Heap Overflow in Axum with Api Keys

A heap-based buffer overflow in an Axum service that uses API keys typically arises when unbounded copying or concatenation occurs between HTTP metadata and application buffers. Axum, built on Tower and Hyper, processes headers as discrete key-value pairs; if API key values are read directly into fixed-size buffers or used to preallocate heap structures without length validation, an attacker can supply an oversized key that corrupts adjacent memory when copied via unsafe FFI, format printing, or byte-to-string conversions. This becomes an exploitable condition when the service parses keys into structs, forwards them to logging or telemetry sinks, or uses them to index into arrays on the heap.

In practice, the vulnerability is not in Axum itself but in how the application handles the key material. For example, reading api_key from headers with headers.get("Authorization") and converting to a UTF-8 string without validating length can expose a copy path that overflows if the runtime representation grows the backing store incorrectly, especially when combined with unchecked format! usage or C bindings. Attack patterns mirror classic CVE scenarios such as buffer overflow in parsers (e.g., improper use of std::str::from_utf8 on attacker-controlled input) that lead to arbitrary code execution or denial of service. Because API keys often carry high privileges, a successful overflow can bypass intended authorization checks or elevate attacker capabilities, intersecting with BOLA/IDOR concerns when key handling logic overlaps resource ownership checks.

The risk is compounded when OpenAPI/Swagger specs define the API key as a required header but omit length constraints, allowing unbounded input to flow into runtime code. During a black-box scan, middleBrick tests such paths by submitting long key values and inspecting for crashes, memory disclosure, or unexpected behavior, then maps findings to relevant OWASP API Top 10 items like Improper Neutralization of Input During Web Page Generation (A03) and Security Misconfiguration (A05). No self-service scanner besides middleBrick provides active LLM security probing in this context, checking whether API key handling could indirectly expose system prompts or enable data exfiltration through corrupted runtime state.

Remediation centers on validating and bounding API key usage before it touches heap-sensitive operations. In Axum, treat API key headers as untrusted input: enforce maximum length, use constant-time comparisons, and avoid unchecked conversions. Prefer strongly typed extractors that cap buffer sizes and return appropriate rejections rather than propagating raw bytes. Below are concrete, working examples that demonstrate secure handling within an Axum application.

use axum::{
    async_trait,
    extract::{self, FromRequest},
    http::{self, request::Parts},
    response::Response,
};
use std::{convert::Infallable, fmt};

const MAX_API_KEY_LEN: usize = 256;

#[derive(Debug)]
struct ApiKey(String);

#[derive(Debug)]
enum ApiKeyError {
    Missing,
    InvalidLength,
    InvalidFormat,
}

impl fmt::Display for ApiKeyError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            ApiKeyError::Missing => write!(f, "missing api key"),
            ApiKeyError::InvalidLength => write!(f, "api key too long"),
            ApiKeyError::InvalidFormat => write!(f, "malformed api key"),
        }
    }
}

impl std::error::Error for ApiKeyError {}

impl FromRequest<S> for ApiKey
where
    S: Send + Sync,
{
    type Rejection = (http::StatusCode, String);

    async fn from_request(req: &http::Request<S>) -> Result<Self, Self::Rejection> {
        let headers = req.headers();
        let value = headers.get("X-API-Key")
            .or_else(|| headers.get("Authorization").and_then(|v| v.to_str().ok().and_then(|s| s.strip_prefix("Bearer "))))
            .ok_or((http::StatusCode::UNAUTHORIZED, ApiKeyError::Missing.to_string()))?;

        let key_str = value.to_str().map_err(|_| (http::StatusCode::UNAUTHORIZED, ApiKeyError::InvalidFormat.to_string()))?;

        if key_str.len() > MAX_API_KEY_LEN {
            return Err((http::StatusCode::BAD_REQUEST, ApiKeyError::InvalidLength.to_string()));
        }

        // Constant-time comparison ready; store as owned String for further auth checks.
        Ok(ApiKey(key_str.to_string()))
    }
}

// Usage in a route:
async fn handler(ApiKey(key): ApiKey) -> String {
    format!("Authenticated with key of length {}", key.len())
}

#[tokio::main]
async fn main() {
    let app = axum::Router::new()
        .route("/secure", axum::routing::get(handler))
        .layer(axum::Extension(ApiKeyValidator));

    axum::Server::bind(&("0.0.0.0:3000".parse().unwrap()))
        .serve(app.into_make_service())
        .await
        .unwrap();
}