Formula Injection in Axum with Dynamodb

Formula Injection occurs when user-controlled data is interpreted as a formula (e.g., Excel or Google Sheets expressions) and executed by a downstream system. In an Axum application that stores or exports data to DynamoDB, this typically arises when user input is placed into items that are later exported to spreadsheets or used in contexts where formulas are evaluated. DynamoDB itself does not evaluate formulas, but the integration pattern — accepting user input, persisting it in DynamoDB, and later rendering it in CSV or spreadsheet exports — creates a path for malicious payloads to reach end users’ calculation environments.

Consider an Axum handler that accepts a name and amount from a request and stores them in DynamoDB. If the name field is later exported to a CSV file without proper escaping, an input like =cmd|' /C calc'!A0 can trigger execution when the file is opened in Excel. DynamoDB stores the value as-is, so the vulnerability is not in DynamoDB’s behavior but in how the data is consumed downstream. Axum applications that generate reports, receipts, or exports from DynamoDB items must treat all stored values as potentially hostile when rendered in spreadsheet formats.

The risk is compounded when the Axum service exposes administrative or debug endpoints that export DynamoDB contents directly to users without validation. Attackers may probe these endpoints or use authenticated sessions to retrieve items containing formula payloads, then deliver the malicious exports to other users. Since DynamoDB does not sanitize content for spreadsheet contexts, the responsibility falls to the Axum application to neutralize formula-like inputs before export. This makes input validation and output encoding in Axum critical controls when DynamoDB is the persistence layer.

To mitigate Formula Injection in Axum with DynamoDB, ensure that any user-supplied data intended for spreadsheet or export contexts is sanitized and escaped before rendering. In Axum, this is typically done in serialization layers or handler logic. Below is a realistic Axum handler that stores user input in DynamoDB and later exports items safely, with emphasis on escaping values that may be interpreted as formulas.

use axum::{routing::post, Router, Json};
use aws_sdk_dynamodb::Client;
use serde::{Deserialize, Serialize};

#[derive(Deserialize, Debug)]
struct InputData {
    name: String,
    amount: String,
}

#[derive(Serialize, Debug)]
struct SafeExportData {
    name: String,
    amount: String,
}

async fn store_and_export(
    Json(payload): Json,
    client: &Client,
) -> Json {
    // Store raw input in DynamoDB (acceptable if properly validated elsewhere)
    let item = aws_sdk_dynamodb::types::PutItemInput::builder()
        .table_name("app_data")
        .set_item(Some(
            [
                ("name".to_string(), aws_sdk_dynamodb::types::AttributeValue::S(payload.name.clone())),
                ("amount".to_string(), aws_sdk_dynamodb::types::AttributeValue::S(payload.amount.clone())),
            ]
            .into_iter()
            .collect(),
        ))
        .build();
    // client.put_item().send().await; // omitted for brevity

    // Escape values for safe export: neutralize leading equals and control characters
    let safe_name = escape_spreadsheet_value(&payload.name);
    let safe_amount = escape_spreadsheet_value(&payload.amount);

    Json(SafeExportData {
        name: safe_name,
        amount: safe_amount,
    })
}

fn escape_spreadsheet_value(value: &str) -> String {
    let trimmed = value.trim();
    if trimmed.starts_with('=') || trimmed.starts_with('+') || trimmed.starts_with('-') || trimmed.starts_with('@') {
        // Prepend a single quote to force literal interpretation in spreadsheet software
        format!("'{}", trimmed)
    } else {
        trimmed.to_string()
    }
}

In this example, escape_spreadsheet_value neutralizes formula injection by prepending a single quote when the value begins with characters commonly used in formulas. This approach is lightweight and effective for CSV or Excel exports generated from DynamoDB-stored data. For more robust protection, consider a dedicated library for CSV generation that handles escaping automatically, and apply strict allow-lists for expected input patterns in Axum validators.

Additionally, apply principle of least privilege to the DynamoDB integration in Axum. Use IAM policies that restrict the actions and resources available to the service, reducing the impact of any compromised input handling. Combine these measures with regular scanning using middleBrick — for example, the CLI middlebrick scan <url> or the GitHub Action to add API security checks to your CI/CD pipeline — to detect and track related issues like Input Validation and Data Exposure across your API surface.