Crlf Injection in Actix with Dynamodb

Crlf Injection occurs when user-controlled data is reflected in HTTP headers without sanitization, allowing an attacker to inject additional headers or split the response. In an Actix web service that uses Amazon DynamoDB as a backend, risk arises when data fetched from or stored in DynamoDB is later used to construct HTTP response headers. Actix applications often pass item attributes from DynamoDB records directly into headers such as x-request-id or x-user-note. If the DynamoDB item contains carriage return (\r) or line feed (\n) characters, and the Actix handler forwards them unvalidated into headers, an attacker can inject new headers or perform HTTP response splitting. This can lead to cache poisoning, cross-site scripting via injected headers, or bypassing browser security policies.

The exposure is not inherent to DynamoDB but stems from unsafe handling in Actix after reading data. DynamoDB itself stores strings as provided; it does not interpret CRLF. Therefore, if an Actix service stores user-supplied values into DynamoDB and later uses them in headers without validation, the stored payload becomes a stored vector. Additionally, DynamoDB scan or query results can include user-controlled fields that feed into dynamically constructed headers in Actix routes. The combination therefore creates a practical path for injection: attacker input enters via API, persists in DynamoDB, and re-emerges in HTTP responses via Actix, enabling Crlf Injection without direct user-supplied header manipulation in the immediate request.

Consider an endpoint in Actix that retrieves a user profile from DynamoDB and sets a custom header x-profile-summary from an attribute summary. If the summary attribute contains newline sequences, and the handler uses HttpResponse::Ok() and manually appends the header without sanitization, the injected newline can split the response and smuggle a crafted header or response body. Because DynamoDB can be queried unauthenticated in some configurations (depending on IAM), the unauthenticated attack surface may be expanded if the Actix service exposes an endpoint that reads public items and reflects them in headers.

Remediation focuses on sanitization at the boundary where DynamoDB data enters HTTP headers in Actix. Never directly forward DynamoDB attribute values into headers. Instead, enforce strict allowlists, remove or replace CRLF characters, and prefer structured header values. Below are concrete patterns for Actix with DynamoDB item handling.

Example 1: Safe header assignment with sanitization

When constructing an Actix response from a DynamoDB item, sanitize string values before placing them in headers. Remove \r and \n and reject inputs that contain them if they are not expected.

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

fn sanitize_header_value(value: &str) -> String {
    value.replace("\r", "").replace("\n", "")
}

async fn get_profile(user_id: String, dynamodb_client: web::Data) -> Result {
    let resp = dynamodb_client
        .get_item()
        .table_name("UserProfiles")
        .key("user_id", AttributeValue::S(user_id.clone()))
        .send()
        .await?;

    let item = resp.item().ok_or_else(|| actix_web::error::ErrorNotFound("Not found"))?;
    let summary = item.get("summary")
        .and_then(|v| v.as_s().ok())
        .unwrap_or("");

    let safe_summary = sanitize_header_value(summary);
    let response = HttpResponse::Ok()
        .insert_header(("x-profile-summary", safe_summary.as_str()))
        .json(item);
    Ok(response)
}

Example 2: Rejecting invalid characters early

For fields that must not contain newlines (e.g., IDs or tokens), explicitly reject or transform dangerous characters and return a 400 instead of trying to sanitize.

fn validate_no_crlf(value: &str, field_name: &str) -> Result<(), actix_web::Error> {
    if value.contains('\r') || value.contains('\n') {
        return Err(actix_web::error::ErrorBadRequest(format!("{} contains invalid characters", field_name)));
    }
    Ok(())
}

async fn create_item(payload: web::Json, dynamodb_client: web::Data) -> Result {
    let user_note = payload["note"].as_str().ok_or_else(|| actix_web::error::ErrorBadRequest("note must be a string"))?;
    validate_no_crlf(user_note, "note")?;

    // Proceed to store item into DynamoDB
    // ...
    Ok(HttpResponse::Created().finish())
}

Example 3: Using builder-style header insertion with checked conversion

Actix provides header types that can reduce risk; use them where applicable. For free-form strings, still sanitize.

use actix_web::http::header::{HeaderValue, CONTENT_TYPE};

let raw = "text/plain; charset=utf-8";
if let Ok(hv) = HeaderValue::from_str(raw) {
    response.headers_mut().typed_insert(hv);
} else {
    // handle invalid header value
}

Operational guidance

• Validate on input: enforce format rules for fields that will later become headers.
• Sanitize on output: strip or reject CRLF characters from any data sourced from DynamoDB before using it in headers.
• Leverage Actix’s typed headers where possible to avoid manual string concatenation.
• Audit your DynamoDB-backed endpoints with a scanner such as middleBrick to detect header-injection risks; the CLI can be run with middlebrick scan <url> to surface unvalidated reflections.