Beast Attack in Axum with Basic Auth

A Beast Attack (Browser Exploit Against SSL/TLS) exploits the way block ciphers in CBC mode process IVs when TLS session renegotiation occurs. In Axum applications that terminate TLS at a load balancer or reverse proxy and then forward traffic over plain HTTP to the application, an active attacker on the network can inject chosen plaintext into an encrypted request and iteratively recover plaintext by observing changes in the server’s behavior or timing. This becomes significantly more practical when Basic Authentication is used over such a path.

When Basic Auth is sent over HTTP (even if the outer transport uses TLS), the credentials are base64-encoded but not encrypted. If an attacker can force a client to make a request that includes the Authorization header—for example via a malicious image tag, script, or crafted form—the attacker can participate in a CBC IV manipulation attack across renegotiated sessions. In Axum, if the application relies on per-request authentication parsing and does not enforce strict transport integrity, an attacker may be able to correlate request changes with authentication success or failure responses. This can reveal information about the Authorization header byte by byte, especially when the server’s error handling or response codes differ based on whether credentials are valid.

In an Axum service that parses Basic Auth manually from headers, the server may process the credentials early and then proceed to business logic. If session renegotiation is allowed or TLS is misconfigured, the attacker can leverage the predictable IV chaining in CBC to submit known plaintext and infer the base64-encoded credentials. Because Basic Auth credentials are static across requests until changed, an attacker has a consistent target across multiple malicious interactions. Axum’s composability and state handling mean that if the authentication check is not isolated and enforced before any side effects, an attacker can observe timing differences or branching behavior that aids the recovery process.

middleBrick detects scenarios where unauthenticated or insufficiently protected endpoints expose authentication-related behavior differences that can be leveraged in such adaptive chosen-ciphertext attacks. The scanner flags inconsistent responses for the same request when authentication state changes, which is a potential indicator that error handling or status codes leak information useful in a Beast-style context. By correlating findings from the Authentication and Input Validation checks, middleBrick highlights whether Basic Auth handling in Axum could aid protocol downgrade or injection paths that make CBC-based attacks more feasible.

To reduce risk, ensure TLS is enforced end-to-end, disallow renegotiation, and avoid branching logic based on authentication outcomes. Validate and sanitize all inputs, and prefer stronger mechanisms such as token-based authentication with short-lived, scoped credentials. middleBrick’s OpenAPI/Swagger analysis resolves $ref definitions and cross-references runtime behavior to detect mismatches between documented authentication expectations and actual implementation, helping identify endpoints where Basic Auth over non-secure processing paths could be abused.

Remediation focuses on eliminating cleartext credential transmission, removing branching behavior based on authentication validity, and enforcing strict transport security. Do not rely on obscurity or custom parsing; use established libraries and enforce HTTPS for every request that carries credentials.

1. Use HTTPS everywhere and disable renegotiation

Ensure your reverse proxy or load balancer terminates TLS and is configured to disallow TLS renegotiation. In Axum, you can enforce that all requests are served over HTTPS by using a middleware layer that checks the forwarded headers correctly and rejects cleartext HTTP. While Axum does not directly manage TLS, your deployment configuration should terminate TLS before requests reach the Rust service.

2. Avoid manual Basic Auth parsing and branching on auth validity

Do not manually split and decode the Authorization header in route handlers. Instead, validate credentials before business logic executes and return a uniform error response for any failure. This prevents attackers from distinguishing between missing, malformed, and valid credentials based on status code or timing.

use axum::{
    async_trait,
    extract::{FromRequest, Request},
    http::StatusCode,
    response::IntoResponse,
};
use base64::decode;
use std::fmt;

#[derive(Debug)]
enum AuthError {
    MissingHeader,
    InvalidFormat,
    InvalidCredentials,
}

impl fmt::Display for AuthError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "auth error")
    }
}

impl IntoResponse for AuthError {
    fn into_response(self) -> axum::response::Response {
        let (status, body) = match self {
            AuthError::MissingHeader => (StatusCode::UNAUTHORIZED, "Unauthorized"),
            AuthError::InvalidFormat => (StatusCode::BAD_REQUEST, "Bad Request"),
            AuthError::InvalidCredentials => (StatusCode::UNAUTHORIZED, "Unauthorized"),
        };
        (status, body).into_response()
    }
}

struct AuthenticatedUser {
    pub subject: String,
}

#[async_trait]
impl FromRequest for AuthenticatedUser
where
    S: Send + Sync,
{
    type Rejection = AuthError;

    async fn from_request(req: Request, _state: &S) -> Result {
        let header = req.headers().get("authorization")
            .ok_or(AuthError::MissingHeader)?
            .to_str()
            .map_err(|_| AuthError::InvalidFormat)?;

        if !header.starts_with("Basic ") {
            return Err(AuthError::InvalidFormat);
        }

        let encoded = header.trim_start_matches("Basic ").trim();
        let decoded = decode(encoded).map_err(|_| AuthError::InvalidCredentials)?;
        let creds = String::from_utf8(decoded).map_err(|_| AuthError::InvalidCredentials)?;

        // Split only once in case password contains ':'
        let mut parts = creds.splitn(2, ':');
        let (user, pass) = match (parts.next(), parts.next()) {
            (Some(u), Some(p)) => (u, p),
            _ => return Err(AuthError::InvalidCredentials),
        };

        // Replace with constant-time validation in production
        if validate_credentials(user, pass) {
            Ok(AuthenticatedUser { subject: user.to_string() })
        } else {
            Err(AuthError::InvalidCredentials)
        }
    }
}

fn validate_credentials(user: &str, pass: &str) -> bool {
    // Use a constant-time comparison in real applications
    user == "admin" && pass == "s3cur3P@ss!"
}

// Example handler using the extractor
async fn handler(user: AuthenticatedUser) -> impl IntoResponse {
    format!("Authenticated as: {}", user.subject)
}