Data Exposure in Axum with Dynamodb

When building a web service with the Axum framework in Rust and persisting data in Amazon DynamoDB, data exposure risks arise from mismatches between application-layer handling and DynamoDB’s access patterns. Even when the database enforces IAM policies, an API can inadvertently leak data through insufficient authorization checks at the application level, verbose error messages, or improper serialization.

Consider an endpoint that retrieves a user profile by ID. If the handler uses a path parameter such as user_id to build a DynamoDB key but does not verify that the authenticated subject is allowed to access that specific partition key, the request may return another user’s item. This is a classic broken object-level authorization (BOLA) scenario, often cataloged as IDOR in the OWASP API Top 10. DynamoDB does not understand application ownership semantics; it returns items based on key queries and IAM policy evaluation. If the application layer fails to enforce ownership, DynamoDB will return data that the client should not see, resulting in data exposure.

Serialization issues can also lead to data exposure. In Axum, handlers typically serialize responses into JSON using Serde. If structs include fields that contain sensitive information such as internal identifiers, emails, or temporary credentials, and those fields are accidentally included in the serialized output, the data will be exposed to any client who can read the response. For example, a DynamoDB item may contain a password_hash attribute that should never leave the backend. If the Rust struct used for serialization does not explicitly exclude this field with [serde(skip_serializing)], the field can be unintentionally exposed in API responses.

Error handling amplifies the risk. Detailed DynamoDB errors, such as conditional check failures or validation exceptions, may reveal internal schema details or the presence of specific attributes. If an Axum handler forwards low-level error messages to the client, an attacker can learn about data existence or structure, aiding further exploitation. Insecure default configurations in DynamoDB, such as tables with broad IAM permissions for debugging, can also contribute to exposure when those permissions are accidentally used in production code.

The combination of Axum’s type-driven routing and DynamoDB’s key-value model requires disciplined authorization, careful data modeling, and strict output filtering to prevent data exposure. Without explicit checks on every request and conservative serialization practices, sensitive data can move from a controlled store to an uncontrolled response.

To mitigate data exposure when using DynamoDB with Axum, implement precise authorization, conservative serialization, and safe error handling. The following patterns demonstrate concrete fixes.

1. Enforce ownership at the handler level

Always validate that the authenticated subject matches the partition key of the requested item. Use a typed key structure and compare the subject identifier before querying DynamoDB.

use aws_sdk_dynamodb::Client;
use axum::{extract::State, Json};
use serde::{Deserialize, Serialize};

#[derive(Debug, Deserialize)]
struct GetProfilePath {
    user_id: String,
}

#[derive(Debug, Serialize)]
struct SafeProfile {
    user_id: String,
    display_name: String,
    // Sensitive fields are omitted or explicitly marked to skip serialization
    #[serde(skip_serializing)]
    password_hash: String,
}

async fn get_profile(
    State(db_client): State<Client>,
    axum::extract::Path(params): axum::extract::Path<GetProfilePath>,
    auth: Option<AuthExtraction>, // custom extractor providing authenticated subject
) -> Result<Json<SafeProfile>, (axum::http::StatusCode, String)> {
    let subject = auth.ok_or((axum::http::StatusCode::UNAUTHORIZED, "missing auth"))?;
    // Enforce ownership: subject must match user_id
    if subject.user_id != params.user_id {
        return Err((axum::http::StatusCode::FORBIDDEN, "access denied".to_string()));
    }

    let key = aws_sdk_dynamodb::types::AttributeValue::S(params.user_id.clone());
    let resp = db_client
        .get_item()
        .table_name("users")
        .set_key(Some(
            ["user_id"]
                .into_iter()
                .zip([key]
                    .into_iter()
                    .map(|v| v.into()))
                .collect(),
        ))
        .send()
        .await
        .map_err(|e| (axum::http::StatusCode::INTERNAL_SERVER_ERROR, e.to_string()))?;

    let item = resp.item().ok_or((axum::http::StatusCode::NOT_FOUND, "not found".to_string()))?;
    let profile = SafeProfile {
        user_id: item.get("user_id").and_then(|v| v.as_s().ok()).cloned().unwrap_or_default(),
        display_name: item.get("display_name").and_then(|v| v.as_s().ok()).cloned().unwrap_or_default(),
        password_hash: String::new(), // intentionally empty, never serialized
    };
    Ok(Json(profile))
}

2. Use conservative serialization with Serde attributes

Ensure that sensitive fields are omitted from JSON responses by default. Use skip_serializing for secrets and prefer DTOs (data transfer objects) that only include intended public fields.

#[derive(Debug, Serialize)]
struct UserSettings {
    theme: String,
    #[serde(skip_serializing)]
    api_key: String,
    #[serde(skip_serializing)]
    internal_role: String,
}

// Only expose non-sensitive fields via API
async fn get_settings(
    State(db_client): State<Client>,
    // ...
) -> Result<Json<UserSettings>, (StatusCode, String)> {
    // fetch item from DynamoDB
    let settings = UserSettings {
        theme: "dark".to_string(),
        api_key: "secret123".to_string(),
        internal_role: "admin".to_string(),
    };
    Ok(Json(settings))
}

3. Sanitize errors and avoid information leakage

Do not expose raw DynamoDB error details to clients. Map them to generic responses and log details internally for investigation.

async fn safe_get_item(
    db_client: &Client,
    key: &HashMap<String, AttributeValue>
) -> Result<HashMap<String, AttributeValue>, (StatusCode, String)> {
    let resp = db_client.get_item()
        .table_name("items")
        .set_key(Some(key.clone()))
        .send()
        .await;

    match resp {
        Ok(output) => Ok(output.item().cloned().unwrap_or_default()),
        Err(e) => {
            // Log full error internally, return generic message to client
            tracing::error!("dynamodb error: {:?}", e);
            Err((StatusCode::INTERNAL_SERVER_ERROR, "request failed".to_string()))
        }
    }
}

4. Apply least-privilege IAM policies at runtime

Ensure the runtime credentials used by the service only allow required actions on specific resources. For example, use a policy that restricts dynamodb:GetItem to items where the partition key equals the authenticated subject’s ID. This limits the impact of any application-level mistake.

By combining these patterns—explicit ownership checks, safe serialization, and sanitized error handling—you reduce the likelihood of data exposure when Axum interacts with DynamoDB.