flawopen.com/bola-idor/Python

● CWE-639 · 緊急
セキュリティ研究 · FlawOpen

Python におけるオブジェクトレベルの認可不備 (BOLA / IDOR) 対策

Python および FastAPI における BOLA (CWE-639) と IDOR 脆弱性の根本的対策。リクエストの ID を鵜呑みにせず、認証されたテナント ID をクエリに強制バインドする手法を解説。

💡 わかりやすい解説 (ELI5)

ホテルの 204 号室の鍵を受け取った客が、ドアの数字を 205 に書き換えるだけで隣室を開けられてしまう状態です。フロントは宿泊客であること（認証）は確認しましたが、その部屋の持ち主か（認可）を確認していませんでした。

主要な概念と専門用語

Authentication vs. Authorization
主要概念 (Authentication vs. Authorization)：Authentication confirms WHO the user is (valid login). Authorization confirms WHAT resources that specific user is permitted to view or modify.
BOLA (Broken Object Level Authorization)
主要概念 (BOLA (Broken Object Level Authorization))：The #1 OWASP API vulnerability where an endpoint exposes an object reference (e.g. /api/invoices/1042) without verifying tenant ownership.
IDOR (Insecure Direct Object Reference)
主要概念 (IDOR (Insecure Direct Object Reference))：The broader vulnerability class (CWE-639) where internal implementation objects are exposed directly to users without access control checks.
Tenancy Scope Injection
主要概念 (Tenancy Scope Injection)：An architectural pattern where database queries automatically enforce WHERE organization_id = :auth_user_org_id at the ORM layer.
Non-Enumerable Identifiers (UUIDv4)
主要概念 (Non-Enumerable Identifiers (UUIDv4))：Using cryptographically random 128-bit identifiers instead of sequential auto-incrementing integers (1, 2, 3...) to prevent predictable enumeration.

ステップ・バイ・ステップの攻撃フロー

Step 1

Legitimate Sign-In

An attacker creates a legitimate account on the platform and receives a valid JWT authentication bearer token.

Step 2

Resource Inspection

The attacker accesses their own billing invoice via GET /api/invoices/1042 and observes sequential database identifiers in use.

Step 3

Identifier Perturbation

The attacker modifies the URL to request GET /api/invoices/1041 using their own valid authentication token.

Step 4

Flawed Controller Logic

The FastAPI backend confirms the token is valid, but queries the database solely by Invoice.id == 1041 without verifying tenant ownership.

Step 5

Cross-Tenant Exfiltration

The server returns confidential invoice records, billing details, and personal data belonging to another organization.

Step 6

Automated Harvest

The attacker scripts a loop iterating over IDs 1 through 100,000, draining the entire multi-tenant database.

ソースコード比較：脆弱 vs 堅牢化

✕ 脆弱な実装
# VULNERABLE: Verifies user login, but queries object solely by client ID
from fastapi import FastAPI, Depends, HTTPException, status
from sqlalchemy.orm import Session

@app.get("/api/invoices/{invoice_id}")
def get_invoice(invoice_id: int, current_user: User = Depends(get_current_user), db: Session = Depends(get_db)):
    # Flaw: Attacker passes invoice_id belonging to another tenant
    invoice = db.query(Invoice).filter(Invoice.id == invoice_id).first()
    
    if not invoice:
        raise HTTPException(status_code=404, detail="Invoice not found")
        
    # Leaks confidential billing records of competitor organizations!
    return invoice
✓ 堅牢化されたセキュアパッチ
# HARDENED: Query is strictly bound to the authenticated tenant's organization ID
from fastapi import FastAPI, Depends, HTTPException, status
from sqlalchemy.orm import Session

@app.get("/api/invoices/{invoice_id}")
def get_invoice(invoice_id: int, current_user: User = Depends(get_current_user), db: Session = Depends(get_db)):
    # Defense-in-depth: query filters by BOTH invoice_id AND authenticated organization_id
    invoice = db.query(Invoice).filter(
        Invoice.id == invoice_id,
        Invoice.organization_id == current_user.organization_id
    ).first()
    
    if not invoice:
        # Return 404 rather than 403 to prevent object existence enumeration
        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="Invoice not found")
        
    return invoice

エンジニアリング＆システム堅牢化チェックリスト

References