Mass Assignment in Actix with Dynamodb

Mass assignment occurs when an API binds incoming JSON directly to a data model or structure without filtering individual fields. In an Actix-web service that uses DynamoDB as the persistence layer, this typically manifests in deserialization helpers that map HTTP request bodies into DynamoDB AttributeValue expressions or into strongly-typed Rust structs that are later passed to DynamoDB PutItem or UpdateItem calls. Because DynamoDB is schemaless at the wire level, it is straightforward to store fields not anticipated by the application, and Actix’s flexible extraction patterns can inadvertently allow client-supplied keys to map into DynamoDB item attributes that drive privilege escalation or unintended data writes.

Consider an endpoint that accepts a JSON payload to create a user profile and directly forwards the parsed values to DynamoDB without field-level allowlisting. An attacker can inject additional attributes such as admin, permissions, or even __meta__ that the Rust struct does not model but that DynamoDB happily stores. Later reads may return these injected attributes, and if the application uses DynamoDB expressions for updates, those expressions can reference maliciously injected keys. This maps to the BFLA/Privilege Escalation and Property Authorization checks in middleBrick’s 12 security checks, because the unchecked binding effectively grants broader data manipulation than intended.

With DynamoDB, there is no server-side schema enforcement at write time; the onus is on the client and application code to validate shape. Actix typically uses serde for deserialization, and if the developer relies on #[serde(flatten)] or derives Deserialize with permissive maps (e.g., HashMap), an attacker can supply unexpected keys that propagate into the item. middleBrick’s unauthenticated scan surface can surface this by correlating runtime behavior (observed item attributes) with the OpenAPI spec and identifying missing authorization on properties, a finding that aligns with BOLA/IDOR and Property Authorization checks.

Remediation centers on strict input validation, explicit field mapping, and avoiding pass-through deserialization into DynamoDB AttributeValue maps unless absolutely necessary. Prefer strongly-typed structs with serde and reject unknown fields using #[serde(deny_unknown_fields)]. When you must work with dynamic attributes, parse into an intermediate map and apply an allowlist before constructing DynamoDB items.

Example 1: Strongly-typed struct with deny_unknown_fields

use actix_web::{post, web, HttpResponse};
use serde::Deserialize;
use aws_sdk_dynamodb::types::AttributeValue;
use aws_sdk_dynamodb::Client as DynamoClient;

#[derive(Deserialize, Debug)]
#[serde(deny_unknown_fields)]
struct CreateUser {
    user_id: String,
    email: String,
    display_name: String,
}

#[post("/users")]
async fn create_user(
    body: web::Json,
    dynamo: web::Data,
) -> HttpResponse {
    let item = aws_sdk_dynamodb::types::PutItemInput::builder()
        .table_name("users")
        .set_item(Some({
            let mut map = std::collections::HashMap::new();
            map.insert("user_id".to_string(), AttributeValue::S(body.user_id.clone()));
            map.insert("email".to_string(), AttributeValue::S(body.email.clone()));
            map.insert("display_name".to_string(), AttributeValue::S(body.display_name.clone()));
            map
        }))
        .build()
        .expect("valid");

    // Assume client put_item call here
    HttpResponse::Ok().finish()
}

Example 2: Dynamic mode with allowlist

use actix_web::{post, web, HttpResponse};
use serde_json::Value;
use aws_sdk_dynamodb::types::AttributeValue;
use aws_sdk_dynamodb::Client as DynamoClient;
use std::collections::HashMap;

async fn build_item_with_allowlist(
    raw: serde_json::Map,
    dynamo: &DynamoClient,
) -> HashMap {
    let allowed = ["email", "display_name", "profile_version"];
    let mut item = HashMap::new();
    for key in allowed {
        if let Some(v) = raw.get(key) {
            item.insert(
                key.to_string(),
                AttributeValue::S(v.as_str().unwrap_or("").to_string()),
            );
        }
    }
    item
}

#[post("/profile")]
async fn update_profile(
    body: web::Json,
    dynamo: web::Data,
) -> HttpResponse {
    let item = build_item_with_allowlist(body.as_object().cloned().unwrap_or_default(), &dynamo);
    let put = aws_sdk_dynamodb::types::PutItemInput::builder()
        .table_name("profiles")
        .set_item(Some(item))
        .build()
        .expect("valid");
    HttpResponse::Ok().finish()
}

These patterns ensure that only expected fields reach DynamoDB, reducing the risk of privilege escalation via mass assignment. For scans that include OpenAPI/Swagger spec analysis, middleBrick can highlight mismatches between declared request schemas and runtime item attributes, providing actionable guidance aligned with OWASP API Top 10 and relevant compliance mappings.