Out Of Bounds Write in Axum with Basic Auth

An Out Of Bounds Write occurs when a program writes data before or after the intended buffer, which in managed languages like Rust typically surfaces as an unsafe operation or a misuse of collections that permit unchecked indexing. In Axum, this risk arises not from the framework’s routing or extractor design itself, but from how you handle byte buffers, streams, and mutable state in combination with authentication. When Basic Auth is used, Axum commonly extracts credentials via headers and passes them into business logic that may perform in-place mutations of arrays, vectors, or custom buffers. If length checks are omitted or trust is placed in attacker-controlled header values (e.g., Content-Length–derived sizes, header-derived offsets, or parsed numeric parameters), an attacker can supply crafted values that cause writes beyond allocated memory regions.

Consider an endpoint that reads a JSON payload after validating a Basic Auth token. If the handler copies data from the request into a fixed-size array using unchecked indexing derived from header values, an oversized payload or manipulated numeric header can shift the write target outside the intended bounds. In Axum, this often manifests when developers use Vec::set_len, raw pointer writes in unsafe blocks, or unchecked slice operations while processing authenticated requests. Because Basic Auth transmits credentials in headers rather than in a cryptographically bound session, there is no built-in integrity check on the size or origin of the data being processed, making it easier for an attacker to chain malformed headers with body content to trigger an out-of-bounds condition.

Real-world patterns that amplify the risk include custom parsers that interpret header-provided lengths or offsets without validation, deserialization logic that trusts input sizes, and stateful buffers reused across authenticated calls. For example, if an endpoint exposes a PATCH-style update that applies deltas to a buffer, and the delta length is taken from a header without verifying it against the actual buffer capacity, an authenticated user can send a length that causes the write to overflow into adjacent memory. Although Rust’s safety guarantees prevent some classes of memory corruption, unsafe code paths and unchecked indexing can bypass these protections, especially when combined with Basic Auth’s lack of built-in bounds on semantic parameters.

In the context of API security scanning, middleBrick checks for signs of unchecked buffer manipulation and missing length validation in authenticated flows. It examines whether Axum handlers properly validate sizes derived from headers, whether slice operations include explicit bounds checks, and whether unsafe blocks are necessary and correctly guarded. The scanner does not assume that framework defaults prevent misuse; it tests the unauthenticated surface and maps findings to relevant categories such as Input Validation and Unsafe Consumption, providing remediation guidance specific to Rust-based services using Basic Auth.

To mitigate Out Of Bounds Write risks in Axum with Basic Auth, focus on strict validation of all size- or offset-providing inputs, avoiding unchecked operations, and isolating sensitive logic behind verified extractors. Always treat header-derived values as untrusted and enforce explicit bounds before any mutation of buffers or vectors.

1. Validate header-provided lengths before buffer operations

If your handler uses a header to indicate a length or offset, ensure it is within acceptable limits before using it to index or slice. Example of unsafe code and its corrected form:

// ❌ Unsafe: using header value directly
async fn unsafe_update(
    headers: HeaderMap,
    body: Bytes,
) -> Result {
    let raw = headers.get("X-Data-Length").and_then(|v| v.to_str().ok())?;
    let len: usize = raw.parse().map_err(|_| (StatusCode::BAD_REQUEST, "invalid length"))?;
    let mut buffer: [u8; 1024] = [0; 1024];
    // Potential out-of-bounds write if len > buffer.len()
    buffer[..len].copy_from_slice(&body[..len]);
    Ok(...)
}

// ✅ Safe: validate length against buffer capacity
async fn safe_update(
    headers: HeaderMap,
    body: Bytes,
) -> Result {
    let raw = headers.get("X-Data-Length").and_then(|v| v.to_str().ok())
        .ok_or((StatusCode::BAD_REQUEST, "missing length"))?;
    let len: usize = raw.parse().map_err(|_| (StatusCode::BAD_REQUEST, "invalid length"))?;
    if len > body.len() || len > 1024 {
        return Err((StatusCode::LENGTH_REQUIRED, "length exceeds limits").into());
    }
    let mut buffer: [u8; 1024] = [0; 1024];
    buffer[..len].copy_from_slice(&body[..len]);
    Ok(...)
}

2. Use safe abstractions for mutable state

Prefer Vec::resize or iterator-based construction instead of set_len and raw pointer manipulation. If you must use unsafe, wrap it with explicit length checks and document the invariants.

// ❌ Risky: set_len without validation
let mut v = Vec::with_capacity(100);
let claimed: usize = /* from header */;
v.set_len(claimed); // unsafe if claimed > capacity

// ✅ Recommended: resize or reject
let mut v = Vec::new();
let claimed: usize = /* from header */;
if claimed <= 100 {
    v.resize(claimed, 0u8);
    // proceed safely
} else {
    return Err(...);
}

3. Combine Basic Auth extraction with validation

Use Axum extractors to parse credentials, then apply business logic only after confirming that all derived parameters respect bounds. Example of a minimal, safe handler:

use axum::{routing::post, Router, extract::State, http::HeaderMap, response::IntoResponse};
use tower_http::auth::{AuthLayer, credentials::BasicCredentials};

async fn update_handler(
    State(_auth): State<BasicCredentials>,
    headers: HeaderMap,
    body: String,
) -> Result {
    let len_header = headers.get("X-Data-Length")
        .ok_or((StatusCode::BAD_REQUEST, "X-Data-Length required"))?
        .to_str()
        map_err(|_| (StatusCode::BAD_REQUEST, "invalid header"))?;
    let len: usize = len_header.parse()
        map_err(|_| (StatusCode::BAD_REQUEST, "not a number"))?;
    if len > body.len() || len > 4096 {
        return Err((StatusCode::LENGTH_REQUIRED, "invalid length".into()));
    }
    let data = &body[..len];
    // process data safely
    Ok((StatusCode::OK, format!("processed {} bytes", data.len())))
}

pub fn app() -> Router {
    Router::new()
        .route("/update", post(update_handler))
        .layer(AuthLayer::basic("my_realm", |_user, _pass| async { Some(()) }))
}

These practices ensure that even when Basic Auth is present, all user-influenced parameters are validated before any buffer mutation, reducing the likelihood of Out Of Bounds Writes in Axum services.