Injection Flaws in Actix with Dynamodb

When building Actix-web services that interact with DynamoDB, injection flaws arise when untrusted input is passed into DynamoDB operations without proper validation or parameterization. DynamoDB’s expression-based APIs, such as ConditionExpression, FilterExpression, and KeyConditionExpression, accept attribute values separately from expression strings, which allows safe usage when the SDK APIs are used as designed. However, if developers mistakenly interpolate user input directly into expression strings or construct expressions by string concatenation, injection-like behavior can occur. This can lead to unintended data access, privilege escalation, or data exposure.

For example, consider an endpoint that accepts a user-supplied sort key value to query a table. If the application builds a KeyConditionExpression by concatenating the user value into the string, an attacker could supply a value like id = '123' AND attribute_exists(aws_access_key_id) to probe for additional conditions. Because DynamoDB does not support parameterized conditionals the way SQL does, the responsibility falls on the application to avoid injecting raw input into expressions. Unsafe handling can map to the OWASP API Top 10 category API1:2023 – Broken Object Level Authorization when combined with BOLA/IDOR, enabling attackers to access other users’ data by manipulating key expressions.

In Actix, a typical handler might deserialize JSON into a struct and forward values to the AWS SDK for Rust (aws-sdk-dynamodb). If the SDK call uses .key_condition_expression() or .filter_expression() with strings built from unchecked input, the API surface includes injection risks. The DynamoDB backend will evaluate the expression as written, which can bypass intended filters. This is not a syntax injection in the SQL sense, but it is an injection flaw because the expression’s semantics are altered by untrusted data. Findings from middleBrick’s checks for Input Validation and Property Authorization highlight these patterns when expressions are constructed dynamically without strict allowlists or schema validation.

Additional risk occurs when developers use DynamoDB’s Scan with FilterExpression and permit user input to dictate filter logic. An attacker could supply values designed to always evaluate to true, causing excessive data retrieval and potential Data Exposure. middleBrick’s checks for Data Exposure and Input Validation aim to detect such patterns by correlating runtime behavior with the OpenAPI spec and flagging endpoints that accept untrusted input into authorization or filtering logic without safeguards.

Remediation centers on keeping expression strings static and treating all user input strictly as values. Use the SDK’s built-in expression builders and never concatenate or interpolate user-controlled data into KeyConditionExpression, FilterExpression, or ConditionExpression. For Actix handlers, validate and allowlist inputs before constructing requests, and prefer using ExpressionAttributeNames and ExpressionAttributeValues to avoid naming conflicts and injection-style manipulation.

Example: Safe KeyConditionExpression with partition and sort keys

use aws_sdk_dynamodb::types::AttributeValue;
use aws_sdk_dynamodb::Client;
use actix_web::{web, HttpResponse, Result};

async fn get_item(
    client: web::Data,
    path: web::Path<(String, String)>>, // (partition_key_value, sort_key_value)
) -> Result {
    let (pk_value, sk_value) = path.into_inner();
    // Validate format/size before use
    if !pk_value.chars().all(|c| c.is_alphanumeric()) {
        return Ok(HttpResponse::BadRequest().body("Invalid partition key"));
    }

    let output = client
        .query()
        .table_name("MyTable")
        .key_condition_expression("pk = :pk AND sk = :sk")
        .expression_attribute_values({
            let mut map = std::collections::HashMap::new();
            map.insert(":pk".to_string(), AttributeValue::S(pk_value));
            map.insert(":sk".to_string(), AttributeValue::S(sk_value));
            map
        })
        .send()
        .await
        .map_err(|err| actix_web::error::ErrorInternalServerError(err.to_string()))?;

    Ok(web::Json(output.items))
}

Example: Safe FilterExpression with allowlisted field and value placeholder

use aws_sdk_dynamodb::types::AttributeValue;
use aws_sdk_dynamodb::Client;
use actix_web::web;

async fn scan_filtered(
    client: web::Data,
    field: web::Query>,
) -> Result {
    let field_name = match field.get("field") {
        Some(f) if f == "status" || f == "priority" => f.clone(),
        _ => return Ok(HttpResponse::BadRequest().body("Invalid field")),
    };
    let raw_value = match field.get("value") {
        Some(v) => v,
        None => return Ok(HttpResponse::BadRequest().body("Missing value")),
    };

    let output = client
        .scan()
        .table_name("MyTable")
        .filter_expression(format!("#fld = :val").as_str())
        .expression_attribute_names({
            let mut map = std::collections::HashMap::new();
            map.insert("#fld".to_string(), field_name.as_str());
            map
        })
        .expression_attribute_values({
            let mut map = std::collections::HashMap::new();
            map.insert(":val".to_string(), AttributeValue::S(raw_value.to_string()));
            map
        })
        .limit(100)
        .send()
        .await
        .map_err(|err| actix_web::error::ErrorInternalServerError(err.to_string()))?;

    Ok(web::Json(output.items))
}

Remediation best practices

• Keep expression strings constant and supply all conditional values via ExpressionAttributeValues.
• Use ExpressionAttributeNames for attribute names that come from non-fully-trusted sources (e.g., query parameters) to avoid name-based injection or reserved-word errors.
• Apply strict allowlists on any user input used in expression components (field names, key formats).
• Validate input length, type, and format before passing it to the SDK; reject malformed or overly complex inputs early in the Actix handler.
• Prefer Query over Scan where possible; when using Scan, restrict FilterExpression to simple equality/inequality on indexed attributes and enforce server-side limits.

By designing handlers this way, you ensure DynamoDB receives safe, static expressions and only values change at runtime. This reduces the attack surface aligned with Input Validation, Property Authorization, and Data Exposure checks that middleBrick evaluates during scans.