Api Key Exposure in Phoenix with Mongodb

When a Phoenix application uses MongoDB as its primary data store, mishandling API keys can lead to their unintended exposure through logs, error messages, or misconfigured database operations. MongoDB operations in Phoenix typically rely on connection strings and collection-level permissions that may inadvertently reveal sensitive values if not carefully controlled.

In a Phoenix+Elixir context, API keys are often stored in configuration files or injected via environment variables. If these keys are passed directly into MongoDB queries or used to construct database connection strings without sanitization, they may be exposed in several ways:

• Error traces: If a MongoDB driver raises an exception due to authentication failure or invalid permissions, the full connection string—including the embedded API key—can appear in logs or crash reports.
• Logging misconfiguration: Elixir’s Logger or telemetry handlers that capture query metadata might inadvertently log parameters that include API keys when developers use unparameterized concatenation or inspect raw connection details.
• Insecure defaults: Using the same MongoDB user for both application logic and administrative tasks increases the blast radius if a key is exposed, as it may grant broader database access than necessary.

Consider a scenario where a Phoenix controller builds a MongoDB filter by interpolating a client-supplied token into a query: this pattern can expose the token in logs if the query fails and the parameters are dumped for debugging. Even when using parameterized queries, developers might mistakenly pass an API key as part of a document payload that gets persisted or returned in error responses, creating a leakage path.

Because MongoDB supports rich query expressions and flexible schemas, developers sometimes embed metadata—including authentication tokens or temporary credentials—directly into documents for convenience. Without strict schema validation and access controls, these embedded keys can be read by any process with read access to the collection, effectively exposing the key to unauthorized consumers.

middleBrick’s unauthenticated scan can detect indicators of such exposure by analyzing the API surface and configuration patterns, flagging endpoints where sensitive values might be reflected in responses or logs. This is especially relevant when OpenAPI specifications describe MongoDB-driven endpoints without clearly separating authentication material from business data.

To prevent API key exposure when using MongoDB in Phoenix, apply defensive coding practices and configuration hygiene. The goal is to ensure keys never enter the data plane, are never logged, and are isolated from application logic that interacts with MongoDB.

First, keep API keys out of MongoDB documents entirely. Store them only in runtime secure storage (e.g., system environment variables or a secrets manager) and reference them via configuration at startup. Use Elixir’s System.get_env/1 with strict validation, and avoid constructing connection strings through string interpolation that could be logged.

# config/runtime.exs
import Config

mongodb_uri =
  case System.fetch_env("MONGODB_URI") do
    {:ok, uri} -> uri
    :error -> raise "MONGODB_URI environment variable is missing"
  end

config :my_app, MyApp.Repo,
  url: mongodb_uri,
  pool_size: 10

Second, use parameterized queries and avoid embedding keys in filter documents. When querying collections, pass keys as separate arguments rather than concatenating them into BSON structures.

# Correct: key used for authentication only at connection level, not in queries
from_collection = "users"
filter = %{status: "active"}

case MongoDB.find(MyApp.MongoClient, from_collection, filter) do
  {:ok, cursor} ->
    # process cursor
  {:error, reason} ->
    # log reason without exposing connection details
    Logger.error("MongoDB query failed: #{inspect(reason, limit: :infinity)}")
end

Third, sanitize logs and telemetry to exclude sensitive fields. Configure your Logger backends to filter known key names and connection parameters, and avoid calling IO.inspect on structures that may contain raw API values.

# config/config.exs
config :logger, :console,
  format: "[$level] $message\n",
  metadata_filter: ["authorization"]

# Use a custom formatter to redact sensitive keys
defmodule MyApp.SensitiveDataFilter do
  def redact(key, value) when key in ~w(api_key access_token secret)a, do: {key, "[FILTERED]"}
  def redact(_key, value), do: {key, value}
end

Fourth, enforce principle of least privilege for MongoDB database users. Create distinct roles for read-only operations and administrative tasks, and bind API keys to the minimal role required. This limits exposure impact if a key is inadvertently surfaced.

// MongoDB role definition example
use app_db
db.createRole({
  role: "app_reader",
  privileges: [
    {
      resource: { db: "app_db", collection: "" },
      actions: ["find"]
    }
  ],
  roles: []
})

Finally, validate and version your OpenAPI specs to ensure that MongoDB-driven endpoints do not describe or return authentication material. middleBrick’s spec analysis can highlight inconsistencies between declared parameters and runtime behavior, helping you catch risky definitions before deployment.