Broken Access Control in Axum with Mongodb

Broken Access Control occurs when an API fails to enforce proper authorization checks between subjects and resources. In an Axum service that uses MongoDB as the backend, the risk emerges at the boundary where HTTP requests are translated into database operations. If route-level authorization is missing or misapplied, an authenticated user can manipulate identifiers in URLs or headers to access records that belong to other users.

Consider an endpoint like GET /users/{user_id}. If the handler retrieves the target user_id from the path and directly uses it in a MongoDB query such as { "_id": ObjectId("...") } without verifying that the authenticated subject owns or is permitted to view that user_id, the system exhibits a BOLA/IDOR flaw. Axum does not provide built-in authorization; it relies on developers to implement checks. When those checks are absent or incorrectly scoped, the database returns data the client should not see.

Another common pattern is filtering by a tenant or organization ID stored in the MongoDB document (e.g., tenant_id). If the query does not include the tenant constraint, a user who knows or guesses another user’s document ID can read or write across tenant boundaries. MongoDB’s flexible schema can exacerbate this when fields like role or is_admin are interpreted by application code but not enforced server-side through strict query filters. Attackers may also probe for IDOR via method tampering, where a GET is changed to PUT or DELETE on the same route, and the handler applies insufficient checks before performing write operations.

Middleware in Axum can mitigate some risks by attaching identity and permissions to the request extensions, but the developer must ensure every handler that interacts with MongoDB includes authorization logic. Missing checks in even a single handler create a pathway for unauthorized data access. Because MongoDB returns documents that match the query, the database itself is not at fault; the exposure arises when application code omits required filters or relies solely on client-supplied identifiers without validating them against the authenticated subject’s permissions.

To secure Axum routes that use MongoDB, always couple route parameters with the authenticated subject’s identity or role, and embed these checks directly in the database query. Below are concrete, idiomatic examples using the official MongoDB Rust driver and common Axum patterns.

First, ensure your data model includes an owner or tenant field. For a user-centric resource:

// In your MongoDB document
{
  "_id": ObjectId("..."),
  "user_id": "usr_abc123",
  "email": "[email protected]",
  "role": "user"
}

Next, in your Axum handler, derive the subject from your authentication layer and use it to constrain the query:

use axum::extract::State;
use mongodb::{bson::{doc, oid::ObjectId}};
use serde::Deserialize;

#[derive(Deserialize)]
pub struct UserParams {
    pub user_id: String,
}

pub async fn get_user_by_id(
    State(db): State<mongodb::Database>,
    params: axum::extract::Path<UserParams>,
    subject: Subject, // your auth-derived type with user_id
) -> Result<Json<User>, (StatusCode, String)> {
    // Enforce ownership: only allow access if the path user_id matches the authenticated user
    if params.user_id != subject.user_id {
        return Err((StatusCode::FORBIDDEN, "Unauthorized".into()));
    }

    let collection = db.collection("users");
    let oid = ObjectId::parse_str(¶ms.user_id).map_err(|_| (StatusCode::BAD_REQUEST, "Invalid ID"))?;
    let filter = doc! {
        "_id": oid,
        "user_id": subject.user_id, // redundant safety net
    };
    let user = collection.find_one(filter, None).await.map_err(|e| (StatusCode::INTERNAL_SERVER_ERROR, e.to_string()))?;
    match user {
        Some(u) => Ok(Json(u)),
        None => Err((StatusCode::NOT_FOUND, "User not found".into())),
    }
}

This approach ensures that even if an attacker guesses or iterates over valid ObjectIds, the query will return no document unless the user_id in the path matches the authenticated subject. For tenant-based models, include the tenant identifier in both the route or body and the filter:

let filter = doc! {
    "_id": oid,
    "tenant_id": subject.tenant_id,
};

Additionally, avoid exposing internal IDs in URLs when possible; use opaque identifiers and map them to MongoDB ObjectIds after authorization checks. Combine these handler-level checks with Axum middleware that validates tokens and populates a subject, and ensure every MongoDB operation includes the appropriate ownership or role filter. This reduces the attack surface and aligns with OWASP API Top 10 controls for Broken Object Level Authorization.