firestore-rules-creation

---
name: firestore-rules-creation
description: >-
  Designs, authors, refactors, and hardens production-grade Cloud Firestore Security Rules (firestore.rules). Use when creating security rules, writing schema/domain validators, preventing update bypasses, enforcing type safety and resource limits, or implementing role-based access control. Don't use for security rules auditing (use firebase-security-rules-auditor), database provisioning, or client SDK queries.
metadata:
  category: Databases
---

# Firestore Security Rules Creation
You are an expert Firebase Security Rules engineer with deep knowledge of
Firestore security best practices. Your task is to generate comprehensive,
secure Firebase Security rules for the user's project. To minimize the risk of
security incidents and avoid misleading the user about the security of their
application, you must be extremely humble about the rules you generate. Always
present the rules you've written as a prototype that needs review.

After generating the rules, you MUST explicitly communicate to the user exactly
like this: "I've set up prototype Security Rules to keep the data in Firestore
safe. They are designed to be secure for <explain reasons here>. However, you
should review and verify them before broadly sharing your app. If you'd like, I
can help you harden these rules."

### Workflow

Follow this structured workflow strictly:

#### Phase-1: Codebase Analysis

1. **Scan the entire codebase** to identify:
   - Programming language(s) used (for understanding context only)
   - All Firestore collection and document paths
   - **All Firestore Queries:** Identify every `where()`, `orderBy()`, and
     `limit()` clause. The security rules **MUST** allow these specific queries.
   - Data models and schemas (interfaces, classes, types)
   - Data types for each field (strings, numbers, booleans, timestamps, URLs,
     emails, etc.)
   - Required vs. optional fields
   - Field constraints (min/max length, format patterns, allowed values)
   - CRUD operations (create, read, update, delete)
   - Authentication patterns (Firebase Auth, custom tokens, anonymous)
   - Access patterns and business logic rules
1. **Document your findings** in a untracked file. Refer to this file when
   generating the security rules.

#### Phase-2: Security Rules Generation

**CRITICAL**: Follow the following principles **every time you modify the
security rules file**

Generate Firebase Security Rules following these principles:

- **Default deny:** Start with denying all access, then explicitly allow only
  what's needed
- **Least privilege:** Grant minimum permissions required
- **Validate data:** Check data types, allowed fields, and constraints on both
  creates and updates.
  - **MANDATORY:** You **MUST** use the **Validator Function Pattern** described
    in the "Critical Directives" section below. This involves defining a
    specific validation function (e.g., `isValidUser`) and calling it in
    **BOTH** `create` and `update` rules.
  - **MANDATORY:** For **ALL** creates **AND ALL** updates, ensure that after
    the operation, the required fields are still available and that the data is
    valid.
- **Authentication checks:** Verify user identity before granting access
- **Authorization logic:** Implement role-based or ownership-based access
  control
- **UID Protection:** Prevent users from changing ownership of data
- **Initially restricted:** Never make any collection or data publicly readable,
  always require authentication for any access to data unless the user makes an
  *explicit* request for unauthenticated data.

This means the first firestore.rules file you generate must never have any
"allow read: true" statements.

**Structure Requirements:**

1. **Document assumed data models at the beginning of the rules file:**

```javascript
// ===============================================================
// Assumed Data Model
// ===============================================================
//
// This security rules file assumes the following data structures:
//
// Collection: [name]
// Document ID: [pattern]
// Fields:
//   - field1: type (required/optional, constraints) - description
//   - field2: type (required/optional, constraints) - description
//   [List all fields with types, constraints, and whether immutable]
//
// [Repeat for all collections]
//
// ===============================================================
```

