Insecure Design in Axum (Rust)

Insecure design in an Axum service written in Rust often stems from mismatches between Rust’s type safety and how endpoints model business rules and authorization. Unlike languages without a strong type system, Rust can encode invariants at compile time, but developers may still bypass those safeguards at runtime when designing endpoints.

Consider an endpoint that accepts a resource identifier and returns data scoped to the current user. If the design relies solely on path parameters (e.g., /users/{user_id}/documents/{document_id}) without verifying that the authenticated user owns the document, an Insecure Design flaw (BOLA/IDOR) emerges. Axum’s extractor model makes it easy to bind parameters and middleware, but it does not automatically enforce ownership checks. A developer might write:

async fn get_document(
    user: UserPrincipal,
    document_id: Path<Uuid>,
) -> Result<impl IntoResponse, (StatusCode, String)> {
    let doc = repo.get_document(*document_id).await?;
    // Missing: verify that doc.user_id == user.id
    Ok(Json(doc))
}

Here, the type of user signals authentication but not authorization scope. The design assumes the client-supplied document_id is safe to fetch, which is an insecure design pattern commonly leading to IDOR. This becomes critical when combined with overly permissive route definitions or missing grouping of authorization logic in extractors.

Another insecure design pattern involves state transitions without validating preconditions. For example, an endpoint that changes an order to cancelled might accept a JSON payload with an optional reason, but fail to enforce that only certain roles can cancel or that cancellation rules align with business policy. In Rust with Axum, this can manifest as:

async fn cancel_order(
    order_id: Path<Uuid>,
    body: Json<CancelOrder>, // { reason: Option<String> }
) -> Result<impl IntoResponse, (StatusCode, String)> {
    let mut order = repo.get_order(*order_id).await?;
    order.status = OrderStatus::Cancelled; // No transition checks
    repo.update_order(&order).await?;
    Ok(Accepted)
}

This design omits checks such as: is the order already shipped (invalid transition), does the actor have permission, and is the reason within allowed values. Because Rust does not prevent this at compile time, the flaw is an insecure design choice rather than a language limitation.

Input validation design also plays a role. Accepting loosely typed JSON and passing it directly to downstream services or databases can expose injection or parsing issues, even when using strongly typed structs. If deserialization does not enforce strict constraints (e.g., bounded string lengths, enum validation, or numeric ranges), an attacker may supply unexpected values that violate assumptions elsewhere in the system.

In summary, insecure design in Axum with Rust arises when developers rely on routing and extraction without embedding authorization and state-machine rules into the handler logic or into reusable extractors/middleware. The language and framework provide tools to reduce risk, but the design must intentionally leverage them to avoid IDOR, privilege escalation, and invalid state transitions.

To remediate insecure design in Axum, encode authorization and state rules into the type-driven flow of your handlers. Use extractor-based authorization that combines authentication with ownership checks, and enforce state transitions explicitly.

First, build an authorization extractor that validates ownership before returning data:

struct DocumentAuth {
    document: Document,
}

async fn document_auth(
    Extension(repo): Extension<Arc<dyn DocumentRepo>>,
    UserPrincipal(user_id): UserPrincipal,
    document_id: Path<Uuid>,
) -> Result<DocumentAuth, (StatusCode, String)> {
    let doc = repo.get_document(*document_id).await?;
    if doc.user_id != user_id {
        return Err((StatusCode::FORBIDDEN, "Not authorized".to_string()));
    }
    Ok(DocumentAuth { document: doc })
}

async fn get_document(
    auth: DocumentAuth,
) -> Result<impl IntoResponse, (StatusCode, String)> {
    Ok(Json(auth.document))
}

This pattern ensures that every request proving ownership is validated at the extractor layer, making insecure designs harder to implement by accident.

Second, model state transitions as explicit checks or state-machine methods rather than direct field assignment:

enum CancelError {
    InvalidTransition,
    Forbidden,
}

impl Order {
    fn cancel(&self, user_role: Role, reason: &str) -> Result<Order, CancelError> {
        if self.status == OrderStatus::Shipped {
            return Err(CancelError::InvalidTransition);
        }
        if user_role != Role::Manager && user_role != Role::Admin {
            return Err(CancelError::Forbidden);
        }
        // Validate reason if needed
        let mut updated = self.clone();
        updated.status = OrderStatus::Cancelled;
        Ok(updated)
    }
}

async fn cancel_order(
    Extension(repo): Extension<Arc<dyn OrderRepo>>,
    user: UserPrincipal,
    order_id: Path<Uuid>,
    body: Json<CancelOrder>,
) -> Result<impl IntoResponse, (StatusCode, String)> {
    let order = repo.get_order(*order_id).await?;
    let updated = order.cancel(user.role, &body.reason)
        .map_err(|e| match e {
            CancelError::InvalidTransition => (StatusCode::CONFLICT, "Cannot cancel this order"),
            CancelError::Forbidden => (StatusCode::FORBIDDEN, "Insufficient permissions"),
        })?;
    repo.update_order(&updated).await?;
    Ok(Accepted)
}

This approach moves transition logic into the domain model, making invalid states unrepresentable and ensuring that authorization and precondition checks are centralized.

Third, tighten input validation by using strong types and runtime constraints in your structs:

#[derive(Deserialize)]
struct CancelOrder {
    #[serde(deserialize_with = "crate::valid_reason")]
    reason: String,
}

fn valid_reason<'de, D>(deserializer: D) -> Result<String, D::Error>
where
    D: Deserializer<'de>,
{
    let s = String::deserialize(deserializer)?;
    if s.len() > 200 {
        return Err(serde::de::Error::custom("reason too long"));
    }
    // Optionally validate against a set of allowed reasons
    Ok(s)
}

By combining scoped authorization extractors, explicit state methods, and strict deserialization rules, you address insecure design at the architectural level in Axum services written in Rust.