Formula Injection in Actix with Api Keys

Formula Injection occurs when user-controlled data is interpreted as a formula (e.g., Excel, CSV, or similar formats) and executed, leading to unintended behavior or data leakage. In Actix web applications, this risk can emerge when API keys or other sensitive values are dynamically inserted into downloadable files or responses without proper sanitization.

Consider an Actix endpoint that generates a CSV report and embeds an API key for downstream service authentication. If the API key or any user-influenced field is not escaped, an attacker may supply input such as =cmd|' /C calc'!A0 or a formula expression that causes the consuming application to execute commands when the file is opened. This becomes particularly dangerous when the generated file is opened in spreadsheet software that evaluates formulas automatically.

In a typical Actix handler, you might construct a CSV using string concatenation or a serializer and include an API key retrieved from environment configuration or a secure vault. The vulnerability arises not from Actix itself but from the downstream usage of the exported data. For example:

// Risky: directly embedding an API key into CSV content
async fn export_report(data: web::Data<AppState>, api_key: String) -> impl Responder {
    let csv = format!("api_key,report_id,amount\n{},{},100\n", api_key, data.report_id);
    HttpResponse::Ok()
        .content_type("text/csv")
        .body(csv)
}

If the api_key contains or is concatenated with untrusted input that includes formula syntax, and the CSV is consumed in an environment that evaluates formulas, this can lead to Formula Injection. Moreover, if the API key is treated as data rather than a secret and exposed in such outputs, it can be exfiltrated when the file is shared or processed by third-party tools.

middleBrick identifies this pattern during unauthenticated scans by analyzing OpenAPI specifications and runtime behavior. It flags endpoints that expose sensitive values like API keys in downloadable resources or responses where injection into structured formats (CSV, JSON, XML) is possible. The scanner’s LLM/AI Security checks also probe for indirect exposure paths, such as logs or error messages that might reveal API keys in formula-exploitable contexts.

Because middleBrick tests the unauthenticated attack surface, it can detect endpoints that inadvertently leak API keys through report exports, logs, or error payloads. Findings include severity ratings and remediation guidance, helping teams understand whether the exposure relates to improper data handling, missing output encoding, or unsafe consumption patterns.

To mitigate Formula Injection risks involving API keys in Actix, ensure that sensitive values are never directly interpolated into formats that may be interpreted as formulas. Treat API keys as secrets, not data, and avoid including them in outputs that could be opened in spreadsheet or document applications.

When you must include an API key for operational purposes, encode or escape it appropriately for the target format. For CSV exports, avoid embedding raw API keys. Instead, use opaque references or tokens that map to server-side secrets. If an API key must be transmitted, use HTTP headers or secure request parameters rather than embedding it in the body.

The following example demonstrates a safer approach in Actix, using environment-based configuration and avoiding direct inclusion of the API key in user-facing output:

// Safer: do not embed API key in CSV; use a token or reference
async fn export_report_safe(
    data: web::Data<AppState>,
    token: web::Query<TokenRequest>,
) -> Result<HttpResponse, Error> {
    // Validate and map token to an internal reference
    let api_key = data.secrets.get(&token.0.reference).ok_or_else(|| {
        error::ErrorBadRequest("Invalid reference")
    })?;

    // Construct CSV with safe, non-formula content
    let csv = format!(
        "report_id,amount,key_ref\n{},{},{}
  

use actix_web::{web, App, HttpResponse, HttpServer, Responder};
use std::env;

#[derive(serde::Deserialize)]
struct TokenRequest {
    reference: String,
}

struct AppState {
    secrets: std::collections::HashMap<String, String>,
}

#[actix_web::main]
async fn main() -> std::io::Result<()> {
    let mut secrets = std::collections::HashMap::new();
    secrets.insert("rep_001".to_string(), env::var("API_KEY_EXTERNAL").unwrap_or_default());

    let data = web::Data::new(AppState { secrets });

    HttpServer::new(move || {
        App::new()
            .app_data(data.clone())
            .route("/export", web::get().to(export_report_safe))
    })
    .bind("127.0.0.1:8080")?
    .run()
    .await
}