Mass Assignment in Buffalo with Jwt Tokens

Mass Assignment in the Buffalo web framework occurs when user-supplied parameters are directly bound to model fields without explicit allowlisting. This becomes high risk when JWT tokens are used for authentication and the token payload is merged into request parameters or bound into structs that also accept user input. If a developer decodes a JWT and copies claims such as user_id or role into a params map or a changeset without restricting which fields can be set, an attacker who can influence the token (e.g., via a compromised client or a weaker signing key) can escalate privileges or overwrite sensitive fields.

Consider a scenario where an endpoint decodes a JWT to extract user_id, then uses that value together with JSON body parameters to update a record. In Buffalo, if the developer does not restrict which keys are permitted, a crafted request can include extra fields such as is_admin or permissions, and the mass assignment will apply them if the code passes all decoded JWT claims and body params into the same changeset. This combination means that trust placed in the JWT (e.g., identity and role claims) is directly coupled with untrusted body input, expanding the attack surface beyond what a typical mass assignment issue would allow.

For example, an attacker who manages to tamper with a JWT (or obtain a token for a low-privilege account) could send a PATCH request with additional fields that the Buffalo handler inadvertently accepts. Because the handler might use a single struct binding for both token-derived values and body data, the changeset can become polluted. Even if the JWT itself is cryptographically verified, the application must still enforce a strict allowlist for which fields can be mass-assigned; otherwise the token’s convenience becomes a vector for privilege escalation or unintended data modification.

In practice, this manifests as a BOLA/IDOR or Privilege Escalation finding in a middleBrick scan, because the endpoint allows modification of sensitive attributes through mass assignment when JWT context is incorporated into the binding logic. The scanner does not assume trust in authentication claims and will flag the absence of explicit field permission checks when user-influenced data and token-derived data are combined for persistence.

To remediate mass assignment when using JWT tokens in Buffalo, strictly separate token-derived data from user-supplied parameters and explicitly permit only safe fields. Avoid merging the entire JWT claims map into the same params struct used for mass assignment. Instead, extract only the claims you need (such as subject or role) and pass them explicitly to business logic, while using a permitted parameters list for any user input.

Example of an unsafe pattern to avoid:

defmodule MyAppWeb.UserController do
  use MyAppWeb, :controller

  def update(conn, %{"id" => id, "user" => user_params}) do
    # UNSAFE: decoding JWT and merging claims into user_params
    with {:ok, claims} <- MyApp.Jwt.verify(conn), do
      changeset = User.changeset(%User{}, Map.merge(claims, user_params))
      # ... update logic
    end
  end
end

The above merges decoded JWT claims directly into user-provided parameters, which enables mass assignment across all claims present in the token.

Safer approach — extract only required claims and apply them explicitly:

defmodule MyAppWeb.UserController do
  use MyAppWeb, :controller

  def update(conn, %{"id" => id, "user" => user_params}) do
    with {:ok, claims} <- MyApp.Jwt.verify(conn),
         %{sub: subject} <- claims,
         user <- User |> User.get_user!(subject),
         permitted <- User.permitted_fields(user_params),
         changeset <- User.update_changeset(user, permitted, subject) do
      # ... apply update
    end
  end
end

defmodule MyApp.User do
  def permitted_fields(params) do
    # Explicitly allowlist fields that can be mass-assigned
    params
    |> Map.take(~w(name email)a)
  end

  def update_changeset(user, params, updater_id) do
    user
    |> Ecto.Changeset.cast(params, [:name, :email])
    |> Ecto.Changeset.put_change(:updated_by_id, updater_id)
  end
end

In this safer version, Map.take/2 implements a clear allowlist for mass assignment, and only the :name and :email fields can be set from user input. The JWT-subject is used for authorization and audit tracking but is not merged into the changeset. This prevents an attacker from injecting extra claims into persisted records even if the token is compromised.

When using JWT tokens, also ensure that tokens are treated as credentials and not as a source of mass-assignable data. Validate token signatures, keep scopes minimal, and avoid placing mutable business fields inside the token payload. Combine this discipline with Buffalo’s changeset permissions so that the API surface exposed to user input remains strictly separated from authentication-derived metadata.