Insecure Design in Axum
How Insecure Design Manifests in Axum
Insecure Design in Axum applications typically emerges from architectural decisions that expose excessive functionality or fail to implement proper access controls at the framework level. Unlike implementation bugs, these are design flaws that persist even in perfectly coded applications.
One common pattern involves improper extraction of request data. Axum's extractors provide convenient access to request information, but developers often extract more than necessary:
async fn get_user_profile(
Path((user_id, secret_token)): Path<(u32, String)>,
Extension(db): Extension<PgPool>
) -> impl IntoResponse { ... }This route handler accepts a secret_token parameter that should never be exposed in URLs. An attacker can manipulate this token to escalate privileges or bypass authentication. The design flaw is accepting sensitive data in predictable locations rather than using secure headers or session management.
Another manifestation occurs with improper use of Axum's Extract trait. Developers sometimes create custom extractors that perform no validation:
#[derive(Extract)]
struct AdminOnly {
user: User
}
impl Extract for AdminOnly
where
B: axum::body::HttpBody + Send,
B::Data: Send,
B::Error: Into<Response>,
{
type Error = Response;
async fn extract(req: &mut RequestParts<B>) -> Result<Self, Self::Error> {
let user = req.extensions_mut().get_mut::<User>().unwrap().clone();
Ok(AdminOnly { user })
}
}This extractor assumes the user is already authenticated and authorized, but provides no enforcement mechanism. Any route using this extractor becomes vulnerable if the middleware chain is broken or bypassed.
Excessive data exposure through response types is another critical issue. Axum's response system makes it easy to accidentally expose internal structures:
async fn list_users(
Extension(db): Extension<PgPool>
) -> impl IntoResponse {
let users = sqlx::query_as!("SELECT * FROM users")
.fetch_all(db)
.await?;
Json(users) // Exposes password hashes, internal IDs, etc.
}The response serializes the entire user struct, potentially including sensitive fields like password hashes, internal database IDs, or system metadata. This violates the principle of least privilege at the data exposure level.
Finally, insecure default configurations in Axum middleware chains can create design vulnerabilities. The order of middleware matters significantly:
let app = Router::new()
.route("/admin/*", get(admin_dashboard))
.layer(CorsLayer::new())
.layer(Logger::default());If CORS is applied before authentication middleware, unauthenticated requests might receive preflight responses that reveal API structure and available endpoints. The design flaw is the middleware ordering that allows information disclosure before proper access controls are applied.
Axum-Specific Detection
Detecting insecure design patterns in Axum requires examining both the route structure and the middleware composition. middleBrick's black-box scanning approach is particularly effective here because it tests the actual exposed API surface without requiring source code access.
For Axum applications, middleBrick performs several specific checks. It analyzes the HTTP method usage patterns - a GET endpoint that modifies data or a POST endpoint that only retrieves information indicates potential design confusion. The scanner tests parameter extraction patterns by sending requests with unexpected parameter types and observing how the application responds.
The tool specifically looks for Axum's Extract trait usage patterns. When it encounters endpoints that accept complex path parameters or query parameters without validation, it flags these as potential design issues. For example:
Path<(u32, String, String)> // Multiple parameters in path
Query<AdminFilter> // Complex query structures without validationmiddleBrick tests these by sending malformed or unexpected parameter combinations to see if the application properly validates or sanitizes input before processing.
The scanner also examines response serialization patterns. It captures API responses and analyzes the JSON structure to identify potential over-exposure of internal data. If a /users endpoint returns fields like is_admin, internal_id, or creation_timestamp when these aren't necessary for the client, middleBrick flags this as a data exposure design issue.
For middleware chain analysis, middleBrick tests the actual request flow by attempting authenticated and unauthenticated access to various endpoints. It specifically looks for endpoints that should require authentication but don't enforce it, or endpoints that expose sensitive functionality without proper authorization checks.
The tool's LLM security checks are particularly relevant for Axum applications that might integrate AI features. It tests for system prompt leakage by sending specially crafted requests that could extract model instructions or training data. This is a design vulnerability if the application exposes AI endpoints without proper rate limiting or content filtering.
middleBrick's OpenAPI spec analysis complements the black-box scanning by comparing the documented API structure with the actual runtime behavior. If the spec shows authentication requirements that don't match the actual implementation, this indicates a design documentation gap that could lead to security issues.
Axum-Specific Remediation
Remediating insecure design in Axum applications requires architectural changes rather than simple code patches. The first principle is to implement proper separation of concerns using Axum's middleware system.
Create dedicated middleware for authentication and authorization:
async fn auth_middleware(
mut req: RequestParts<Body>
) -> Result<RequestParts<Body>, Response> {
// Extract token from Authorization header
let auth_header = req.headers().get("Authorization");
if let Some(auth_header) = auth_header {
if let Ok(token) = auth_header.to_str() {
if let Ok(user) = validate_token(token) {
req.extensions_mut().insert(user);
return Ok(req);
}
}
}
Err(Response::builder()
.status(StatusCode::UNAUTHORIZED)
.body(Body::from("Unauthorized")))
}This middleware should be applied to the entire application or specific route groups, never implemented inline in individual handlers where it can be bypassed.
For parameter extraction, use Axum's type system to enforce validation at the extraction layer:
#[derive(Deserialize)]
#[serde(rename_all = "camelCase")]
struct UserIdPath {
#[serde(deserialize_with = "validate_user_id")]
user_id: u32,
}
fn validate_user_id<'de, D>(deserializer: D) -> Result<u32, D::Error>
where
D: serde::Deserializer<'de>,
{
let s = String::deserialize(deserializer)?;
match s.parse::() {
Ok(num) if num > 0 => Ok(num),
_ => Err(serde::de::Error::custom("Invalid user ID"))
}
} This ensures that only valid user IDs reach your route handlers, preventing IDOR attacks at the design level.
Implement proper response data shaping using dedicated DTOs (Data Transfer Objects):
#[derive(Serialize)]
struct UserResponse {
id: u32,
username: String,
email: String,
}
async fn get_user(
Path(user_id): Path<u32>,
Extension(db): Extension<PgPool>
) -> Result<Json<UserResponse>, StatusCode> {
let user = sqlx::query_as!(User, "SELECT id, username, email FROM users WHERE id = $1", user_id)
.fetch_one(db)
.await?;
Ok(Json(UserResponse {
id: user.id,
username: user.username,
email: user.email
}))
}This pattern ensures you only expose the data fields that are actually needed by clients, never internal implementation details.
For middleware ordering, establish a clear security layer stack:
let app = Router::new()
.route("/api/*", api_routes)
.route("/admin/*", admin_routes)
.layer(Recovery::new())
.layer(CorsLayer::new())
.layer(Logger::default())
.layer(auth_middleware_layer())
.layer(rate_limit_layer());Authentication and authorization should come after CORS but before logging and rate limiting. This ensures all security checks are applied before any processing occurs.
Finally, implement comprehensive input validation using Axum's extractors combined with custom validation logic:
async fn create_post(
Json(payload): Json<CreatePost>,
Extension(db): Extension<PgPool>
) -> Result<Json<PostResponse>, StatusCode> {
// Validate payload content
if payload.title.len() < 5 || payload.title.len() > 200 {
return Err(StatusCode::BAD_REQUEST);
}
// Additional business logic validation
if !is_allowed_to_post(&payload.user_id, &db).await? {
return Err(StatusCode::FORBIDDEN);
}
// Process the request
Ok(Json(PostResponse::from(post)))
}This approach validates data at the point of entry, preventing malformed or malicious data from propagating through your application's design.