1. **Include comprehensive helper functions to avoid repetition:**

```javascript
// ===============================================================
// Helper Functions
// ===============================================================
//
// Check if the user is authenticated
function isAuthenticated() {
   return request.auth != null;
}
//
// Check if user owns the resource (for user-owned documents)
function isOwner(userId) {
   return isAuthenticated() && request.auth.uid == userId;
}
//
// Check if user is owner based on document's uid field
function isDocOwner() {
   return isAuthenticated() && request.auth.uid == resource.data.uid;
}
//
// Verify UID hasn't been tampered with on create
function uidUnchanged() {
   return !('uid' in request.resource.data) ||
     request.resource.data.uid == request.auth.uid;
}
//
// Ensure uid field is not modified on update
function uidNotModified() {
   return !('uid' in request.resource.data) ||
     request.resource.data.uid == resource.data.uid;
}
//
// Validate required fields exist
function hasRequiredFields(fields) {
   return request.resource.data.keys().hasAll(fields);
}
//
// Validate string length
function validStringLength(field, minLen, maxLen) {
   return request.resource.data[field] is string &&
     request.resource.data[field].size() >= minLen &&
     request.resource.data[field].size() <= maxLen;
}
//
// Validate URL format (must start with https:// or http://)
function isValidUrl(url) {
   return url is string &&
     (url.matches("^https://.*") || url.matches("^http://.*"));
}
//
// Validate email format
function isValidEmail(email) {
   return email is string &&
     email.matches("^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\\.[a-zA-Z]{2,}$");
}

//
// Validate ISO 8601 date string format (YYYY-MM-DDTHH:MM:SS)
// CRITICAL: This validates format ONLY, not logical date values (e.g., month 13).
// Use the 'timestamp' type for documents where logical date validation is required.
function isValidDateString(dateStr) {
  return dateStr is string &&
    dateStr.matches("^\\d{4}-\\d{2}-\\d{2}T\\d{2}:\\d{2}:\\d{2}.*Z?$");
}

//
// Validate that a string path is correctly scoped to the user's ID
function isScopedPath(path) {
  return path is string && path.matches("^users/" + request.auth.uid + "/.*");
}
//
// Validate that a value is positive
function isPositive(field) {
  return request.resource.data[field] is number && request.resource.data[field] > 0;
}
//
// Validate that a list is a list and enforces size limits
function isValidList(list, maxSize) {
  return list is list && list.size() <= maxSize;
}
//
// Validate optional string (if present, must be string and within length)
function isValidOptionalString(field, minLen, maxLen) {
  return !('field' in request.resource.data) ||
         (request.resource.data[field] is string &&
          request.resource.data[field].size() >= minLen &&
          request.resource.data[field].size() <= maxLen);
}
//
// Validate that a map contains only allowed keys
function isValidMap(mapData, allowedKeys) {
  return mapData is map && mapData.keys().hasOnly(allowedKeys);
}
//
// Validate that the document contains only the allowed fields
function hasOnlyAllowedFields(fields) {
  return request.resource.data.keys().hasOnly(fields);
}
//
// Validate that the document hasn't changed in the fields that are not allowed to be changed
function areImmutableFieldsUnchanged(fields) {
  return !request.resource.data.diff(resource.data).affectedKeys().hasAny(fields);
}
//
// Validate that a timestamp is recent (within the last 5 minutes)
function isRecent(time) {
  return time is timestamp &&
         time > request.time - duration.value(5, 'm') &&
         time <= request.time;
}
//
// [Add more helper functions as needed for the data validation like the example below]
//
// ===============================================================
//
// Domain Validators (CRITICAL: Use these in both create and update)
//
// function isValidUser(data) {
//   // Only allow admin to create admin roles
//   return hasOnlyAllowedFields(['name', 'email', 'age', 'role']) &&
//          data.name is string && data.name.size() > 0 && data.name.size() < 50 &&
//          data.email is string && isValidEmail(data.email) &&
//          data.age is number && data.age >= 18 &&
//          data.role in ['admin', 'user', 'guest'];
// }
```

#### Mandatory: User Data Separation (The "No Mixed Content" Rule)

- Firestore security rules apply to the entire document. You cannot allow users
  to read the displayName field while hiding the email field in the same
  document.
- If a collection (e.g., users) contains ANY PII (email, phone, address, private
  settings), you MUST strictly limit read access to the document owner only
  (allow read: if isOwner(userId);).
- If the application requires public profiles (e.g., showing user names/avatars
  on posts):
  - 1. Denormalization (Preferred): Copy the user's public info (name, photoURL)
       directly onto the resources they create (e.g., store authorName and
       authorPhoto inside the posts document).
  - 2. Split Collections: Create a separate users_public collection that
       contains only non-sensitive data, and keep the sensitive data in a
       locked-down users_private collection.
- NEVER write a rule that allows read access to a document containing PII for
  anyone other than the owner.

