Prototype Pollution in Axum with Basic Auth

Prototype pollution in Axum with Basic Auth typically arises when user-controlled data from authenticated requests is merged into server-side objects used to build responses. In JavaScript-based server-side environments that use prototype-based inheritance, an attacker can supply specially crafted input such as __proto__, constructor.prototype, or other inherited properties. If Axum handlers deserialize JSON bodies or query parameters and then merge them into shared templates or configuration objects, the polluted prototype can affect all subsequent objects, leading to unauthorized behavior or information exposure.

When Basic Auth is in play, the presence of authentication may create a false sense of security and encourage developers to pass authenticated user attributes into shared objects without strict validation. For example, a handler might read a username from a Basic Auth header and then merge it into a response-building object like this:

use axum::{routing::get, Router, extract::Extension, http::HeaderMap};
use serde_json::json;

async fn profile_handler(
    Extension(state): Extension,
    headers: HeaderMap,
) -> impl IntoResponse {
    // Extract Basic Auth username (simplified)
    let username = extract_username(&headers);
    // Dangerous merge into a shared template-like structure
    let profile = json!({
        "greeting": "Hello",
        "user": username,
        "settings": state.shared_settings,
    });
    (StatusCode::OK, Json(profile))
}

If state.shared_settings or the merge logic indirectly allows user properties to overwrite inherited prototypes, an attacker authenticated with Basic Auth could inject fields that propagate across object instances. This is not a flaw in Axum itself but a risk pattern enabled by unsafe data handling when combining authenticated context with mutable prototypes.

In the context of middleBrick’s 12 security checks, this scenario maps to Input Validation and Property Authorization. middleBrick would flag unsanitized user data being merged into objects with prototype reach, especially when credentials are present but validation is missing. The scanner tests unauthenticated attack surfaces, but when authentication mechanisms like Basic Auth are used, they can expand the attack surface if handlers fail to isolate user identity from object construction.

Real-world examples like CVE-2021-44906 illustrate how prototype pollution in libraries such as lodash can be leveraged when input is not strictly constrained. In Axum applications, similar outcomes can occur when JSON deserialization or manual merging does not reject or sanitize inherited properties. middleBrick’s cross-referencing of OpenAPI specs with runtime findings helps identify endpoints where authentication and object merging intersect without adequate validation.

To prevent prototype pollution in Axum when using Basic Auth, ensure that user-controlled data never directly mutates shared objects or prototypes. Instead, validate and sanitize all inputs, use isolated data structures, and avoid merging authenticated metadata into generic templates. Below are concrete, safe patterns.

1. Validate and extract Basic Auth credentials without merging into response objects

use axum::{routing::get, Router, extract::Extension, http::HeaderMap, response::Json};
use serde_json::Value;

fn extract_username(headers: &HeaderMap) -> Option {
    headers.get("authorization")
        .and_then(|v| v.to_str().ok())
        .filter(|s| s.starts_with("Basic "))
        .map(|s| &s[6..])
        .and_then(|credentials| base64::decode(credentials).ok())
        .and_then(|bytes| String::from_utf8(bytes).ok())
        .map(|s| s.split(':').next().unwrap_or("").to_string())
}

async fn profile_handler(
    Extension(state): Extension,
    headers: HeaderMap,
) -> Result, (StatusCode, String)> {
    let username = extract_username(&headers).ok_or_else(|| (StatusCode::UNAUTHORIZED, "Missing auth".into()))?;
    // Build a clean, isolated response without merging shared mutable state
    let response = serde_json::json!({
        "authenticated_as": username,
        "greeting": "Hello",
        "settings": state.public_settings.clone(), // ensure public_settings is cloned or copied
    });
    Ok(Json(response))
}

This pattern avoids polluting prototypes by never passing user-derived values into shared prototype chains. public_settings should be a read-only or cloned structure that does not expose inherited properties.

2. Use strict schema validation for JSON inputs

use axum::{routing::post, Json};
use serde::{Deserialize, Serialize};

#[derive(Deserialize, Serialize)]
struct ProfileUpdate {
    display_name: String,
    theme: String,
}

async fn update_profile_handler(
    Json(payload): Json,
) -> Result {
    // Process validated payload; no prototype pollution risk from user fields
    Ok(format!("Updated {} with theme {}", payload.display_name, payload.theme))
}

By using strongly typed deserialization with serde, you ensure that only expected fields are accepted, and no unexpected prototype properties can be injected. middleBrick’s Input Validation checks would verify that such strict schemas are in place and that OpenAPI definitions align with runtime behavior.

For continuous assurance, consider integrating the middleBrick CLI to scan your endpoints regularly:

middlebrick scan https://api.example.com/openapi.json

Or add the GitHub Action to fail builds if security scores degrade, ensuring that prototype pollution risks tied to authentication are caught early.