LatticeStripe.Webhook.CacheBodyReader (LatticeStripe v0.2.0)

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Body reader that caches the raw request body for webhook signature verification.

Stripe signs the raw, unmodified request body using HMAC-SHA256. Most frameworks (Phoenix included) parse the body via Plug.Parsers, which reads and discards the raw bytes. By the time your controller or Plug runs, the original body is gone — and signature verification fails.

CacheBodyReader solves this by acting as a drop-in body reader for Plug.Parsers. It reads the body normally, then stashes the raw bytes in conn.private[:raw_body] before returning. LatticeStripe.Webhook.Plug reads from that private key automatically.

Setup

In your Phoenix endpoint.ex, configure Plug.Parsers to use CacheBodyReader as the body reader:

plug Plug.Parsers,
  parsers: [:urlencoded, :multipart, :json],
  pass: ["*/*"],
  json_decoder: Jason,
  body_reader: {LatticeStripe.Webhook.CacheBodyReader, :read_body, []}

Then mount the webhook plug in your router:

forward "/webhooks/stripe", LatticeStripe.Webhook.Plug,
  secret: System.fetch_env!("STRIPE_WEBHOOK_SECRET"),
  handler: MyApp.StripeHandler

Alternative: Mount before Plug.Parsers

If you cannot use CacheBodyReader (e.g., conflicting body reader config), mount LatticeStripe.Webhook.Plug before Plug.Parsers in your endpoint. The plug falls back to Plug.Conn.read_body/2 directly when conn.private[:raw_body] is not set.

# endpoint.ex — mount BEFORE plug Plug.Parsers
plug LatticeStripe.Webhook.Plug,
  at: "/webhooks/stripe",
  secret: System.fetch_env!("STRIPE_WEBHOOK_SECRET"),
  handler: MyApp.StripeHandler

plug Plug.Parsers,
  parsers: [:json],
  pass: ["application/json"],
  json_decoder: Jason

Summary

Functions

read_body(conn, opts)

Reads the request body and caches the raw bytes in conn.private[:raw_body].

Functions

read_body(conn, opts)

@spec read_body(
  Plug.Conn.t(),
  keyword()
) ::
  {:ok, binary(), Plug.Conn.t()}
  | {:more, binary(), Plug.Conn.t()}
  | {:error, term()}

Reads the request body and caches the raw bytes in conn.private[:raw_body].

This function is a drop-in replacement for Plug.Conn.read_body/2 and is intended to be used as the :body_reader option for Plug.Parsers.

After this function runs, conn.private[:raw_body] contains the raw body bytes that were read.