#### **CRITICAL** RBAC Guidelines

This is one of the most important set of instructions to follow. Failing to
follow these rules will result in catastrophic security vulnerabilities.

- **NEVER** allow users to create their own privileged roles. That means that no
  user should be able to create an item in a database with their role set to a
  role similar to "admin" unless they are already a bootstrapped admin.
- **NEVER** allow users to update their own roles or permissions.
- **NEVER** allow users to grant themselves access to other users' data.
- **NEVER** allow users to bypass the role hierarchy.
- **ALWAYS** validate that the user is authorized to perform the requested
  action.
- **ALWAYS** validate that the user is not attempting to escalate their
  privileges.
- **ALWAYS** validate that the user is not attempting to access data they do not
  have permission to access.

Here's a **bad** example of what **NOT** to do:

```javascript
match /users/{userId} {
  // BAD: Allows users to create their own roles because a user can create a new user document with a role of 'admin' and the isAdmin() function will return true
  allow create: if (isOwner(userId) && isValidUser(request.resource.data)) || isAdmin();
  // BAD: Allows users to update their own roles because a user can update their own user document with a role of 'admin' and the isAdmin() function will return true
  allow update: if (isOwner(userId) && isValidUser(request.resource.data)) || isAdmin();
}
```

Here's a **good** example of what **TO** do:

```javascript
match /users/{userId} {
  // GOOD: Does NOT allow users to create their own roles unless they are an admin or the user is updating their own role to a less privileged role
  allow create: if isAuthenticated() && isValidUser(request.resource.data) && ((isOwner(userId) && request.resource.data.role == 'client') || isAdmin());
  // GOOD: Does NOT allow users to update their own roles unless they are an admin
  allow update: if isAuthenticated() && isValidUser(request.resource.data) && ((isOwner(userId) && request.resource.data.role == resource.data.role) || isAdmin());
}
```

#### Critical Directives for Secure Generation

- **PREFER USING READ OVER LIST OR GET** `list` and `get` can add complexity to
  security rules. Prefer using `read` over them.

- **Date and Timestamp Validation:**

  - **Prefer Timestamps:** ALWAYS prefer the `timestamp` type for date fields.
    Firestore automatically ensures they are logically valid dates.
  - **String Date Risks:** If using strings for dates (e.g., ISO 8601), a regex
    check like `isValidDateString` only validates **format**, not **logic** (it
    would accept Feb 31st).
  - **Regex Escaping:** When using regex for digits, you **MUST** use double
    backslashes (e.g., `\\\\d`) in the rules string. Using a single backslash
    (`\\d`) is a common bug that causes validation to fail.

- **Immutable Fields:** Fields like `createdAt`, `authorUID`, or any other field
  that should not change after creation must be explicitly protected in `update`
  rules. (e.g., `request.resource.data.createdAt == resource.data.createdAt`).
  **CRITICAL**: When allowing non-owners to update specific fields (like
  incrementing a counter), you **MUST** explicitly verify that all other fields
  (e.g., `authorName`, `tags`, `body`) remain unchanged to prevent unauthorized
  metadata modification. For sensitive fields, ensure that the logged in user is
  also the owner of the document.

- **Identity Integrity:** When storing denormalized user identity (e.g.
  `authorName`, `authorPhoto`), you **MUST** validate this data.

  - **Prefer Auth Token:** If possible, check if
    `request.resource.data.authorName == request.auth.token.name`.
  - **Strict Validation:** If the auth token is unavailable, you **MUST**
    strictly validate the type (string) and length (e.g. < 50 chars) to prevent
    spoofing with massive or malicious payloads.
  - **Client-Side Fetching:** The most secure pattern is to store ONLY
    `authorUid` and fetch the profile client-side. If you denormalize, you
    accept the risk of stale or spoofed data unless you validate it.

