Cors Wildcard in Axum with Dynamodb

A CORS wildcard in an Axum service that uses DynamoDB as a backend can unintentionally expose sensitive data and amplify authorization risks. When cors.allow_any_origin or a broad wildcard such as * is set in the Axum CORS layer, the service permits any origin to perform authenticated requests on behalf of users. If the Axum application then forwards those requests to DynamoDB using AWS credentials attached to the service (for example, via an IAM role assigned to an EC2 instance, ECS task, or Lambda runtime), there is no additional origin check before data is read or written.

This combination creates a CORS-based privilege escalation path. A malicious site can send cross-origin requests to the Axum endpoint; because CORS allows any origin, the browser includes credentials (cookies or Authorization headers). The Axum service, authorized via its DynamoDB credentials, processes the request and returns data that the end user is not entitled to view. This violates the principle of user-level authorization and can lead to sensitive DynamoDB records being exposed through BOLA/IDOR patterns facilitated by the wildcard CORS rule.

Moreover, preflight requests (OPTIONS) with wildcard origins can cause frameworks to fall back to permissive default headers, which may include broad Access-Control-Allow-Headers and Access-Control-Allow-Methods values. Attackers can craft requests that trigger unsafe methods or headers, increasing the risk of unintended DynamoDB operations if the Axum routes are not strictly limited. The issue is not DynamoDB itself but the permissive CORS configuration that allows untrusted origins to invoke backend actions that ultimately query or modify DynamoDB items.

In a security scan, such misconfigurations are flagged under the Authentication and BOLA/IDOR checks. The scanner observes that the CORS policy permits arbitrary origins while the application performs DynamoDB operations without validating that the requesting user is the owner of the targeted item. This mapping to OWASP API Top 10 A01: Broken Object Level Authorization and A05: Broken Access Control highlights how a frontend-style CORS issue can lead to backend data exposure when integrated with a datastore like DynamoDB.

MiddleBrick detects this pattern by correlating CORS findings with IAM-aware checks and DynamoDB-specific inventory. The report includes prioritized findings with severity and remediation guidance, emphasizing that CORS should never use wildcards in production services that interact with backend data stores, and that origins must be explicitly enumerated and validated before any DynamoDB call is performed.

To secure an Axum service that interacts with DynamoDB, CORS must be configured with explicit origins and strict headers, and every DynamoDB operation must enforce user-level ownership checks. Below are concrete, syntactically correct examples for Axum using the official AWS SDK for Rust.

1. Configure strict CORS in Axum

Replace wildcard CORS with an allowlist of known frontend origins and limit headers and methods.

use axum::middleware::from_fn_with_state;
use axum::routing::get;
use axum::Router;
use tower_http::cors::{CorsLayer, Any};

let cors = CorsLayer::new()
    .allow_origin(vec!["https://app.yourcompany.com".parse().unwrap(), "https://staging.yourcompany.com".parse().unwrap()])
    .allow_methods(vec![axum::http::Method::GET, axum::http::Method::POST])
    .allow_headers(vec![axum::http::HeaderName::from_static("content-type")])
    .expose_headers(vec![])
    .max_age(Some(std::time::Duration::from_secs(600)));

let app = Router::new()
    .route("/api/profile", get(get_profile))
    .layer(from_fn_with_state((), move |_, state| async move { Ok::<_, ())>((state, cors)) }));

2. Enforce ownership checks before DynamoDB access

Always verify that the requesting user owns the item in DynamoDB. Use the AWS SDK for Rust with proper error handling and avoid scanning entire tables.

use aws_sdk_dynamodb::Client;
use aws_sdk_dynamodb::types::AttributeValue;
use std::error::Error;

async fn get_user_item(client: &Client, user_id: &str, item_id: &str) -> Result, Box> {
    let resp = client.get_item()
        .table_name("UserItems")
        .key("user_id", AttributeValue::S(user_id.to_string()))
        .key("item_id", AttributeValue::S(item_id.to_string()))
        .send()
        .await?;

    Ok(resp.item)
}

3. Use condition expressions to prevent unauthorized updates

When modifying DynamoDB items, include a condition that the user_id matches the caller to avoid BOLA.

async fn update_user_item_if_owner(
    client: &Client,
    user_id: &str,
    item_id: &str,
    new_value: &str,
) -> Result<(), Box> {
    client.update_item()
        .table_name("UserItems")
        .key("user_id", AttributeValue::S(user_id.to_string()))
        .key("item_id", AttributeValue::S(item_id.to_string()))
        .set_expression_attribute_values(Some({
            let mut map = std::collections::HashMap::new();
            map.insert(":val".to_string(), AttributeValue::S(new_value.to_string()));
            map
        }))
        .update_expression("SET value = :val")
        .condition_expression("user_id = :uid")
        .expression_attribute_values({
            let mut map = std::collections::HashMap::new();
            map.insert(":uid".to_string(), AttributeValue::S(user_id.to_string()));
            map
        })
        .send()
        .await?;
    Ok(())
}

4. Combine CORS state with request validation in handlers

Ensure handlers validate the authenticated user identity before invoking the DynamoDB client. Do not rely on CORS alone for authorization; treat CORS as a network-level control only.

async fn get_profile(
    State(cors): State,
    Extension(user_id): Extension,
    Path(item_id): Path,
) -> Result, (StatusCode, String)> {
    let item = get_user_item(&client, &user_id, &item_id).await.map_err(|e| (StatusCode::INTERNAL_SERVER_ERROR, e.to_string()))?;
    match item {
        Some(val) => Ok(Json(val)),
        None => Err((StatusCode::NOT_FOUND, "Item not found or access denied".into())),
    }
}

5. Infrastructure and scanning recommendations

Use MiddleBrick’s CLI to verify that CORS headers are not permissive and that no wildcard origins remain. Run middlebrick scan <url> regularly to detect exposed CORS policies and misconfigured authorization around DynamoDB endpoints. The dashboard and GitHub Action integrations can fail builds if the score drops below your chosen threshold, providing continuous monitoring for these classes of misconfiguration.