Dangling Dns in Axum with Firestore

A dangling DNS record occurs when a DNS entry (such as a CNAME or A record) points to a destination that no longer exists or is misconfigured. In an Axum service that integrates with Google Firestore, this misconfiguration can expose internal or restricted endpoints to unintended network paths. If Axum routes requests based on hostname or subdomain and passes derived values to Firestore operations without strict validation, an attacker may supply a hostname that resolves through a dangling record to an unexpected internal or third-party endpoint.

Consider an Axum handler that uses the request host to select a Firestore project or document path. If the application constructs Firestore resource names using unchecked host input, a dangling DNS entry could redirect resolution to a different project or service, bypassing intended isolation. For example, a developer might assume that a CNAME for analytics.example.com points to a controlled analytics backend, but if that CNAME becomes dangling and resolves to an internal Firestore emulator or a legacy project, requests from external clients could reach unintended Firestore instances. This becomes a boundary confusion issue: the application trusts DNS as an authorization boundary, which it is not.

In practice, this can manifest when Axum dynamically builds Firestore client calls using request metadata. If the code uses the Host header to pick a Firestore collection or project ID and then performs read or write operations, an attacker can supply a hostname that resolves via a dangling record to a project with overly permissive rules. Because the scan tests unauthenticated attack surfaces and checks for BOLA/IDOR and insecure direct object references across API boundaries, middleBrick can flag scenarios where host-based routing intersects with Firestore resource access without proper authorization checks.

Additionally, in environments that use Firebase emulators during development, a dangling DNS or misconfigured hosts file can cause emulator endpoints to be referenced in production code paths. If Axum services conditionally point to emulator hosts based on environment variables that are overridden or if deployment configurations are inconsistent, requests might route to local or external emulator instances that expose Firestore data in non-production formats. The LLM/AI security checks in middleBrick specifically look for system prompt leakage and output exposure; while this scenario does not directly leak prompts, it demonstrates how misconfigured routing and trust in DNS can lead to data exposure and incorrect data destinations.

To contextualize this within compliance mappings, such misconfigurations can relate to OWASP API Top 10 API2:2023 Broken Object Level Authorization and Data Exposure risks, where missing authorization on resource identifiers allows access to data across tenants or projects. middleBrick’s per-category breakdowns and prioritized findings include these concerns when runtime behavior deviates from expected authorization boundaries, providing remediation guidance to tighten validation and remove reliance on implicit trust in network naming.

Remediation centers on never trusting DNS or request metadata to determine Firestore project or document scope. Validate and constrain all inputs, use static configuration for critical identifiers, and enforce authorization checks before any Firestore operation. Below are concrete Axum examples that demonstrate secure handling.

First, avoid using the Host header to select Firestore projects or collections. Instead, map authenticated user or service identity to a fixed project ID. For example, use a configuration-driven approach where the Firestore project is set from environment variables at startup and not derived from requests:

use axum::{routing::get, Router};
use google_cloud_rust::firestore::client::Client;
use std::net::SocketAddr;

async fn build_client() -> Client {
    // Initialize once, e.g., via lazy_static or tokio::sync::OnceCell
    let project_id = std::env::var("FIRESTORE_PROJECT_ID")
        .expect("FIRESTORE_PROJECT_ID must be set");
    Client::new(project_id).await.unwrap()
}

async fn get_document_handler() -> String {
    let client = build_client().await;
    // Use a static collection and validated document ID
    let doc = client.collection("public_data").doc("metadata").get().await;
    format!("{:?}", doc)
}

#[tokio::main]
async fn main() {
    let app = Router::new().route("/meta", get(get_document_handler));
    let addr = SocketAddr::from(([127, 0, 0, 1], 3000));
    axum::Server::bind(&addr).serve(app.into_make_service()).await.unwrap();
}

Second, if multi-tenancy is required, enforce strict allowlists and perform server-side authorization rather than relying on hostname patterns. Use a mapping validated at deployment time and reject any identifiers not in the allowlist:

use axum::{routing::post, Json, http::StatusCode};
use serde::{Deserialize, Serialize};

#[derive(Deserialize)]
struct TenantRequest {
    tenant_key: String,
}

#[derive(Serialize)]
struct DataResponse {
    data: String,
}

fn is_valid_tenant(tenant_key: &str) -> bool {
    const ALLOWED_TENANTS: &[&str] = &["tenant_a", "tenant_b"];
    ALLOWED_TENANTS.contains(&tenant_key)
}

async fn query_data(Json(payload): Json) -> Result, (StatusCode, String)> {
    if !is_valid_tenant(&payload.tenant_key) {
        return Err((StatusCode::FORBIDDEN, "Invalid tenant".into()));
    }
    // Here, use tenant_key to safely scope Firestore reads with server-side checks
    let client = build_client().await;
    let doc = client.collection(&payload.tenant_key).doc("settings").get().await;
    Ok(Json(DataResponse { data: format!("{:?}", doc) }))
}

Third, sanitize and validate any identifiers used in Firestore paths, and prefer constant-time comparisons where applicable. Never concatenate raw user input into document paths without strict allowlisting and encoding:

use axum::extract::Path;

async fn get_user_document(Path(user_id): Path) -> Result {
    // Strict pattern validation: alphanumeric and underscores, max length 64
    if !user_id.chars().all(|c| c.is_ascii_alphanumeric() || c == '_') || user_id.len() > 64 {
        return Err((StatusCode::BAD_REQUEST, "Invalid user_id".into()));
    }
    let client = build_client().await;
    let doc = client.collection("users").doc(&user_id).get().await;
    Ok(format!("{:?}", doc))
}

By combining environment-based configuration, strict allowlists, and input validation, you reduce the risk that dangling DNS or misconfigured routing leads to unauthorized Firestore access. middleBrick’s scans, including its checks for BOLA/IDOR and Data Exposure, can help detect deviations in runtime behavior, and the Pro plan’s continuous monitoring can alert you if new endpoints or misconfigurations appear over time.