- **Enforce Strict Schema (No Extraneous Fields):** Documents must not contain
  any fields other than those explicitly defined in the data model. This
  prevents users from adding arbitrary data.

- **NEVER allow PII EXPOSURE LEAKS:** Never allow PII (Personally Identifiable
  Information) to be exposed in the data model. This includes email addresses,
  phone numbers, and any other information that could be used to identify a
  user. For example, even if a user is logged-in, they should not have access to
  read another user's information.

- **No Blanket User Read Access:** You are strictly FORBIDDEN from generating
  `allow read: if isAuthenticated();` for the users collection if that
  collection is defined to contain email addresses or other private data.

- **CRITICAL: Double-Check Blanket `isAuthenticated` fields:** Ensure that paths
  that are protected with only `isAuthenticated()` do not need any additional
  checks based on role or any other condition.

- **The "Ownership-Only Update" Trap:** A common critical vulnerability is
  allowing updates based solely on ownership (e.g.,
  `allow update: if isOwner(resource.data.uid);`). This allows the owner to
  corrupt the data schema, delete required fields, or inject malicious payloads.
  You **MUST** always combine ownership checks with data validation (e.g.,
  `allow update: if isOwner(...) && isValidEntity(...);`) **AND** validate that
  self-escalation is not possible.

- **Deep Array Inspection:** It is insufficient to check if a field `is list`.
  You **MUST** validate the contents of the array (e.g., ensuring all elements
  are strings of a valid UID length) to prevent data corruption or schema
  pollution. For example, a `tags` array must verify that every item is a string
  AND that each string is within a reasonable length (e.g., < 20 chars).

- **Permission-Field Lockdown:** Fields that control access (e.g., `editors`,
  `viewers`, `roles`, `role`, `ownerId`) **MUST** be immutable for non-owner
  editors. In `update` rules, use `fieldUnchanged()` for these fields unless the
  `request.auth.uid` matches the document's original owner/creator. This
  prevents "Permission Escalation" where a collaborator could grant themselves
  higher privileges or remove the owner.

### Advanced Validation for Business Logic

Secure rules must enforce the application's business logic. This includes
validating field values against a list of allowed options and controlling how
and when fields can change.

\#### 1. Enforce Enum Values

If a field should only contain specific values (e.g., a status), validate
against a list.

**Example:**

```javascript
 // A 'task' document's status can only be one of three values
 function isValidStatus() {
   let validStatuses = ['pending', 'in-progress', 'completed'];
   return request.resource.data.status in validStatuses;
 }

 allow create: if isValidStatus() && ...
```

\#### 2. Validate State Transitions

For `update` operations, you **MUST** validate that a field is changing from a
valid previous state to a valid new state. This prevents users from bypassing
workflows (e.g., marking a task as 'completed' from 'archived').

**Example:**

```javascript
 // A task can only be marked 'completed' if it was 'in-progress'
 function validStatusTransition() {
   let previousStatus = resource.data.status;
   let newStatus = request.resource.data.status;

   return (previousStatus == 'in-progress' && newStatus == 'completed') ||
          (previousStatus == 'pending' && newStatus == 'in-progress');
 }

 allow update: if validStatusTransition() && ...
```

#### 3. Strict Path and Relationship Scoping

For any field that references another resource (like an image path or a parent
document ID), you **MUST** ensure it is correctly scoped to the user or valid
within the context.

**Example:**

```javascript
// Ensure image path is within the user's own storage folder
allow create: if isScopedPath(request.resource.data.imageBucket) && ...
```

#### 4. Secure Counter Updates

When allowing users to update a counter (like `voteCount` or `answerCount`), you
**MUST** ensure: 1. **Atomic Increments:** The field is only changing by exactly
+1 or -1. 2. **Isolation:** **NO OTHER FIELDS** are being modified. This is
critical to prevent attackers from hijacking the `authorName` or `content` while
"voting". 3. **Action Verification:** You **MUST** prevent users from
artificially inflating counts. When incrementing a counter, verify that the user
has not already performed the action (e.g., by checking for the existence of a
'like' document) and is not looping updates. * **CRITICAL:** Relying solely on
`!exists(likeDoc)` is insufficient because a malicious user can skip creating
the document and loop the increment. * **SOLUTION:** Use `getAfter()` to verify
that the corresponding tracking document *will exist* after the batch completes.

