Insecure Deserialization in Chi with Basic Auth

Insecure deserialization occurs when an application processes untrusted data without validating its type or integrity after deserialization. In the Chi web framework for OCaml, this commonly arises when endpoints accept serialized payloads (e.g., JSON, MessagePack, or custom binary formats) and reconstruct values without strict type checks. When Basic Auth is used, developers may mistakenly believe that the presence of an Authorization header provides sufficient protection, leading to insecure handling of deserialized data. For example, an endpoint that parses a Base64-encoded cookie or a JSON body containing serialized user state may instantiate objects or records that include sensitive permissions or session identifiers. If the deserialization logic does not enforce schema validation or whitelisted types, an attacker can supply crafted payloads that execute unintended code or alter control flow upon deserialization, potentially bypassing Basic Auth checks if authorization decisions are derived from the deserialized content.

Chi routes typically pattern-match on request parameters and headers; if a route decodes and deserializes data before verifying credentials or scope, the attack surface expands. Consider an API that uses Basic Auth for initial access but then relies on deserialized claims for authorization. An attacker who can influence the deserialization process might forge tokens or elevate privileges by embedding malicious types or references. Because Chi does not inherently protect against malformed or malicious serialized inputs, the framework relies on developer practices. Real-world deserialization vulnerabilities often map to OWASP API Top 10:2023 A05 (Security Misconfiguration) and A03 (Injection), where improper validation of external data leads to remote code execution or data tampering. Tools like middleBrick can detect such risks by correlating OpenAPI specifications with runtime behavior, highlighting endpoints that accept serialized inputs without adequate safeguards.

For instance, an endpoint defined with Cohttp and Yojson might deserialize a JSON payload into a variant type without checking for unexpected constructors. If Basic Auth is handled via a header parser that sets user roles in a session, deserializing attacker-controlled data after authentication could overwrite those roles. MiddleBrick tests this scenario by probing endpoints with manipulated serialized payloads and inspecting whether deserialization logic intersects with authentication decisions. The scanner checks for missing type constraints, reflective deserialization, and improper error handling that could leak stack traces or aid further exploitation. By combining real OpenAPI specs with active runtime checks, middleBrick identifies gaps where deserialization occurs in trust boundaries, providing specific remediation guidance aligned with compliance frameworks such as OWASP API Top 10 and SOC2.

Secure deserialization in Chi with Basic Auth requires strict validation of input types and separation of authentication from data processing. Always parse and validate credentials before deserializing any user-controlled data, and avoid using deserialized values to make authorization decisions. Prefer explicit, schema-based parsing with libraries such as Yojson for JSON or Bin_prot with strict type definitions for binary formats. Define closed variant types and use functions that reject unknown tags or fields. For Basic Auth, decode the header once, validate credentials against a trusted source, and store minimal claims in a secure session or context that is not derived from deserialized content.

Below are concrete Chi code examples that demonstrate secure handling. The first example shows safe Basic Auth parsing with fixed credentials and no reliance on deserialized data for authorization:

let parse_auth_header (h : Header.t) : (string * string) option =
  match Header.get h "authorization" with
  | Some auth when String.is_prefix ~prefix:"Basic " auth ->
    let encoded = String.drop_prefix auth 6 in
    (try
       let decoded = Base64.decode_exn encoded in
       match String.split ~on:':' decoded with
       | [user; pass] -> Some (user, pass)
       | _ -> None
     with _ -> None)
  | _ -> None

let require_auth req k =
  match parse_auth_header (Server_request.headers req) with
  | Some ("admin", "secret123") -> k (Server_request.set_auth_status req `Authenticated)
  | _ -> Server_response.unauthorized ()

The second example shows how to deserialize JSON safely in a Chi route after authentication, using a closed Yojson type and rejecting malformed inputs:

type user_action =
  | View
  | Edit

let action_of_yojson = function
  | `String "view" -> Ok View
  | `String "edit" -> Ok Edit
  | _ -> Error "invalid action"

let handle_request req =
  let* () = require_auth req (fun _ -> ()) in
  let body = Cohttp_lift_body.to_string (Server_request.meth_body req) in
  match Yojson.Safe.from_string body with
  | exception Yojson.Json_error msg -> Server_response.bad_request ~msg
  | json ->
    match action_of_yojson json with
    | Ok action -> Server_response.ok (Format.asprintf "Action: %a" Fmt.(to_to_string user_action) action)
    | Error msg -> Server_response.bad_request ~msg

These patterns ensure that deserialization never occurs before authentication and that only explicitly allowed values are accepted. middleBrick can verify such implementations by scanning endpoints for missing validation layers and improper data flows, helping teams align with secure coding standards.