Header Injection in Actix with Dynamodb

Header Injection occurs when user-controlled data is placed into HTTP response headers without validation or sanitization. In an Actix web service that integrates with Amazon DynamoDB, this typically arises when values derived from DynamoDB item attributes are used to construct response headers without proper encoding or filtering. For example, if a scan report pulls a user-supplied metadata field such as x-request-id or x-correlation-id from a DynamoDB-stored object and writes it directly into a response header, an attacker can inject newline sequences to split headers and smuggle in additional headers or malicious content.

With DynamoDB, the risk surface is shaped by how items are modeled and retrieved. Suppose a table stores custom metadata as string attributes, and an Actix handler deserializes a DynamoDB stream or query response into a struct, then forwards a field like description into a HeaderMap. A payload such as value X-Injected: true in that attribute results in header injection because Actix (or more precisely, the underlying HTTP layer) treats the newline as a header delimiter. This can lead to response splitting, cache poisoning, or the addition of security-relevant headers like Set-Cookie or Location.

The combination of Actix's asynchronous handler model and DynamoDB's schemaless string storage increases the chance that untrusted data reaches headers. Developers may assume DynamoDB validates or constrains content, but the database simply returns what is stored. If validation, normalization, and header-safe encoding are omitted in the Actix layer, a scan by middleBrick will flag this as a BFLA/Privilege Escalation or Property Authorization issue tied to improper data handling. middleBrick’s checks include input validation and property authorization, which can surface places where DynamoDB-supplied fields are used in headers without sanitization.

An illustrative, insecure Actix handler in Rust might look like this, demonstrating the hazard:

use actix_web::{web, HttpResponse};
use aws_sdk_dynamodb::Client;

async fn get_item(
    client: web::Data,
    path: web::Path,
) -> actix_web::Result {
    let key = path.into_inner();
    let output = client.get_item()
        .table_name("Items")
        .key("id", aws_sdk_dynamodb::types::AttributeValue::S(key))
        .send()
        .await?
        .item()
        .ok_or_else(|| actix_web::error::ErrorNotFound("not found"))?;

    // Risky: pulling an attribute directly into a header
    if let Some(val) = output.get("x_custom_header") {
        if let Some(s) = val.as_s() {
            let mut resp = HttpResponse::Ok();
            resp.insert_header(("X-Custom", s.to_string()));
            return Ok(resp.finish());
        }
    }
    Ok(HttpResponse::Ok().finish())
}

In this snippet, the value of the DynamoDB attribute x_custom_header is used verbatim in an HTTP response header. An attacker who can control what is stored in DynamoDB can set x_custom_header to foo Set-Cookie: session=stolen, causing the response to include an unauthorized cookie. middleBrick’s tests for Input Validation and Property Authorization would highlight this pattern, emphasizing the need to sanitize and strictly scope data from DynamoDB before it reaches headers.

Remediation centers on treating all data from DynamoDB as untrusted when it flows into HTTP headers. Apply canonicalization, strict allowlists, and header-safe encoding in the Actix layer. Below is a secure rewrite of the earlier handler that demonstrates a robust approach.

First, define a sanitization routine that removes or replaces newline and carriage return characters and enforces a safe character set for header values. Then, use this routine before inserting into headers.

use actix_web::{web, HttpResponse, http::header::HeaderValue};
use aws_sdk_dynamodb::Client;

fn sanitize_header_value(input: &str) -> Option {
    // Remove newline and carriage return to prevent header injection
    let cleaned = input.replace(|c: char| c == '\r' || c == '\n', "");
    // Optionally enforce a length limit and allowlist characters
    if cleaned.len() > 256 || !cleaned.chars().all(|c| c.is_ascii_alphanumeric() || c == '-' || c == '_' || c == '.') {
        return None;
    }
    HeaderValue::from_str(&cleaned).ok()
}

async fn get_item(
    client: web::Data,
    path: web::Path,
) -> actix_web::Result {
    let key = path.into_inner();
    let output = client.get_item()
        .table_name("Items")
        .key("id", aws_sdk_dynamodb::types::AttributeValue::S(key))
        .send()
        .await?
        .item()
        .ok_or_else(|| actix_web::error::ErrorNotFound("not found"))?;

    // Safe: sanitize DynamoDB attribute before using in header
    if let Some(val) = output.get("x_custom_header") {
        if let Some(s) = val.as_s() {
            if let Some(header_value) = sanitize_header_value(s) {
                let mut resp = HttpResponse::Ok();
                resp.insert_header(("X-Custom", header_value));
                return Ok(resp.finish());
            } else {
                // Log and reject invalid header value
                return Err(actix_web::error::ErrorBadRequest("invalid header value"));
            }
        }
    }
    Ok(HttpResponse::Ok().finish())
}

This approach ensures that values sourced from DynamoDB are stripped of dangerous characters and conform to a strict allowlist before being placed in headers. It aligns with secure coding practices that treat external data as hostile, addressing risks highlighted in scans performed by tools like middleBrick.

Additionally, prefer storing minimal metadata in DynamoDB that could ever be reflected in headers. If header-like behavior is required, consider using standard, well-defined headers with controlled values rather than passing raw DynamoDB attributes. When using the middleBrick Pro plan, continuous monitoring can detect regressions where unsanitized DynamoDB fields reappear in headers, and the GitHub Action can fail builds if risk scores degrade.