Format String in Actix with Dynamodb

A format string vulnerability occurs when user-controlled input is directly used in functions that interpret format specifiers, such as format! or write! in Rust, without proper sanitization or explicit formatting. In an Actix web service that interacts with Amazon DynamoDB, this typically arises when constructing log messages, error responses, or debug strings using data extracted from DynamoDB item attributes (for example, a user-supplied identifier or metadata field).

Consider an Actix handler that retrieves an item from DynamoDB and directly includes a potentially malicious attribute in a logging or response string. If the attribute contains format specifiers like %s, %x, or additional format directives, and the logging or formatting call does not use a strict format string, the application may read from or write to the stack, leading to information disclosure or code execution. The DynamoDB value is treated as a format string rather than as data, which is the core of the vulnerability.

For example, an attacker might store a value such as %s %s %s in a DynamoDB attribute named username. When the Actix service later logs this value using a macro that implicitly treats the input as a format string, the application may attempt to read arbitrary memory addresses from the stack. Because DynamoDB is the source of the attacker-controlled data and Actix handles the request, this combination creates a clear path for an attacker to probe the process memory or trigger unintended behavior through crafted input stored in the database.

Another scenario involves constructing HTTP responses or error messages by interpolating DynamoDB attribute values into format strings. If the format string is not explicitly provided as a literal, and the attribute value controls the format specification, an attacker can influence how subsequent arguments are interpreted. In the context of an Actix service, this could lead to memory disclosure or instability, especially when the service relies on structured logging or custom debug formatting that inadvertently trusts stored data.

Because middleBrick tests unauthenticated attack surfaces and includes input validation checks, such a vulnerability would be flagged under the Input Validation category, with a corresponding risk score and guidance to ensure strict formatting and avoid passing user data directly into formatting functions.

To remediate format string risks when working with DynamoDB data in Actix, ensure that all data from DynamoDB is treated strictly as data and never as a format string. Use explicit format strings in logging and output construction, and validate or sanitize attribute values before use.

Example vulnerable code

The following example demonstrates an unsafe pattern where a DynamoDB attribute is used directly in a logging call, risking format string abuse:

use actix_web::{get, web, HttpResponse, Responder};
use rusoto_dynamodb::{DynamoDb, DynamoDbClient, GetItemInput};

#[get("/user/{id}")]
async fn get_user(
    id: web::Path,
    client: web::Data,
) -> impl Responder {
    let input = GetItemInput {
        table_name: "users".to_string(),
        key: [("id".to_string(), rusoto_dynamodb::AttributeValue { s: Some(id.into_inner()), ..Default::default() })].into_iter().collect(),
        ..Default::default()
    };
    match client.get_item(input).await {
        Ok(output) => {
            if let Some(item) = output.item {
                if let Some(username) = item.get("username").and_then(|v| v.s.as_ref()) {
                    // UNSAFE: username may contain format specifiers
                    log::info!("{}", username);
                }
            }
            HttpResponse::Ok().finish()
        }
        Err(_) => HttpResponse::InternalServerError().finish(),
    }
}

Secure remediation with explicit formatting

Use an explicit format string and treat the DynamoDB attribute as a single argument:

use actix_web::{get, web, HttpResponse, Responder};
use rusoto_dynamodb::{DynamoDb, DynamoDbClient, GetItemInput};

#[get("/user/{id}")]
async fn get_user(
    id: web::Path,
    client: web::Data<DynamoDbClient>,
) -> impl Responder {
    let input = GetItemInput {
        table_name: "users".to_string(),
        key: [("id".to_string(), rusoto_dynamodb::AttributeValue { s: Some(id.into_inner()), ..Default::default() })].into_iter().collect(),
        ..Default::default()
    };
    match client.get_item(input).await {
        Ok(output) => {
            if let Some(item) = output.item {
                if let Some(username) = item.get("username").and_then(|v| v.s.as_ref()) {
                    // SAFE: explicit format string prevents interpretation of username as format specifiers
                    log::info!("User username: {}", username);
                }
            }
            HttpResponse::Ok().finish()
        }
        Err(_) => HttpResponse::InternalServerError().finish(),
    }
}

Additionally, apply input validation to restrict the content of attributes used in logs or outputs. For instance, reject or encode values that contain format specifiers when they are not expected. The DynamoDB AttributeValue should be handled as raw data, and serialization or logging should always specify the format explicitly.

For comprehensive protection, integrate middleBrick scans into your workflow. The CLI tool (middlebrick scan <url>) can be run locally during development, while the GitHub Action can enforce security gates in CI/CD pipelines. If you use AI coding assistants, the MCP Server allows middleBrick to scan APIs directly from your development environment, helping catch issues like improper formatting before deployment.