Mass Assignment in Actix with Basic Auth

Mass assignment occurs when an API endpoint binds user-supplied JSON directly to a server-side data model, allowing attackers to set fields they should not control. In Actix web, this typically happens when a handler deserializes request bodies into a struct using web::Json<T> without filtering fields. When Basic Auth is used, the request carries an Authorization: Basic base64(username:password) header, but authentication and authorization decisions are often handled separately from data binding. This separation can create a chain of risk: the endpoint trusts the authenticated identity (derived from Basic Auth) and then blindly updates account or role fields based on user input, effectively letting an authenticated user modify privileged attributes.

For example, an authenticated handler might deserialize a user profile update like this:

#[derive(Deserialize)]
struct UpdateUser {
    email: String,
    role: String,  // attacker-controlled if not filtered
    is_admin: bool,
}

async fn update_profile(
    payload: web::Json<UpdateUser>,
    req: HttpRequest,
) -> impl Responder {
    // Basic Auth extracted elsewhere, identity used to locate user
    // but payload.role can overwrite admin status
    HttpResponse::Ok().finish()
}

An authenticated attacker with Basic Auth credentials can send an email and also set role or is_admin, escalating privileges. Because Actix does not automatically ignore unknown or sensitive fields during deserialization, the bound struct becomes a conduit for privilege manipulation. The vulnerability is not in Basic Auth itself, but in the combination of permissive data binding and an assumption that authentication implies safe update scopes. This maps to the BOLA/IDOR and BFLA/Privilege Escalation checks in middleBrick’s scan, which would flag such unvalidated mass assignment as a high-severity finding.

Moreover, if the OpenAPI spec defines request models with sensitive fields but server-side code does not apply a denylist or explicit allowlist, the spec-to-runtime mismatch discovered by middleBrick’s OpenAPI/Swagger analysis (with full $ref resolution) will highlight the drift. The scanner runs 12 checks in parallel, including Property Authorization and Input Validation, to detect when authenticated endpoints accept unrestricted property updates.

To prevent mass assignment with Basic Auth in Actix, use explicit field selection and avoid binding raw user input to domain models. Always deserialize into a dedicated DTO that contains only the fields you intend to update, and apply changes selectively to the entity.

1) Use a restricted DTO without sensitive fields:

#[derive(Deserialize)]
struct ProfileUpdateDto {
    email: String,
    // do not include role or is_admin here
}

async fn update_profile(
    dto: web::Json<ProfileUpdateDto>,
    user_identity: Identity, // extracted from Basic Auth
    // service to fetch and update user by identity
) -> impl Responder {
    let user_id = user_identity.id(); // your own extraction logic
    // update only allowed fields
    repo.update_email(user_id, dto.email).map(|_| HttpResponse::Ok().finish())
}

2) If you must accept a broader model, selectively apply changes:

#[derive(Deserialize)]
struct RawUserUpdate {
    email: Option<String>,
    role: Option<String>,
    is_admin: Option<bool>,
}

async fn safe_update(
    raw: web::Json<RawUserUpdate>
    user_identity: Identity,
) -> impl Responder {
    let user_id = user_identity.id();
    let mut updates = UserPatch::default();
    if let Some(email) = &raw.email {
        updates.email = Some(email.clone());
    }
    // Ignore role and is_admin from raw input
    repo.patch_user(user_id, updates).map(|_| HttpResponse::NoContent().finish())
}

3) Basic Auth extraction example (kept separate from update logic):

use actix_web::http::header;
use actix_web::{HttpRequest, Error};
use base64::prelude::*;

fn extract_basic_identity(req: &HttpRequest) -> Option<String> {
    req.headers().get(header::AUTHORIZATION).and_then(|h| {
        h.to_str().ok().and_then(|s| {
            if s.starts_with("Basic ") {
                let encoded = s.trim_start_matches("Basic ");
                let decoded = BASE64_STANDARD.decode(encoded).ok()?;
                String::from_utf8(decoded).ok()
            } else {
                None
            }
        })
    })
}

By decoupling authentication (Basic header parsing) from authorization (what the identity is allowed to change) and using strict DTOs, you eliminate mass assignment paths. middleBrick’s CLI (middlebrick scan <url>) can verify that endpoints with authentication do not bind sensitive fields, and the GitHub Action can fail builds if findings include BFLA or Property Authorization violations.