Mass Assignment in Chi with Api Keys

Mass assignment in a Chi-based API occurs when user-controlled input is directly bound to server-side structures, such as structs or maps, without explicit field filtering. When API keys are handled through request parameters or headers and are bound implicitly, an attacker can assign unexpected fields that alter privilege or behavior. For example, an endpoint that decodes JSON into a user struct and merges in an api_key field from the request body may allow a caller to overwrite administrative flags or impersonate other users.

Consider a Chi route that uses wrap-params to coerce and merge query, body, and header data into a single map intended for database insertion. If the map is later used to both create a session and look up an API key, an attacker can inject fields like is_admin or role by including them in the request body. Because the API key is often used for authorization decisions, an attacker who can indirectly influence which key is used may bypass intended access controls.

In practice, this vector combines two concerns: the mechanics of Chi routing and parameter coercion, and the lifecycle of API key validation. If key validation occurs after mass binding, malicious fields may have already modified the data model or context. Even when keys are extracted early, logging or error paths that echo bound parameters can inadvertently expose key material or confirm the existence of specific fields to unauthenticated callers.

Real-world patterns mirror OWASP API Top 10 #1 (Broken Object Level Authorization) and map to the BOLA/IDOR and Property Authorization checks performed by middleBrick. A scanner exercising unauthenticated endpoints can detect whether mass assignment permits fields like api_key, role, or scopes to be set by the client, and whether those fields affect authorization decisions.

An OpenAPI 3.0 spec that lacks explicit required and additionalProperties: false on request bodies increases the risk. middleBrick tests these scenarios by comparing the spec’s defined schema with runtime behavior, identifying endpoints where bound input can modify sensitive attributes tied to API key usage.

Remediation focuses on strict input separation and explicit key handling. API keys should be extracted from a single trusted source—typically an Authorization header—before any request body or query parameters are bound to domain models. Never allow user-supplied fields to influence which key is used or whether it is considered valid.

Use Chi’s ring-middleware to parse headers independently and validate the key against a datastore before routing to handlers that perform sensitive operations. Keep domain structs distinct from parameter maps; avoid merging user input directly into structs intended for database writes.

;; Bad: merging request body into a user map that includes api_key
(let [body (parse-body request)
      user (merge {:id nil :email "" :api_key nil} body)]
  (insert-user user))

;; Good: extract api_key separately, validate, then handle user data
(let [api-key (get-in request [:headers "x-api-key"])
      user-body (-> request :params :body (dissoc :api_key))]
  (if (valid-api-key? api-key)
      (let [user (validate-user-params user-body)]
        (respond-ok (create-user user)))
      (respond-unauthorized "Invalid API key")))

In the good example, dissoc ensures no client-supplied :api_key can pollute the user map. The key is validated first, and only after authorization succeeds are user parameters processed. This pattern aligns with middleBrick’s findings and remediation guidance, reducing the likelihood of privilege escalation via mass assignment.

For JSON APIs using middleware like wrap-key-params, explicitly whitelist permitted fields and reject extras rather than relying on automatic coercion. Combine this with runtime scanning—such as that offered by middleBrick’s CLI tool—to continuously verify that endpoints adhere to the intended separation.

When using the middleBrick Dashboard or the Pro plan’s continuous monitoring, configure scans to flag endpoints where request schemas contain fields like api_key, role, or permissions that should be server-managed. The GitHub Action can fail builds if such risky definitions are detected, enforcing secure design before deployment.