Integrity Failures in Axum with Dynamodb

Integrity failures occur when an API allows unauthorized changes to data or fails to verify that data remains consistent and trustworthy across layers. In an Axum service that uses Amazon DynamoDB as the primary datastore, several factors in how requests are handled can expose integrity weaknesses.

Axum is a Rust web framework that relies on extractors to bind request data into strongly typed structures. When those structures map directly to DynamoDB item shapes without additional validation, unchecked fields can lead to tampering through path-based or object-level authorization issues (BOLA/IDOR). For example, an endpoint that derives a DynamoDB key from a route parameter but also accepts an item version or conditional token from the client may overwrite state if the client-supplied value is not independently verified.

DynamoDB’s schema-less design and flexible attribute types amplify these risks. If an Axum handler trusts client-provided attribute values for critical fields such as tenant identifiers, ownership markers, or state flags, an attacker can inject values that bypass intended scoping or privilege boundaries. This is especially relevant when conditional writes are used without strict pre-checks; missing or malformed condition expressions can allow concurrent updates to violate business rules, leading to lost updates or inconsistent aggregates.

Another contributing factor is insufficient serialization checks. Axum often uses JSON payloads that are deserialized into Rust structs and then marshaled into DynamoDB attribute values via the AWS SDK. If field-level validation is omitted or performed only at the JSON layer, the resulting DynamoDB item may contain unexpected types or missing required attributes, which can later cause partial writes or misinterpreted queries. For instance, a numeric quantity field expected to be a positive integer may be deserialized as a string or negative number, and DynamoDB’s lack of schema enforcement permits this, enabling downstream logic errors or privilege escalations when the value is used in authorization decisions.

The interaction also surfaces in conditional logic and error handling patterns. An Axum handler that performs a GetItem or UpdateItem without robust pre-flight validation of item existence and ownership may inadvertently expose whether specific keys exist, aiding enumeration attacks. Misconfigured retry logic or partial failure handling can leave items in a partially updated state, breaking invariants. Because DynamoDB does not support multi-document transactions in all modes, compensating logic in Axum must be carefully designed to preserve integrity across operations.

These combinations highlight why integrity controls must span both the application layer (Axum extractors, validation libraries, and explicit business rules) and the data layer (DynamoDB condition expressions, item keys, and attribute-level checks). Relying on defaults or implicit trust in either side of the integration creates gaps that can be leveraged to modify data in unauthorized ways or to bypass intended access controls.

Remediation centers on strict validation, explicit condition expressions, and defensive handling of item keys and attributes within Axum handlers. Below are targeted patterns and code examples that reduce integrity risks when using DynamoDB.

• Validate and bind identifiers server-side

Do not rely on client-provided identifiers for DynamoDB keys. Derive tenant and item identifiers from authentication context and route parameters, and enforce ownership checks before any write.

// Axum handler with server-side key derivation
use axum::{extract::State, http::StatusCode, response::IntoResponse};
use aws_sdk_dynamodb::Client;
use serde::{Deserialize, Serialize};

#[derive(Deserialize)]
struct UpdateParams {
    tenant_id: String,
    item_id: String,
}

#[derive(Deserialize)]
struct UpdatePayload {
    quantity: i64,
}

async fn update_handler(
    State(state): State<AppState>,
    params: axum::extract::Path<(String, String)>, // (tenant_id from route, item_id from route)
    payload: axum::extract::Json<UpdatePayload>,
) -> Result<impl IntoResponse, (StatusCode, String)> {
    let (tenant_id, item_id) = params.into_inner();
    // Ensure the authenticated principal matches tenant_id, omitted for brevity
    let condition_expr = "attribute_exists(pk) AND quantity >= :min_qty";
    let condition_expr = condition_expr.to_string();
    let min_qty = 0;

    let pk = format!("TENANT#{}#ITEM#{}", tenant_id, item_id);
    let sk = "METRIC#CURRENT".to_string();

    let resp = state
        .db
        .update_item()
        .table_name(&state.table_name)
        .key("pk", aws_sdk_dynamodb::types::AttributeValue::S(pk))
        .key("sk", aws_sdk_dynamodb::types::AttributeValue::S(sk))
        .update_expression("SET quantity = :q")
        .condition_expression(condition_expr)
        .expression_attribute_values(
            ":q",
            &aws_sdk_dynamodb::types::AttributeValue::N(payload.quantity.to_string()),
        )
        .expression_attribute_values(
            ":min_qty",
            &aws_sdk_dynamodb::types::AttributeValue::N(min_qty.to_string()),
        )
        .send()
        .await
        .map_err(|e| (StatusCode::INTERNAL_SERVER_ERROR, e.to_string()))?;

    Ok((StatusCode::OK, format!("Updated: {:?}", resp)))
}

// Condition expression to enforce tenant ownership and prevent negative quantities
let condition_expr = "tenant_id = :tid AND quantity >= :min_qty";
let resp = client
    .update_item()
    .table_name("Items")
    .key("pk", AttributeValue::S("ITEM#123".to_string()))
    .update_expression("SET quantity = :q")
    .condition_expression(condition_expr.to_string())
    .expression_attribute_values(
        ":tid",
        AttributeValue::S("tenant-a".to_string()),
    )
    .expression_attribute_values(
        ":q",
        AttributeValue::N("42".to_string()),
    )
    .expression_attribute_values(
        ":min_qty",
        AttributeValue::N("0".to_string()),
    )
    .send()
    .await?;

// Validate payload and map to DynamoDB attributes with explicit types
use serde_json::json;
use aws_sdk_dynamodb::types::AttributeValue;

fn prepare_item(payload: &serde_json::Value) -> Result<std::collections::HashMap<String, AttributeValue>, String> {
    let obj = payload.as_object().ok_or("Payload must be an object")?;
    let tenant_id = obj.get("tenant_id").and_then(|v| v.as_str()).ok_or("Missing tenant_id")?;
    let quantity = obj.get("quantity").and_then(|v| v.as_i64()).ok_or("quantity must be an integer")?;
    if quantity < 0 {
        return Err("quantity must be non-negative".into());
    }
    let mut item = std::collections::HashMap::new();
    item.insert("pk".to_string(), AttributeValue::S(format!("TENANT#{}#ITEM#{}", tenant_id, "xyz")));
    item.insert("quantity".to_string(), AttributeValue::N(quantity.to_string()));
    item.insert("tenant_id".to_string(), AttributeValue::S(tenant_id.to_string()));
    Ok(item)
}

// Handle conditional check failures explicitly
match result {
    Err(e) if e.is_conditional_check_failed_exception() => {
        tracing::warn!(?e, "Conditional check failed, possible integrity violation");
        (StatusCode::CONFLICT, "Update conflict".to_string()).into_response()
    }
    _ => Err(e.into()),
};