern.lower():
risk_score += 3
break
if risk_score >= 8:
return "CRITICAL"
elif risk_score >= 5:
return "HIGH"
elif risk_score >= 3:
return "MEDIUM"
return "LOW"
def generate_report(self) -> dict:
"""Generate a comprehensive shadow API discovery report."""
shadows = self.identify_shadow_apis()
total_documented = len(self.documented_endpoints)
total_discovered = len(self.discovered_endpoints)
report = {
"scan_date": datetime.now().isoformat(),
"summary": {
"documented_endpoints": total_documented,
"total_discovered_endpoints": total_discovered,
"shadow_endpoints": len(shadows),
"shadow_ratio": f"{len(shadows)/max(total_discovered,1)*100:.1f}%",
},
"shadow_endpoints": []
}
for endpoint in shadows:
risk = self.classify_risk(endpoint)
report["shadow_endpoints"].append({
"method": endpoint.method,
"path": endpoint.path_pattern,
"risk_level": risk,
"request_count": endpoint.request_count,
"unique_sources": len(endpoint.source_ips),
"authenticated": endpoint.has_auth_header,
"status_codes": sorted(endpoint.status_codes),
"first_seen": endpoint.first_seen,
"last_seen": endpoint.last_seen,
})
return report
def main():
detector = ShadowAPIDetector()
# Load documented API specifications
spec_files = sys.argv[1:] if len(sys.argv) > 1 else ["openapi.yaml"]
for spec in spec_files:
if spec.endswith(('.yaml', '.yml', '.json')):
detector.load_openapi_spec(spec)
# Process access logs
detector.process_access_log("/var/log/api/access.log")
report = detector.generate_report()
print(f"\n{'='*60}")
print(f"SHADOW API DISCOVERY REPORT")
print(f"{'='*60}")
print(f"Documented: {report['summary']['documented_endpoints']}")
print(f"Discovered: {report['summary']['total_discovered_endpoints']}")
print(f"Shadow: {report['summary']['shadow_endpoints']} ({report['summary']['shadow_ratio']})")
print()
for ep in report["shadow_endpoints"]:
risk_marker = {"CRITICAL": "[!!!]", "HIGH": "[!!]", "MEDIUM": "[!]", "LOW": "[.]"}
print(f" {risk_marker.get(ep['risk_level'], '[?]')} {ep['method']} {ep['path']}")
print(f" Risk: {ep['risk_level']} | Requests: {ep['request_count']} | Auth: {ep['authenticated']}")
# Save full report
with open("shadow_api_report.json", "w") as f:
json.dump(report, f, indent=2, default=str)
print(f"\nFull report saved to shadow_api_report.json")
if __name__ == "__main__":
main()2. Cloud Configuration Scanning
# AWS: Discover API Gateway endpoints not in documentation
aws apigateway get-rest-apis --query 'items[*].[name,id]' --output table
# List all routes for each API
aws apigatewayv2 get-apis --query 'Items[*].[Name,ApiId,ProtocolType]' --output table
# AWS Lambda function URLs (potential shadow APIs)
aws lambda list-function-url-configs --function-name "*" 2>/dev/null
# Find ALB listener rules routing to undocumented backends
aws elbv2 describe-rules --listener-arn $LISTENER_ARN \
--query 'Rules[*].[Priority,Conditions[0].Values[0],Actions[0].TargetGroupArn]'3. Source Code Repository Mining
# Search for undocumented route definitions in source code
# Express.js routes
grep -rn "app\.\(get\|post\|put\|delete\|patch\)" --include="*.js" --include="*.ts" src/
# Flask/Django routes
grep -rn "@app\.route\|@api\.route\|path(" --include="*.py" src/
# Spring Boot endpoints
grep -rn "@\(Get\|Post\|Put\|Delete\|Patch\)Mapping\|@RequestMapping" --include="*.java" src/
# Compare found routes against OpenAPI specification
diff <(grep -roh "'/api/[^']*'" src/ | sort -u) \
<(yq '.paths | keys[]' openapi.yaml | sort -u)Prevention and Governance
API Registration Gateway Policy
# Kong plugin configuration - reject unregistered routes
plugins:
- name: request-validator
config:
allowed_content_types:
- application/json
body_schema: null
- name: pre-function
config:
access:
- |
-- Block requests to unregistered endpoints
local registered = kong.cache:get("registered_endpoints")
local path = kong.request.get_path()
local method = kong.request.get_method()
local key = method .. ":" .. path
if not registered[key] then
kong.log.warn("Shadow API access attempt: ", key)
return kong.response.exit(404, {error = "Endpoint not registered"})
endReferences
- APIsec Shadow API Best Practices: https://www.apisec.ai/blog/secure-your-shadow-apis-best-practices-for-api-discovery
- Wiz Shadow API Guide: https://www.wiz.io/academy/api-security/shadow-api
- Checkmarx Shadow and Zombie APIs: https://checkmarx.com/learn/api-security/shadow-zombie-apis-undocumented-api-vulnerabilities-threaten-security-posture/
- Treblle Shadow API Tools: https://treblle.com/blog/top-tools-for-detecting-shadow-apis-and-how-treblle-differs
- SecureLayer7 Shadow APIs: https://blog.securelayer7.net/shadow-apis-explained-risks-detection-and-prevention/