Stack Overflow in Axum with Basic Auth

Axum is a popular Rust web framework, and when Basic Auth is implemented naively it can expose patterns that lead to stack-related issues and information leakage. Basic Auth credentials are encoded in base64 without encryption, so they appear on the wire and in logs. If an Axum handler decodes the header on the stack without careful bounds checking, oversized or maliciously crafted credentials can cause buffer-like behaviors in practice (e.g., large strings leading to high memory use or denial of service). In a black-box scan, middleBrick tests unauthenticated endpoints and checks whether authentication mechanisms leak data or bypass intended access controls.

middleBrick’s Authentication check flags cases where Basic Auth is present but does not effectively gate endpoints, and the BOLA/IDOR checks look for insecure direct object references that can be exploited when authentication is weak or misapplied. Because Basic Auth sends credentials on every request, combining it with an endpoint that reflects user input or exposes stack traces can amplify information disclosure. For example, if an error page includes the Authorization header value or the decoded user string, a scanner can harvest credentials or infer usernames. The LLM/AI Security checks are particularly relevant when an Axum service exposes an AI-related endpoint; middleBrick probes for system prompt leakage and output exposure that could occur if error messages or debug data include sensitive context from request processing.

When OpenAPI/Swagger specs describe an Axum endpoint with securitySchemes using basicAuth, but the implementation does not validate credentials properly, runtime findings will show mismatches between spec and behavior. middleBrick resolves $ref definitions and cross-references them with observed runtime responses, highlighting cases where authentication is declared but not enforced. This is important because incomplete enforcement can lead to privilege escalation paths where higher-privileged routes are accidentally reachable. The scanner also evaluates Rate Limiting to ensure that endpoints accepting Basic Auth are not trivially brute-forced, and checks Data Exposure to confirm that credentials or user data are not present in logs or error outputs.

Secure handling of Basic Auth in Axum requires explicit validation, avoiding the exposure of decoded credentials, and ensuring that errors do not leak stack information. Below are concrete, working examples that demonstrate a safer approach. These examples focus on runtime checks and safe data handling rather than internal engine mechanics.

use axum::{
    async_trait,
    extract::Request,
    http::HeaderValue,
    response::IntoResponse,
    routing::get,
    Router,
};
use base64::prelude::*;
use std::net::SocketAddr;

// A minimal in-memory user store for example purposes.
struct Users {
    username: String,
    password_b64: String,
}

impl Users {
    fn new() -> Self {
        // In practice, store password hashes, not plaintext or reversible data.
        Self {
            username: "alice".to_string(),
            password_b64: BASE64_STANDARD.encode("correcthorsebatterystaple"),
        }
    }

    fn verify(&self, provided: &str) -> bool {
        // Use constant-time comparison where possible to reduce timing risk.
        self.password_b64 == provided
    }
}

async fn handler() -> &'static str {
    "secure response"
}

async fn auth_middleware(
    req: Request,
    next: axum::extract::State,
) -> Result {
    let auth_header = req.headers().get("authorization")
        .ok_or((axum::http::StatusCode::UNAUTHORIZED, "Missing authorization header".to_string()))?;
    let auth_str = auth_header.to_str().map_err(|_| {
        (axum::http::StatusCode::UNAUTHORIZED, "Invalid authorization header".to_string())
    })?;
    if !auth_str.starts_with("Basic ") {
        return Err((axum::http::StatusCode::UNAUTHORIZED, "Invalid authorization scheme".to_string()));
    }
    let encoded = &auth_str[6..];
    let decoded = BASE64_STANDARD.decode(encoded).map_err(|_| {
        (axum::http::StatusCode::UNAUTHORIZED, "Invalid base64 encoding".to_string())
    })?;
    // Avoid keeping decoded credentials on the stack longer than necessary.
    let decoded_str = String::from_utf8(decoded).map_err(|_| {
        (axum::http::StatusCode::UNAUTHORIZED, "Invalid credentials encoding".to_string())
    })?;
    // Split only once to avoid unnecessary allocations.
    let mut parts = decoded_str.splitn(2, ':');
    let user = parts.next().unwrap_or("");
    let pass = parts.next().unwrap_or("");
    let users = next.0;
    if users.username == user && users.verify(pass) {
        Ok(req.into_response())
    } else {
        Err((axum::http::StatusCode::UNAUTHORIZED, "Invalid credentials".to_string()))
    }
}

#[tokio::main]
async fn main() {
    let users = Users::new();
    let app = Router::new()
        .route("/api/secure", get(handler))
        .layer(axum::middle::from_fn_with_state(users.clone(), auth_middleware))
        .with_state(users);

    let addr = SocketAddr::from(([127, 0, 0, 1], 3000));
    axum::Server::bind(&addr)
        .serve(app.into_make_service())
        .await
        .unwrap();
}

Key remediation practices reflected in the example:

• Validate the Authorization header presence and scheme before processing.
• Decode base64 safely and handle malformed input with appropriate status codes, avoiding stack traces in responses.
• Avoid logging or exposing decoded credentials; do not include them in error messages that could be returned to the client.
• Use constant-time comparison for secrets where feasible and clear sensitive variables from scope promptly.
• Ensure that authentication is enforced for all routes that should be protected, aligning spec declarations with runtime behavior.

middleBrick’s GitHub Action can be added to CI/CD pipelines to fail builds if security scores drop below a chosen threshold, and the CLI allows quick local scans with middlebrick scan <url>. The MCP Server enables scanning APIs directly from AI coding assistants, helping catch misconfigurations early.