Broken Authentication in Chi with Api Keys

Chi is a lightweight HTTP framework for the Crystal language. When developers use API keys in Chi without addressing authentication design flaws, they can unintentionally expose or weaken authentication. Broken Authentication in this context refers to weaknesses in how identity and access tokens (here, API keys) are issued, stored, transmitted, and validated, allowing attackers to assume other identities or bypass authorization entirely.

One common pattern is to pass API keys via query parameters or headers without enforcing strict transport security and key rotation. For example, a route defined as get "/api/data", DataHandler might read env.get("HTTP_X_API_KEY") and grant access based solely on a key match. If keys are transmitted over unencrypted HTTP, they can be intercepted via network sniffing. If keys are embedded in URLs or client-side JavaScript, they can leak through logs, browser history, or referrer headers. These issues map to OWASP API Top 10 authentication weaknesses such as broken object level authorization (BOLA) when key-to-owner binding is missing, and improper enforcement of authentication in middleware.

Chi does not enforce authentication out of the box; it relies on developer code. If the validation logic is inconsistent—for instance, checking the key only on entry points but not on subresource requests—an attacker can leverage Insecure Direct Object References (IDOR) to access other users’ data by guessing or iterating identifiers. Similarly, missing rate limiting on authentication endpoints can enable credential stuffing or brute-force attempts against valid keys. Another risk is storing keys in plaintext configuration or logs; if an attacker gains read access to the host or log streams, they can reuse keys. Without auditing and without binding keys to scopes, IPs, or time windows, API keys become long-lived credentials that violate the principle of least privilege, increasing the impact of a compromised key.

Additional factors that amplify broken authentication include insecure dependency versions and misconfigured TLS. Even when keys are validated, failing to reject weak or default keys, not rotating keys after staff turnover, and omitting proper HTTP security headers (such as strict transport security) can undermine the authentication chain. In distributed systems where Chi services call each other, propagating keys without mutual TLS or signed tokens can lead to SSRF or token replay attacks. The lack of built-in session management in Chi shifts responsibility to the developer to design secure key lifecycle and revocation mechanisms. Therefore, authentication in Chi with API keys must be evaluated across transmission integrity, storage protection, binding to identities, and operational practices to avoid Broken Authentication vulnerabilities.

Remediation focuses on secure key handling, transport integrity, binding, and operational hygiene. Use HTTPS for all traffic to prevent interception, store keys outside source code, and validate keys with constant-time comparisons to avoid timing attacks. Bind keys to metadata such as owner ID, scope, and expiration, and enforce authorization checks on every request to subresources.

Below are concrete, syntactically correct examples in Chi that demonstrate secure API key usage.

using HTTP
using Random

# Secure key storage: load from secure environment at startup, never hardcode
# In production, use a secrets manager and rotate keys periodically
ENV["API_KEY"] = "s3cr3t_k3y_3x4mpl3" # placeholder; inject via env in real deployments

struct KeyContext
  key : String
  owner_id : Int64
  scopes : Array(String)
end

# Constant-time comparison to avoid timing leaks
macro constant_time_eq(a, b)
  quote
    begin
      left = {{a}}
      right = {{b}}
      left.size == right.size && left.zip(right).all { |x, y| x == y }
    end
  end
end

# Middleware to extract and validate API key
class AuthMiddleware
  def initialize(@key_ctx : KeyContext)
  end

  def call(env)
    request = HTTP::Request.new(env)
    provided = request.headers["X-API-Key"]? || env.get("HTTP_X_API_KEY")
    if provided && constant_time_eq(provided, @key_ctx.key)
      # Attach identity and scopes to request metadata for downstream handlers
      env["auth"] = @key_ctx
      @app.call(env)
    else
      HTTP::Response.new(401, {"Content-Type" => "application/json"}, {error: "unauthorized"}.to_json)
    end
  end
end

# Example handler with per-route authorization checks
class DataHandler
  def initialize(@db : DB)
  end

  def call(env)
    auth = env["auth"]? || raise "auth missing"
    user_id = env.url_params["user_id"].try(&.to_i64) || 0
    # Enforce BOLA: ensure the requesting key owner can access the requested user_id
    if auth.owner_id != user_id || !auth.scopes.includes?("data:read")
      return HTTP::Response.new(403, {"Content-Type" => "application/json"}, {error: "forbidden"}.to_json)
    end
    # Safe data retrieval
    result = @db.query_one("SELECT id, name, email FROM users WHERE id = $1", user_id)
    {id: result[0], name: result[1], email: result[2]}.to_json
  end
end

# Application setup with TLS enforcement and secure defaults
app = HTTP::Server.new do |env|
  # Enforce HTTPS in production by rejecting non-TLS requests at edge or load balancer;
  # Chi can inspect headers if behind proxy, but prefer transport-layer enforcement.
  if env.get("HTTP_X_FORWARDED_PROTO") != "https"
    HTTP::Response.new(400, {"Content-Type" => "application/json"}, {error: "require HTTPS"}.to_json)
  else
    # Use middleware stack
    AuthMiddleware.new(KeyContext.new(
      key: ENV["API_KEY"].not_nil!,
      owner_id: 1,
      scopes: ["data:read", "data:write"]
    )).call(env)
  end
end

# Configure server with TLS (certificate paths injected via env/flags in real usage)
# app.listen(host: "0.0.0.0", port: 8080, tls: {cert: "cert.pem", key: "key.pem"})
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