Integer Overflow in Axum with Basic Auth

An integer overflow in an Axum service using HTTP Basic Authentication can occur when user-controlled values from authentication headers are parsed into fixed-size integer types without validation. In Rust, arithmetic operations on primitive integer types such as u32 or u64 will wrap around in release builds by default, producing values that may bypass length checks or cause buffer miscalculations later in request handling.

Consider a scenario where the Authorization header contains a base64-encoded credential string whose decoded length is parsed into a numeric variable. If an attacker supplies an extremely long username or password, the base64 decoding step may produce a byte array whose length overflows the integer type used for allocation or validation. A common pattern like let len = decoded.len() as u32; may wrap to a small number, causing an insufficient buffer to be allocated, which can lead to memory corruption or information disclosure downstream.

When combined with Basic Auth, the attack surface includes the parsing logic that extracts the username and password from the header before decoding. If Axum middleware or handler code performs arithmetic on the decoded credential lengths (e.g., computing size limits or offsets) using unchecked casts or unchecked operations, an integer overflow may result in incorrect bounds checks. This can expose internal buffers or lead to unsafe memory operations if the values are later used in APIs that assume correctness. The vulnerability is not in Basic Auth itself but in how the application handles numeric representations of data derived from authentication inputs.

Real-world attack patterns such as buffer overflows or type confusion have been cataloged in vulnerabilities like CVE-2021-21367 and CVE-2021-3156, where incorrect size calculations led to memory safety issues. OWASP API Security Top 10 categories such as Security Misconfiguration and Improper Restriction of Excessive Authentication Attempts can intersect with these flaws when rate limiting or validation logic depends on integer-derived metrics.

middleBrick detects these risks through its 12 parallel security checks, including Input Validation and Property Authorization, by analyzing OpenAPI specifications and runtime behavior. It flags unsafe integer usage in authentication flows and provides remediation guidance. For applications using the Pro plan, continuous monitoring can alert teams when new endpoints introduce unchecked numeric operations, helping prevent regressions in CI/CD pipelines via the GitHub Action.

To mitigate integer overflow risks in Axum with Basic Auth, ensure that all lengths and sizes derived from authentication data are validated before use. Use checked arithmetic and appropriate types such as usize for indexing and allocations, and avoid casting to smaller integer types unless the range is explicitly bounded.

Example of vulnerable code that parses Basic Auth and uses unchecked conversion:

use axum::extract::Request;
use axum::http::header;
use std::convert::TryInto;

async fn auth_middleware(request: Request) -> Result<(), (StatusCode, String)> {
    let auth_header = request.headers().get(header::AUTHORIZATION)
        .ok_or((StatusCode::UNAUTHORIZED, "Missing Authorization header".to_string()))?;
    let header_value = auth_header.to_str().map_err(|_| (StatusCode::BAD_REQUEST, "Invalid header".to_string()))?;
    if !header_value.starts_with("Basic ") {
        return Err((StatusCode::UNAUTHORIZED, "Unsupported scheme".to_string()));
    }
    let encoded = header_value[6..].trim();
    let decoded = base64::decode(encoded).map_err(|_| (StatusCode::BAD_REQUEST, "Invalid base64".to_string()))?;
    // Vulnerable: unchecked cast and unchecked length usage
    let length = decoded.len() as u32;
    if length > MAX_CREDENTIAL_LENGTH {
        return Err((StatusCode::PAYLOAD_TOO_LARGE, "Credential too long".to_string()));
    }
    // Further processing...
    Ok(())
}

Remediation with checked operations and safer patterns:

use axum::extract::Request;
use axum::http::header;
use std::convert::TryInto;

async fn auth_middleware(request: Request) -> Result<(), (StatusCode, String)> {
    const MAX_CREDENTIAL_LENGTH: usize = 1024;
    let auth_header = request.headers().get(header::AUTHORIZATION)
        .ok_or((StatusCode::UNAUTHORIZED, "Missing Authorization header".to_string()))?;
    let header_value = auth_header.to_str().map_err(|_| (StatusCode::BAD_REQUEST, "Invalid header".to_string()))?;
    if !header_value.starts_with("Basic ") {
        return Err((StatusCode::UNAUTHORIZED, "Unsupported scheme".to_string()));
    }
    let encoded = header_value[6..].trim();
    let decoded = base64::decode(encoded).map_err(|_| (StatusCode::BAD_REQUEST, "Invalid base64".to_string()))?;
    // Safe: use usize and checked comparison
    let length = decoded.len();
    if length > MAX_CREDENTIAL_LENGTH {
        return Err((StatusCode::PAYLOAD_TOO_LARGE, "Credential too long".to_string()));
    }
    // Further processing...
    Ok(())
}

Additional remediation steps include validating input sizes before allocations, using crates like num-traits for checked arithmetic, and enabling overflow checks during development with RUSTFLAGS="-C overflow-checks=yes". For teams using the CLI, middlebrick scan <url> can be integrated into local workflows, while the Dashboard and MCP Server help track findings across services. The GitHub Action can enforce a maximum risk score threshold to block deployments if unsafe patterns are detected.