Out Of Bounds Write in Actix with Basic Auth

An Out Of Bounds Write occurs when an API writes data past the boundaries of a fixed-length buffer or data structure, which can corrupt memory, crash services, or lead to code execution. When combined with Actix web and HTTP Basic Authentication, the risk profile changes because authentication headers are parsed early and may be processed by downstream handlers that perform unsafe deserialization or unchecked array writes.

Consider an endpoint that expects a JSON payload with a fixed-size list, such as a batch operation for user roles. If the API reads the Authorization header via Basic Auth to identify the actor but then processes an untrusted array from the request body without length validation, an attacker can supply a large or malformed array that writes beyond allocated memory. In Actix, this can surface when handler code uses unchecked indexing or when deserialization libraries are configured to accept oversized inputs. The authentication step does not inherently protect against this; it only identifies the caller, and if the handler assumes the caller is trusted, it may skip bounds checks.

Runtime findings from a middleBrick scan can highlight mismatches between spec definitions and runtime behavior. For example, an OpenAPI schema might declare an array with maxItems: 10, but the Actix implementation might deserialize into a vector without enforcing that limit. middleBrick’s checks for Input Validation and Unsafe Consumption are designed to surface these gaps, especially when authentication headers are present but do not constrain the request body.

In practice, an attacker might send a POST request with a valid Basic Auth credential but a malicious payload:

POST /api/v1/roles HTTP/1.1
Authorization: Basic dXNlcjpwYXNz
Content-Type: application/json

{
  "roles": ["admin", "editor", "viewer", "auditor", "guest", "bot", "service", "monitor", "backup", "operator", "extra"]
}

If the server allocates a fixed buffer or uses an unchecked loop to assign roles, the extra entry can trigger an Out Of Bounds Write. middleBrick’s Inventory Management and Property Authorization checks help detect when the runtime behavior diverges from the declared schema, especially under unauthenticated attack surface testing where Basic Auth is accepted but not enforced for all mutations.

Remediation focuses on validating inputs independent of authentication and ensuring that authorization does not imply trust. Even when Basic Auth is used to identify a user, every request body must be validated against strict bounds and schema rules.

Use strongly typed structures with serde and enforce collection limits explicitly. Avoid relying on default deserialization behavior that may accept arbitrarily large arrays.

use actix_web::{web, HttpResponse, Result};
use serde::Deserialize;

#[derive(Deserialize)]
struct RoleAssignment {
    roles: Vec,
}

// Enforce a maximum length at the handler level
async fn assign_roles(payload: web::Json) -> Result {
    const MAX_ROLES: usize = 10;
    if payload.roles.len() > MAX_ROLES {
        return Ok(HttpResponse::BadRequest().body("roles array exceeds maximum length"));
    }
    // Process roles safely
    Ok(HttpResponse::Ok().body("roles assigned"))
}

For Basic Auth, parse credentials without granting implicit trust. Validate the decoded username and password, and do not use the presence of valid credentials to bypass input checks.

use actix_web::http::header::HeaderValue;
use actix_web::{HttpRequest, HttpResponse};
use base64::Engine;

fn validate_basic_auth(req: &HttpRequest) -> bool {
    if let Some(auth_header) = req.headers().get("Authorization") {
        if let Ok(auth_str) = auth_header.to_str() {
            if auth_str.starts_with("Basic ") {
                let encoded = auth_str.trim_start_matches("Basic ");
                if let Ok(decoded) = base64::engine::general_purpose::STANDARD.decode(encoded) {
                    if let Ok(credentials) = String::from_utf8(decoded) {
                        let parts: Vec<&str> = credentials.split(':').collect();
                        if parts.len() == 2 {
                            let (user, pass) = (parts[0], parts[1]);
                            // Perform constant-time validation against stored values
                            return user == "legit_user" && pass == "correct_hashed_password";
                        }
                    }
                }
            }
        }
    }
    false
}

Combine these practices: always validate the request body, apply strict length constraints, and treat authentication as identification rather than authorization. middleBrick’s Continuous Monitoring and GitHub Action integrations can help ensure that future changes do not reintroduce boundary violations by failing builds when risk scores degrade.