Xss Cross Site Scripting in Axum with Basic Auth

Cross-site scripting (XSS) in an Axum application that uses HTTP Basic Authentication can arise when user-controlled data is reflected into HTML, JavaScript, or CSS without proper encoding, and when authentication state is handled in a way that amplifies impact. Axum is a Rust web framework, and like any framework, it does not automatically neutralize injection vectors; developers must explicitly encode outputs based on context (HTML body, attribute, JavaScript, URL).

Basic Authentication sends credentials in an Authorization header (Basic base64(username:password)). While the header itself is not directly responsible for XSS, relying on Basic Auth can encourage patterns where applications treat authenticated sessions as inherently trusted and render user data with insufficient escaping. For example, an endpoint that accepts a username query parameter to personalize a dashboard might embed the value in the page. If the username contains <script>alert(1)</script> and the response does not HTML-escape the value, the script executes in the victim’s browser. The presence of Basic Auth does not cause the injection, but it can increase risk if developers assume authenticated requests are safe from output encoding or content security policy considerations.

Consider an Axum handler that reads a user-supplied label and renders it into an HTML response. Without context-aware escaping, an attacker can craft a URL such as /label?name=<img%20onerror=alert(document.cookie)>. If the Basic Auth credentials are valid, the server may render the label into the DOM, and the browser executes the injected image’s onerror handler. This is reflected XSS. In a more advanced scenario, stored XSS can occur if user input is persisted (for example, in a configuration UI) and later rendered without sanitization. Because Basic Auth credentials are base64-encoded rather than encrypted, they can be leaked via logs or referrers, potentially enabling session fixation or credential theft that further aids XSS impact by letting an attacker hijack authenticated sessions.

The 12 security checks in middleBrick test these combinations by probing unauthenticated and authenticated surfaces. For instance, input validation checks examine whether user input is validated and encoded appropriately, while data exposure checks assess whether credentials or sensitive data are inadvertently reflected. LLM/AI Security probes are especially relevant when an Axum endpoint exposes an LLM completion or tool-calling interface; an attacker may try prompt injection or attempt to cause the model output to include executable JavaScript that runs in a privileged context.

middleBrick scans such endpoints and maps findings to frameworks like OWASP API Top 10 and PCI-DSS, providing prioritized remediation guidance rather than attempting to patch. This is important because XSS in Axum is addressed through correct Rust coding practices and response encoding, not through network-level controls like WAFs alone.

Remediation focuses on context-aware output encoding, strict input validation, and avoiding unsafe rendering of user-controlled data in Axum handlers. Below are concrete Rust examples demonstrating secure patterns.

1) HTML-escape user data in responses. Use askama or minijinja for templating, or escape with markup5ever / kuchiki. Example using askama:

[dependencies]
askama = "0.12"
axum = "0.6"

use axum::{routing::get, Router};
use askama::Template;

#[derive(Template)]
#[template(path = "label.html")]
struct LabelTemplate {
    name: String,
}

async fn handler(name: String) -> LabelTemplate {
    // Validate/sanitize input as needed
    LabelTemplate { name }
}

// In main:
// let app = Router::new().route("/label", get(handler));

The label.html template should use Jinja-like autoescaping (enabled by default in Askama), so {{ name }} is HTML-escaped automatically.

2) For JSON APIs, ensure Content-Type is correct and never embed raw user input into JavaScript blocks. If you must embed, use a serialization library that escapes correctly, and enforce strict CSP headers.

3) Basic Auth implementation example with Axum middleware that does not over-trust authenticated identity:

use axum::{
    async_trait,
    extract::{Request, Extension, FromRequest},
    http::{HeaderValue, StatusCode, header::AUTHORIZATION},
    response::IntoResponse,
};
use base64::Engine;

struct AuthenticatedUser {
    username: String,
}

#[async_trait]
impl FromRequest<S> for AuthenticatedUser
where
    S: Send + Sync,
{
    type Rejection = (StatusCode, String);

    async fn from_request(req: Request, _state: &S) -> Result<Self, Self::Rejection> {
        let auth = req.headers().get(AUTHORIZATION)
            .and_then(|v| v.to_str().ok())
            .ok_or((StatusCode::UNAUTHORIZED, "Missing Authorization header".to_string()))?;
        if !auth.starts_with("Basic ") {
            return Err((StatusCode::UNAUTHORIZED, "Only Basic Auth supported".to_string()));
        }
        let encoded = auth.trim_start_matches("Basic ");
        let decoded = base64::engine::general_purpose::STANDARD.decode(encoded)
            .map_err(|_| (StatusCode::UNAUTHORIZED, "Invalid base64".to_string()))?;
        let credentials = String::from_utf8(decoded)
            .map_err(|_| (StatusCode::UNAUTHORIZED, "Invalid UTF-8 credentials".to_string()))?;
        let parts: Vec<&str> = credentials.splitn(2, ':').collect();
        if parts.len() != 2 || parts[0].is_empty() || parts[1].is_empty() {
            return Err((StatusCode::UNAUTHORIZED, "Invalid credentials format".to_string()));
        }
        Ok(AuthenticatedUser { username: parts[0].to_string() })
    }
}

async fn protected_handler(Extension(user): Extension<AuthenticatedUser>) -> impl IntoResponse {
    format!("Hello, {}", user.username)
}