Code Audit: Comprehensive FastAPI Backend Security and Architecture Analysis

[Copilot]

This PR provides a comprehensive code audit of the FastAPI backend implementation, identifying critical security vulnerabilities and architectural inconsistencies that need immediate attention.

Critical Security Issues Identified

🔴 Tenant Isolation Vulnerability

The authentication system has a critical security flaw in app/api/deps.py where the tenant ID is hardcoded:

# SECURITY BUG: Hardcoded tenant_id bypasses multi-tenancy
user = get_user_by_id(db, int(token_data.sub), tenant_id=1)

This completely breaks tenant isolation, allowing users to potentially access data from other tenants.

🔴 CORS Misconfiguration

The CORS middleware in app/main.py is overly permissive:

app.add_middleware(
    CORSMiddleware,
    allow_origins=["*"],  # Allows ANY origin - security risk
    allow_credentials=True,
    allow_methods=["*"],
    allow_headers=["*"],
)

This exposes the API to cross-origin attacks from any domain.

🔴 Weak Default Secrets

The application uses a weak default secret key that poses a production security risk:

SECRET_KEY: str = "your-secret-key"  # Easily guessable default

Architecture Inconsistencies

The audit revealed multiple inconsistent patterns across the codebase:

• Tenant Context: Three different approaches used (hardcoded IDs, current_user.tenant_id, request.state.tenant.id)
• Error Handling: Inconsistent HTTPException patterns and response formats
• Documentation: Varying levels of OpenAPI documentation depth

Data Leak Potential

Several endpoints have tenant isolation issues:

• User listing endpoint doesn't respect middleware tenant context
• CRUD operations use default tenant parameters
• Analytics endpoints have fallback logic that could expose cross-tenant data

Audit Report

A comprehensive audit report has been created at BACKEND_AUDIT_REPORT.md containing:

• Detailed security vulnerability analysis
• Code examples and affected files
• Risk assessment matrix
• Specific remediation recommendations
• Implementation priority guidelines

Risk Summary:

• 3 Critical security issues requiring immediate action
• 2 High-severity data leak vulnerabilities
• 4 Medium-severity validation and error handling issues
• 6 Low-severity code quality improvements

This audit must be reviewed and critical issues addressed before any production deployment.

Fixes #137.

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