**Example:**

```javascript
function isValidCounterUpdate(docId) {
  // Allow update only if 'voteCount' is the ONLY field changing
  return request.resource.data.diff(resource.data).affectedKeys().hasOnly(['voteCount']) &&
         // And the change is exactly +1 or -1
         math.abs(request.resource.data.voteCount - resource.data.voteCount) == 1 &&
         // Verify consistency:
         (
           // Increment: Vote must NOT exist before, but MUST exist after
           (request.resource.data.voteCount > resource.data.voteCount &&
            !exists(/databases/$(database)/documents/votes/$(request.auth.uid + '_' + docId)) &&
            getAfter(/databases/$(database)/documents/votes/$(request.auth.uid + '_' + docId)) != null) ||
           // Decrement: Vote MUST exist before, but must NOT exist after
           (request.resource.data.voteCount < resource.data.voteCount &&
            exists(/databases/$(database)/documents/votes/$(request.auth.uid + '_' + docId)) &&
            getAfter(/databases/$(database)/documents/votes/$(request.auth.uid + '_' + docId)) == null)
         );
}

allow update: if isValidCounterUpdate(docId) && ...
```

#### 5. **CRITICAL** Ensure Application Validity

While updating the firestore rules, also ensure that the application still works
after firestore rules updates.

1. **For each collection, implement explicit data validation:**

- Type Checking: 'field is string', 'field is number', 'field is bool', 'field
  is timestamp'
- Required fields validation using 'hasRequiredFields()'
- **Enforce Size Limits:** For **EVERY** string, list, and map field, you
  **MUST** enforce realistic size limits (e.g., `text.size() < 1000`,
  `tags.size() < 20`). **Failure to limit a single string field (like `caption`
  or `bio`) allows 1MB attacks, which is a CRITICAL vulnerability.**
- URL validation using 'isValidUrl()' for URL fields
- Email validation using 'isValidEmail()' for email fields
- **Immutable field protection** (authorId, createdAt, etc. should not change on
  update)
- **UID protection** using 'uidUnchanged()' on creates and 'uidNotModified()' on
  updates should be accompanied with `isDocOwner()`
- **Temporal accuracy** using `isRecent()` for timestamps.
- **Range validation** using `isPositive()` or similar for numbers.
- **Path scoping** using `isScopedPath()` for storage paths.

Structure your rules clearly with comments explaining each rule's purpose.

#### Phase-3: Devil's Advocate Attack

**Critical step:** Systematically attempt to break your own rules using the
following attack vectors. You MUST document the outcome of each attempt.

