Security Misconfiguration in Axum with Firestore

Security misconfiguration in an Axum service that uses Google Cloud Firestore often stems from overly permissive Firestore rules combined with unvalidated or trusted client input routed through Axum endpoints. When Firestore rules allow read or write access based only on request origin (for example, allowing writes if a request includes a uid claim without verifying ownership), and Axum handlers do not enforce additional authorization checks, the unauthenticated or partially authenticated attack surface expands.

Consider an endpoint like /users/{user_id}/profile implemented in Axum. If the handler retrieves the user_id from path parameters and directly uses it to build a Firestore document reference such as doc("users", user_id) without confirming that the authenticated caller is the same user, an IDOR-like exposure occurs. Misconfigured Firestore rules that permit allow read, write: if request.auth != null; may appear to enforce authentication, but if the Axum service accepts untrusted input for document IDs or query filters, an attacker can iterate over accessible document IDs or inject fields that bypass intended scoping.

Another common misconfiguration is the use of Firestore Admin SDK with elevated privileges from Axum components that are inadvertently exposed through public endpoints. If an Axum route intended for administrative operations lacks proper role checks and instead relies only on network-level protections or a flag set at startup, any request that reaches that route can trigger privileged Firestore operations. This can lead to mass data read, modification, or deletion depending on the rules and the identity used by the service. Additionally, Firestore rules that rely on incomplete resource-level checks, such as verifying document fields only at write time without ensuring they match the caller’s permissions, can allow privilege escalation when combined with insufficient validation in Axum request handling.

Insecure default Firestore rules in development environments can be mistakenly promoted to production alongside Axum services, creating a scenario where unauthenticated access to collections is possible. If Axum endpoints expose Firestore document lists or metadata without validating filters against the caller’s permissions, an attacker can enumerate data and chain it with other vulnerabilities such as excessive data exposure or insecure consumption patterns. The combination of Axum’s routing flexibility and Firestore’s rule-based security requires precise alignment between application-layer checks and database permissions to avoid unintentional exposure.

To remediate misconfiguration, enforce strict ownership checks in Axum handlers and use Firestore rules that scope access to the authenticated user’s own data. For user-specific endpoints, resolve the document reference using the authenticated user’s ID rather than trusting path parameters, and validate that the authenticated identity matches the target document.

Example: a protected profile endpoint in Axum using the Firestore Rust SDK:

use axum::{routing::get, Router, extract::State, http::StatusCode};
use google_cloud_firestore::client::Client;
use google_cloud_firestore::document::Document;
use serde::Deserialize;

struct AppState {
    firestore_client: Client,
}

async fn get_profile(
    State(state): State<AppState>,
    // In practice, extract user identity from a validated auth layer (e.g., session or JWT)
    auth_user_id: String, // derived from authenticated session, not from path
) -> Result<String, (StatusCode, String)> {
    // Use the authenticated identity to scope the document, not user-supplied path params
    let doc_ref = state.firestore_client.doc(&format!("users/{}", auth_user_id));
    let snapshot = doc_ref.get().await.map_err(|e| (StatusCode::INTERNAL_SERVER_ERROR, e.to_string()))?;
    if let Some(content) = snapshot.content() {
        // Ensure the returned document does not contain sensitive fields for this reader
        // Implement field-level filtering as needed
        Ok(serde_json::to_string(&content).unwrap_or_default())
    } else {
        Err((StatusCode::NOT_FOUND, "Profile not found".to_string()))
    }
}

app = Router::new()
    .route("/profile", get(get_profile))
    .with_state(AppState { firestore_client });

On the Firestore rules side, scope reads and writes to the document that matches the authenticated user’s UID and enforce field-level constraints:

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
    match /users/{userId} {
      allow read, write: if request.auth != null && request.auth.uid == userId;
    }
  }
}

For administrative operations, do not expose admin routes through the same handlers as user routes. If Admin SDK usage is required within Axum, isolate those handlers behind strict role checks and avoid using elevated credentials for routine user operations. Validate and sanitize all inputs used to construct document paths or queries, and prefer server-side field filtering to limit data exposure. Use Firestore’s query constraints to ensure that list endpoints only return documents the caller is permitted to see, rather than retrieving full collections and filtering in application code.