Heap Overflow in Chi with Mutual Tls

A heap overflow in a Chi application that uses mutual TLS occurs when unchecked input accumulates on the heap—often in request bodies, headers, or cookies—while the server authenticates clients with mTLS. Because Chi is a lightweight HTTP library for the Crystal language, it does not automatically bound allocations derived from client-supplied data. If a route parses large JSON or form payloads into buffers or custom objects without size limits, an attacker can send oversized, maliciously crafted requests over an mTLS-secured channel. The presence of mutual TLS ensures the client is authenticated, but it does not reduce the size of the data; it only ensures the identity of the peer. This means a trusted client can still trigger heap corruption by exceeding expected structure sizes, leveraging mTLS sessions to bypass IP-based restrictions or network-level authentication checks.

Real-world patterns that amplify risk include using Crystal’s JSON::PullParser or HTTP::FormData to deserialize untrusted input into structs with fixed-size arrays or strings, and storing parsed values in collections that grow on the heap. Common CWE entries relevant here are CWE-122 (Heap-based Buffer Overflow) and CWE-20 (Improper Input Validation). In the context of OWASP API Top 10, this aligns with API1:2023 – Broken Object Level Authorization when over-read data is used to infer or modify object relationships, and API5:2023 – Broken Function Level Authorization if oversized mTLS-authenticated calls trigger unintended logic branches. Examples of triggering payloads include deeply nested JSON with large arrays or strings exceeding typical small-literal defaults, which can corrupt adjacent heap metadata and potentially lead to arbitrary code execution or denial of service.

middleBrick scans unauthenticated attack surfaces and would flag such endpoints under checks like Input Validation and Unsafe Consumption, even when mutual TLS is in use. Its findings include per-category breakdowns tied to frameworks such as OWASP API Top 10 and provide remediation guidance without claiming to fix or block traffic. The scanner runs in 5–15 seconds and does not require credentials, making it suitable for early detection of heap-related issues in Chi services protected by mTLS.

Remediation focuses on bounding and validating data independent of the transport security layer. In Chi, apply strict size limits before deserialization, avoid unchecked heap growth, and use types with predictable memory footprints. Below are concrete, working examples of mutual TLS setup in Chi and safe parsing patterns that mitigate heap overflow risks.

Mutual TLS server setup in Chi

require "openssl"
require "chi"

server = OpenSSL::SSL::Context::Server.new(
  cert_file: "server.crt",
  priv_key:  "server.key",
  verify_mode: OpenSSL::SSL::VerifyMode::PEER,
  ca_file:     "ca.crt"
)

app = Chi.new do
  before do
    # Enforce client certificate presence and verification
    unless env["SSL_CLIENT_VERIFY"] == "SUCCESS"
      halt 401, { error: "client certificate required" }.to_json
    end
  end

  post "/users" do
    # Safe: limit content-length before reading
    request.headers["Content-Length]"?.try do |raw|
      size = raw.to_i64
      if size > 16_384
        halt 413, { error: "payload too large" }.to_json
      end
    end

    # Safe: bounded parsing with explicit limits
    body = request.body.gets_to_end(io_size: 16_384)
    user = User.from_json_safe(body)
    "ok"
  end
end

app.ssl(server, port: 8443)

Safe deserialization and size-bounded structs

class User
  include JSON::Serializable

  # Use fixed-size or constrained fields
  @[JSON::Field(key: "name")]
  getter name : String

  @[JSON::Field(key: "tags")]
  getter tags : Array(String) do |values|
    raise ArgumentError.new("too many tags") if values.size > 10
    values
  end

  def self.from_json_safe(data : String)
    # Enforce a global size guard
    if data.bytesize > 16_384
      raise ArgumentError.new("input exceeds max size")
    end
    JSON.parse(data, object: self)
  end
end

# Usage inside a Chi route
post "/users" do
  body = request.body.gets_to_end(io_size: 16_384)
  user = User.from_json_safe(body)
  { user.name, user.tags }.to_json
end

Additional mitigations

• Set request.headers["Content-Length"] checks or use io_size when reading bodies to avoid unbounded reads.
• Prefer structs with static arrays or Array(T) with validated size in setters.
• Combine mTLS with rate limiting and monitoring; note that middleBrick’s Pro plan includes continuous monitoring and can alert on repeated large payloads across scans.

These patterns ensure that even with mutual TLS protecting channel authenticity, heap allocations remain bounded and input validation is strict, reducing the likelihood of heap overflow conditions in Chi services.