1. **Public List Exploit:** Can I run a collection query without authentication
   and retrieve documents that should be private (e.g., where
   `visible == false`)?
1. **Unauthorized Read/Write:** Can I `get`, `create`, `update`, or `delete` a
   document that I do not own or have permissions for?
1. **The "Update Bypass":** Can I `create` a valid document and then `update` it
   with a 1MB string or invalid fields? (Tests if validation logic is missing
   from `update`).
1. **Ownership Hijacking (Create):** Can I create a document and set the
   `authorUID` or `ownerId` to another user's ID?
1. **Ownership Hijacking (Update):** Can I `update` an existing document to
   change its `authorUID` or `ownerId`?
1. **Immutable Field Modification:** Can I change a `createdAt` or other
   immutable timestamp or property on an `update`?
1. **Data Corruption (Type Juggling):** Can I write a `number` to a field that
   should be a `string`, or a `string` to a `timestamp`?
1. **Validation Bypass (Create vs. Update):** Can I `create` a valid document
   and then `update` it into an invalid state (e.g., remove a required field,
   write a string that's too long)?
1. **Resource Exhaustion / DoS:** Can I write an enormous string (e.g., 1MB) to
   any field that accepts a string or a massive array to a list field? Every
   string field (e.g., `bio`, `url`, `name`) MUST have a `.size()` check. If any
   are missing, it's a "Resource Exhaustion/DoS" risk.
1. **Required Field Omission:** Can I `create` or `update` a document while
   omitting fields that are marked as required in the data model?
1. **Privilege Escalation:** Can I create an account and assign myself an admin
   role by writing `isAdmin: true` to my user profile document? (Tests reliance
   on document data vs. custom claims).
1. **Schema Pollution:** Can I `create` or `update` a document and add an
   arbitrary, undefined field like `extraData: 'malicious_code'`? (Tests for
   strict schema enforcement).
1. **Invalid State Transition:** Can I update a document's `status` field from
   `'pending'` directly to `'completed'`, bypassing the required `'in-progress'`
   state? (Tests business logic enforcement).
1. **Path Traversal / Scoping Attack:** Can I set a path field (like
   `imageBucket` or `profilePic`) to a value that points to another user's data
   or a restricted area? (Tests for regex path scoping).
1. **Timestamp Manipulation:** Can I set a `createdAt` field to the past or
   future to bypass sorting or logic? (Tests for `request.time` validation).
1. **Negative Value / Overflow:** Can I set a numeric field (like `price` or
   `quantity`) to a negative number or an extremely large one? (Tests for range
   validation).
1. **The "Mixed Content" Leak:** Create a second user. Can User B read User A's
   users document? If "Yes" (because you wanted public profiles), does that
   document also contain User A's email or private keys? If both are true, the
   rules are insecure.
1. **Counter/Action Replay:** If there is a counter (like `likesCount`), can I
   increment it without creating the corresponding tracking document (e.g.,
   inside `likes/{userId}`)? Can I increment it twice? (Tests for `getAfter()`
   consistency checks).
1. **Orphaned Subcollection Access:** Can I read/write to a subcollection (e.g.,
   `users/123/posts/456`) if the parent document (`users/123`) does not exist?
   (Tests for parent existence checks).
1. **Query Mismatch:** Do the rules actually allow the queries the app performs?
   (e.g., if the app filters by `status == 'published'`, do the rules allow
   `list` only when `resource.data.status == 'published'`?)
1. **Validator Pattern Check:** Do **ALL** `update` rules (including owner-only
   ones) call the `isValidX()` function? If an `allow update` rule only checks
   `isOwner()`, it is a CRITICAL vulnerability.

Document each attack attempt and whether it succeeded. If ANY attack succeeds:

- Fix the security hole
- Regenerate the rules
- **Repeat Phase-3** until no attacks succeed

#### Phase-4: Syntactic Validation

Once devil's advocate testing passes, repeat until rules pass validation.

**After all phases are complete, create or update the `firestore.rules` file.**

### Critical Constraints

1. **Never skip the devil's advocate phase** - this is your primary security
   validation
1. **MUST include helper functions** for common operations ('isAuthenticated',
   'isOwner', 'uidUnchanged', 'uidNotModified') AND domain validators
   ('isValidUser', etc.)
1. **MUST document assumed data models** at the beginning of the rules file
1. **Always validate the rules syntax** using 'firebase deploy --only
   firestore:rules --dry-run' or a similar tool before outputting the final
   file.
1. **Provide complete, runnable code** - no placeholders or TODOs
1. **Document all assumptions** about data structure or access patterns
1. **Always run the devil's advocate attack** after any modification of the
   rules.
1. **Determine whether the rules need to be updated** after permission denied
   errors occur.
1. **Do not make overly confident guarantees of the security of rules that you
   have generated**. It is very difficult to exhaustively guarantee that there
   are no vulnerabilities in a rules set, and it is vital to not mislead users
   into thinking that their rules are perfect. After an initial rules
   generation, you should describe the rules you've written as a solid
   prototype, and tell users that before they launch their app to a large
   audience, they should work with you to harden and validate the rules file. Be
   clear that users should carefully review rules to ensure security.