Integer Overflow in Actix with Basic Auth

An integer overflow in an Actix web service using HTTP Basic Authentication can occur when user-controlled numeric values derived from authentication or request parameters are used in arithmetic without proper bounds checking. In this context, the server parses the Basic Auth header, decodes the base64 credentials, and may extract a numeric identifier (for example, a user ID or quota value) from the username or password, or from parameters tied to the authenticated identity.

Consider a scenario where the username is structured as user:{id}:{size} and the service decodes this in an Actix handler to allocate a buffer or compute an offset. If {size} is attacker-controlled and used in a computation like let total = id + size; or vec.resize(size, 0), an unchecked large value can wrap around a 32-bit or 64-bit integer. The wrapped result may be much smaller than expected, leading to an undersized buffer or an incorrect memory offset. This can expose memory beyond the intended allocation or allow an attacker to bypass size-based safety checks, potentially enabling out-of-bounds reads or writes when the data is later used in business logic or serialization.

Because Basic Auth credentials are transmitted with every request, a malicious user can craft usernames or passwords that embed large numeric values designed to trigger overflow during parsing or arithmetic. In Actix, if the authentication logic eagerly decodes and processes these values before validation, the overflow may occur early in the request lifecycle. This can corrupt state, bypass authorization checks that depend on numeric invariants, or influence downstream handlers that assume sanitized inputs. The vulnerability is particularly dangerous when combined with unchecked deserialization or when the overflow leads to an integer truncation that changes access control decisions, such as incorrectly granting access based on a small computed role ID after wrap-around.

Real-world patterns that amplify risk include using integer types without saturation or checked arithmetic, concatenating parsed integers into offsets for serialization buffers, or deriving resource limits from authenticated claims without range validation. Because Actix services often handle structured data and strict routing, an overflow in authenticated contexts can silently affect multiple handlers that share the same parsing logic. The interaction between authentication-derived integers and business logic means the overflow may not be immediately visible, but it can distort control flow or data layout in ways that violate the principle of least privilege or enable information leakage through side channels.

To mitigate integer overflow risks when using Basic Authentication in Actix, validate and sanitize all numeric values derived from credentials before using them in arithmetic or memory operations. Prefer bounded integer types and checked operations, and avoid deriving sizes or offsets directly from attacker-controlled strings. Below are concrete, safe patterns and code examples for Actix services.

Example 1: Safe parsing and bounded arithmetic

Parse the Basic Auth credentials, validate numeric fields, and use checked arithmetic to prevent overflow. This example shows extracting user parameters safely and returning a 400 error if values are invalid or would overflow.

use actix_web::{web, HttpRequest, HttpResponse, Result};
use std::num::ParseIntError;

fn parse_basic_auth(req: &HttpRequest) -> Result<(String, u32, u32), HttpResponse> {
    let auth_header = req.headers().get("Authorization")
        .and_then(|v| v.to_str().ok())
        .ok_or_else(|| HttpResponse::BadRequest().body("Missing or invalid Authorization header"))?;
    if !auth_header.starts_with("Basic ") {
        return Err(HttpResponse::BadRequest().body("Invalid authorization scheme"));
    }
    let encoded = auth_header.trim_start_matches("Basic ");
    let decoded = base64::decode(encoded)
        .map_err(|_| HttpResponse::BadRequest().body("Invalid base64"))?;
    let creds = String::from_utf8(decoded)
        .map_err(|_| HttpResponse::BadRequest().body("Invalid UTF-8 credentials"))?;
    let parts: Vec<&str> = creds.split(':').collect();
    if parts.len() != 3 {
        return Err(HttpResponse::BadRequest().body("Invalid credential format, expected user:id:size"));
    }
    let id = parts[1].parse::()
        .map_err(|e: ParseIntError| HttpResponse::BadRequest().body(format!("Invalid id: {}", e)))?;
    let size = parts[2].parse::()
        .map_err(|e: ParseIntError| HttpResponse::BadRequest().body(format!("Invalid size: {}", e)))?;
    // Ensure size is within safe limits to prevent allocation abuse
    const MAX_SIZE: u32 = 1024 * 1024; // 1 MB
    if size > MAX_SIZE {
        return Err(HttpResponse::BadRequest().body("Size exceeds allowed maximum"));
    }
    Ok((parts[0].to_string(), id, size))
}

pub async fn handler(req: HttpRequest) -> Result {
    let (_user, id, size) = parse_basic_auth(&req)?;
    // Use checked arithmetic
    let total = id.checked_add(size)
        .ok_or_else(|| HttpResponse::BadRequest().body("Arithmetic overflow"))?;
    // Use `total` safely, for example to allocate a Vec with bounded size
    let data = vec![0u8; total as usize];
    Ok(HttpResponse::Ok().body(format!(