Broken Authentication in Axum with Mongodb

Broken Authentication in an Axum service that uses MongoDB for persistence typically arises when session handling, credential storage, or authorization checks are implemented inconsistently between the Rust web layer and the database layer. Attackers can exploit gaps such as missing or weak token validation, improper comparison of credentials, or unsafe construction of MongoDB queries that bypass intended access controls.

For example, if an Axum route deserializes a JSON web token (JWT) but does not rigorously validate its signature or claims, an attacker can craft a token with elevated roles. If the route then builds a MongoDB filter using unchecked user input—such as an user_id from the token—without verifying that the requesting user is allowed to access that document, a Broken Object Level Authorization (BOLA) / IDOR condition occurs. A common real-world pattern is constructing a filter like doc! { "_id": oid } without confirming that the authenticated subject owns that _id. This becomes especially risky when the JWT claims are not cross-checked against the MongoDB-stored user record, enabling horizontal privilege escalation across user boundaries.

Another vector involves password handling. If Axum endpoints accept a password, compute a hash, and then query MongoDB with a direct equality check on a non-indexed field or use a MongoDB query that does not enforce uniqueness properly, it can lead to authentication bypass or account enumeration. For instance, using a case-insensitive collation or failing to enforce a unique index on email can allow multiple accounts with effectively the same identity, complicating auditability. Moreover, if session identifiers or impersonation tokens are stored in MongoDB without integrity checks, an attacker who can read or modify that collection may hijack sessions.

Middleware configuration in Axum also plays a role. If authentication middleware is inconsistently applied—covering some routes but not others—or if role checks are performed in application logic rather than enforced through MongoDB access patterns, the attack surface expands. An attacker can probe unauthenticated endpoints that still pull data from MongoDB, combining information leakage from responses with manipulated ObjectIds to infer valid identifiers and escalate privileges. These interactions illustrate why authentication and data access controls must be validated both in the Axum routing layer and in the MongoDB query design to prevent Broken Authentication.

Remediation focuses on strict validation, least privilege database access, and consistent authorization checks. In Axum, implement robust authentication middleware that verifies JWTs using a verified JSON Web Key Set (JWKS), validates all standard claims (iss, aud, exp, nbf), and binds the subject to a canonical user identifier. Always enforce uniqueness constraints in MongoDB for authentication-related fields and use parameterized queries that incorporate the authenticated subject’s ID rather than relying on client-supplied values.

Below is a concrete Axum handler using the jsonwebtoken crate for verification and the official MongoDB Rust driver. The example shows how to safely build a query that scopes documents to the authenticated user, preventing BOLA/IDOR.

use axum::{routing::get, Router};
use mongodb::{bson::{doc, oid::ObjectId}, options::ClientOptions, Client};
use serde::{Deserialize, Serialize};
use jsonwebtoken::{decode, Algorithm, DecodingKey, Validation, TokenData};

#[derive(Debug, Serialize, Deserialize)]
struct Claims {
    sub: String,
    role: String,
    exp: usize,
}

async fn get_user_profile(
    user_id: String,
    token: String,
    db_client: Client,
) -> Result {
    // Verify JWT
    let mut validation = Validation::new(Algorithm::HS256);
    validation.set_audience(&["myapi"]);
    let token_data: TokenData = decode(
        &token,
        &DecodingKey::from_secret("secret".as_ref()),
        &validation,
    ).map_err(|_| (axum::http::StatusCode::UNAUTHORIZED, "Invalid token"))?;

    let claims = token_data.claims;
    // Ensure the requesting subject matches the requested resource
    if claims.sub != user_id {
        return Err((axum::http::StatusCode::FORBIDDEN, "Unauthorized scope".into()));
    }

    let db = db_client.database("appdb");
    let users = db.collection("users");
    // Use the authenticated subject to scope the query
    let filter = doc! { "_id": ObjectId::parse_str(&user_id).map_err(|_| (axum::http::StatusCode::BAD_REQUEST, "Invalid user ID"))? };
    let user_doc = users.find_one(filter, None).await.map_err(|e| (axum::http::StatusCode::INTERNAL_SERVER_ERROR, e.to_string()))?;

    match user_doc {
        Some(doc) => Ok(axum::Json(doc)),
        None => Err((axum::http::StatusCode::NOT_FOUND, "User not found".into())),
    }
}

fn build_router(client: Client) -> Router {
    Router::new()
        .route("/profile/:user_id", get(get_user_profile))
}

Key points in this example:

• JWT validation occurs before any database operation, with audience and issuer checks.
• The authenticated claims.sub is compared directly with the route parameter to enforce ownership.
• The MongoDB filter uses the parsed ObjectId derived from the validated user ID, avoiding injection or malformed object errors.
• Errors are mapped to appropriate HTTP statuses without leaking internal details.

Additionally, ensure MongoDB collections have appropriate unique indexes (e.g., on email) and use role-based access control at the database user level to limit what the connected MongoDB user can read or write. This complements application-level checks and reduces the impact of any logic errors in Axum routes.