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Firebase Security Vulnerability: FBR-ACCESS-002

Name: Implicit Allow Due to Lack of Default Deny

Applicable Services: Cloud Firestore, Firebase Realtime Database, Cloud Storage

Description

Note

This configuration weakness occurs when a security ruleset does not end with an explicit, catch-all deny rule. It relies on the default "implicit deny" behavior, which is a less secure and less maintainable practice.

  • The Technical Flaw: The ruleset defines allow rules for specific paths but omits a final match /{document=**} { allow read, write: if false; }.
  • Intended Behavior: A robust security posture follows the principle of "default deny." Every path in your database should be explicitly denied access unless a specific rule explicitly grants it.
  • The Risk: While Firebase denies access by default if no rule allows it, relying on this is risky. During development, a new data path might be added to the application without a corresponding security rule. Without an explicit global deny, it's easier to overlook this and assume the data is protected when it might fall under a broader, more permissive rule higher up in the ruleset. An explicit deny makes the security posture unambiguous.

Impact

Potential Consequences

The severity of this vulnerability is a note. While it doesn't create a direct flaw, it represents a poor security practice that increases the risk of future vulnerabilities: * Accidental Data Exposure: A developer might add a new collection and forget to secure it. Without an explicit deny-all, if any other rule accidentally matches that path, it could become exposed. * Reduced Maintainability: The security posture is not immediately obvious. A developer must mentally parse all rules to know what is and isn't allowed, rather than seeing a clear default-deny foundation.

Example Attack Scenarios

Exploit Scenario: Accidental Exposure of a New Collection

  • The Vulnerable Rule: The ruleset secures the users and posts collections but lacks a final deny-all rule. 
    // VULNERABLE: This ruleset is missing a default deny.
service cloud.firestore {
  match /databases/{database}/documents {
    match /users/{userId} {
      allow read, write: if request.auth.uid == userId;
    }
    match /posts/{postId} {
      allow read: if true;
    }
    // No final "deny all" rule exists.
  }
}
  • Attacker's Goal: This is a scenario of accidental misconfiguration. A developer adds a new feature that stores sensitive logs in a collection called admin_logs. They forget to add a security rule for this new collection.
  • The Exploit: There is no specific exploit code. The vulnerability is the configuration state. An attacker or any authenticated user could now attempt to access this new collection. 
    // Any authenticated user can attempt this.
db.collection("admin_logs").get().then((snapshot) => {
  // If no other rule blocks this, the implicit deny will apply.
  // BUT, if a broad rule was added elsewhere, this could succeed.
  console.log("Accessed admin logs!");
});
  • Outcome: In the best case, the implicit deny works, and access is denied. However, the security posture is fragile. If a developer had added a broad rule like allow read: if request.auth != null; to a parent path, the admin_logs could become readable by any user. An explicit deny-all prevents this ambiguity.

Mitigation

The vulnerability is resolved by adding a comprehensive, explicit deny-all rule at the end of your ruleset. This ensures that any path not explicitly granted access by a prior rule is definitively denied.

// VULNERABLE: No explicit deny-all.
service cloud.firestore {
  match /databases/{database}/documents {
    match /users/{userId} {
      allow read, write: if request.auth.uid == userId;
    }
    // New collections added without rules are only implicitly denied.
  }
}

// SECURE: Explicitly denies access to all paths by default.
service cloud.firestore {
  match /databases/{database}/documents {
    match /users/{userId} {
      allow read, write: if request.auth.uid == userId;
    }

    // This rule ensures any path not matched above is denied.
    match /{document=**} {
      allow read, write: if false;
    }
  }
}

Best Practice

Always structure your rules with a "default deny" mindset. Start with the most specific rules first and end with the broadest (the deny-all). This makes your ruleset easier to read, maintain, and reason about.

References