Cross-Tenant Vulnerability in SaaS Explained

Cross-Tenant Vulnerability Explained (Critical SaaS Risk) 🚨☁️​

Modern SaaS platforms are usually multi-tenant.
That means many customers share the same infrastructure…
but their data must stay isolated 🔒

A Cross-Tenant Vulnerability happens when one tenant can view, modify, or affect another tenant’s data.
This is considered a high to critical severity security flaw ⚠️

What Is a Tenant? 🧩​

A tenant is an organization or user group that has its own:

• Users
• Data
• Configurations
• Permissions

Example:​

• Tenant A → Company Alpha
• Tenant B → Company Beta

👉 These tenants must never access each other’s data.

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How Cross-Tenant Vulnerabilities Happen 💥​

Most cross-tenant bugs are caused by logic mistakes, not advanced exploits.

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Broken Access Control 🔓​

The backend trusts user input instead of enforcing tenant isolation.

Example API Request​

GET /api/users?tenant_id=123

If an attacker changes it to:

GET /api/users?tenant_id=124

👉 And gets another tenant’s data - that’s a cross-tenant bug.

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IDOR (Insecure Direct Object Reference) 🧨​

The application exposes internal object IDs without tenant checks.

Example​

GET /api/invoices/98765

If invoice 98765 belongs to a different tenant but is still accessible ❌
You’ve got a critical data isolation failure.

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Missing Tenant Validation 🧠❌​

The backend:

• Accepts tenantId from the client
• Does not verify ownership server-side

Bad Practice​

{
  "userId": "123",
  "tenantId": "tenant_B"
}

Never trust tenant context coming from the client.

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JWT or Token Misconfiguration 🎟️⚠️​

Tokens are missing critical claims like:

• tenant_id
• organization_id
• Proper scope validation

Weak Token Example​

{
  "sub": "user123",
  "role": "admin"
}

Without tenant binding, tokens can be reused across tenants 😬

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Shared Admin APIs 🚫​

Admin endpoints that:

• Are shared across tenants
• Lack tenant-based authorization
• Allow cross-organization access

These are gold mines for attackers 💰

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Real-World Bug Bounty Scenario 🧪🎯​

1️⃣ Login as Tenant A
2️⃣ Intercept an API request
3️⃣ Modify:

{
  "organizationId": "tenant_B"
}

4️⃣ Receive Tenant B’s sensitive data
5️⃣ Impact includes:

• Data leakage
• GDPR violations
• Full account takeover risk

💰 These issues are often rated High or Critical in bug bounty programs.

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Security Impact 🛡️💣​

Cross-tenant vulnerabilities can lead to:

• ❌ Unauthorized data access
• ❌ Data modification or deletion
• ❌ Privilege escalation
• ❌ Compliance violations
• ❌ Severe reputation damage

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How to Prevent Cross-Tenant Bugs 🛠️✅​

Strong prevention requires backend enforcement, not frontend checks.

Best Practices:​

• ✅ Enforce tenant isolation server-side
• ✅ Never trust client-supplied tenant IDs
• ✅ Bind tenant context to: Session - JWT - API Gateway
• ✅ Implement RBAC + ABAC
• ✅ Use Row-Level Security (RLS) in databases
• ✅ Add automated tenant-boundary tests

Example: Secure Query Pattern​

SELECT * FROM invoices
WHERE tenant_id = :current_tenant
AND invoice_id = :invoice_id;

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Bug Bounty Hunter Tips 🏹🔍​

Always test:

• API parameters (tenantId, orgId, workspaceId)
• UUID swapping
• JWT claim manipulation
• Bulk export endpoints
• Admin dashboards

🎯 Keywords to watch for: organization, workspace, account, company, team

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Final Warning ⚠️​

Cross-Tenant vulnerabilities are silent killers in SaaS security.
Many companies miss them…
👉 Attackers don’t.