Slack Slash Command

Category: Security & Compliance

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A production-ready Slack slash command endpoint that verifies Slack's request signature before doing any work, logs each command to Postgres, and replies with an ephemeral message. The signature check (is_valid_hmac) is the same pattern you need for any signed webhook (Slack, GitHub, Stripe, Shopify) — swap the base string and prefix for the provider.

Why signature verification matters

Your slash-command URL is public. Without verification, anyone who finds it can forge requests. Slack signs every request with your app's Signing Secret over the base string v0:{timestamp}:{raw_body} and sends it as X-Slack-Signature: v0=<hex>. This pack recomputes that HMAC-SHA256 and rejects anything that doesn't match — before touching your database.

Endpoints

Method	Route	Purpose
POST	/slack/command	Verify the Slack signature, log the command, reply.
POST	/api/seed	Create the slack_commands table (from seed.yml).

Setup

1. Create a Slack app at api.slack.com/apps → your app.
2. Copy Basic Information → App Credentials → Signing Secret.
3. Add a Slash Command (e.g. /status) and set its Request URL to your deployed /slack/command route.
4. Set the managed variables:
   • SLACK_SIGNING_SECRET — the Signing Secret from step 2.
   • DATABASE_URL — your Postgres connection string.
5. Seed the table: POST /api/seed.

Try it

Slack sends application/x-www-form-urlencoded. A genuine request carries a valid X-Slack-Signature; an unsigned or tampered one is rejected with 401:

# Forged / unsigned request → 401 Invalid Slack signature
curl -i -X POST https://<your-host>/slack/command \
  -H 'content-type: application/x-www-form-urlencoded' \
  -H 'x-slack-request-timestamp: 1700000000' \
  -H 'x-slack-signature: v0=deadbeef' \
  --data 'command=/status&text=hello&user_name=alice'

Slack itself supplies the correct signature, so a real slash command from your workspace returns:

{ "response_type": "ephemeral", "text": "✅ Received `/status` — logged as <uuid>" }

How it works

1. ReadHeaders — exposes the request headers so the base string can read the Slack timestamp (interpolation only sees prior actions' output).
2. VerifySignature — is_valid_hmac recomputes HMAC-SHA256(SLACK_SIGNING_SECRET, "v0:{timestamp}:{raw_body}") and compares it constant-time against X-Slack-Signature (stripping the v0= prefix). Mismatch → 401.
3. LogCommand — inserts the command into slack_commands.
4. Respond — returns an ephemeral Slack message.

Adapting to other providers

is_valid_hmac works for any signed webhook — change three fields:

Provider	body (base string)	prefix	signature header
Slack	v0:{timestamp}:{raw_body}	v0=	X-Slack-Signature
GitHub	{raw_body}	sha256=	X-Hub-Signature-256
Stripe	{timestamp}.{raw_body}	v1=	Stripe-Signature
Shopify	{raw_body}	(none)	X-Shopify-Hmac-Sha256

Configuration

config.yml

name: SlackSlashCommand

docs: true

# A verified Slack slash-command handler.
#
# Slack signs every request with your app's Signing Secret over the base string
#   v0:{X-Slack-Request-Timestamp}:{raw_body}
# and sends the result in the X-Slack-Signature header as "v0=<hex>". This pack
# recomputes that HMAC-SHA256 with is_valid_hmac and rejects forged/unsigned
# requests (401) before doing any work — then logs the command and replies with
# an ephemeral message Slack shows only to the invoking user.
#
# Required managed variables:
#   DATABASE_URL          — Postgres connection string (event log)
#   SLACK_SIGNING_SECRET  — Basic Information → App Credentials → Signing Secret

global:
  databases:
    main:
      driver: postgres
      conn_string: "a|ap_var::DATABASE_URL|"

interfaces:

  # POST /slack/command
  # Point your slash command's Request URL here (Slack → Slash Commands).
  # Slack posts application/x-www-form-urlencoded with command/text/user_name/etc.
  slack/command:
    output: http
    method: POST
    summary: Verify a Slack slash command's signature, log it, and reply
    description: >
      Verifies the X-Slack-Signature HMAC over the v0 base string, records the
      command in Postgres, and returns an ephemeral Slack message.
    tags: [slack, webhook, security]

    actions:
      # 1. Expose the request headers so the base string below can read the
      #    Slack timestamp (interpolation only sees PRIOR actions' output).
      - name: ReadHeaders
        input: a|headers|
        hide_data_on_success: true

      # 2. Reject anything not signed by Slack. is_valid_hmac recomputes
      #    HMAC-SHA256(signing_secret, "v0:{timestamp}:{raw_body}") and compares
      #    it constant-time to X-Slack-Signature (after stripping the "v0=" prefix).
      - name: VerifySignature
        input: a|headers|
        run_when_succeeded: [ReadHeaders]
        hide_data_on_success: true
        assert:
          http_code_on_error: 401
          error_message: "Invalid Slack signature"
          tests:
            - value: x-slack-signature
              is_not_null: true
              is_valid_hmac:
                secret: a|ap_var::SLACK_SIGNING_SECRET|
                body: "v0:a|ReadHeaders::x-slack-request-timestamp|:a|raw_body|"
                algorithm: sha256
                prefix: "v0="

      # 3. Log the command. Slack form fields land on the body; ->default('')
      #    keeps the insert valid even for fields a given command omits.
      - name: LogCommand
        run_when_succeeded: [VerifySignature]
        database: main
        query: |-
          INSERT INTO slack_commands (id, command, text, user_name, channel_id, team_id)
          VALUES (
            a|uuid->single_quote|,
            a|body::command->default('')->single_quote|,
            a|body::text->default('')->single_quote|,
            a|body::user_name->default('')->single_quote|,
            a|body::channel_id->default('')->single_quote|,
            a|body::team_id->default('')->single_quote|
          )
          RETURNING id
        post_transforms:
          - extract_value: "[0]"

      # 3. Reply. response_type "ephemeral" shows the message only to the caller.
      - name: Respond
        run_when_succeeded: [LogCommand]
        json_output: |-
          {
            "response_type": "ephemeral",
            "text": "✅ Received `a|body::command->default('')|` — logged as a|LogCommand::id|"
          }

seed.yml

name: SlackSlashCommandSeed
description: Creates the slack_commands table (idempotent). Safe to re-run — starts empty.

docs: true

# Required managed variables:
#   DATABASE_URL — Postgres connection string

global:
  databases:
    main:
      driver: postgres
      conn_string: "a|ap_var::DATABASE_URL|"

interfaces:

  # POST /api/seed
  # Creates the slack_commands table and index.
  api/seed:
    output: http
    method: POST

    actions:
      - name: CreateTable
        database: main
        hide_data_on_success: true
        query: |
          CREATE TABLE IF NOT EXISTS slack_commands (
            id          TEXT         PRIMARY KEY,
            command     TEXT         NOT NULL,
            text        TEXT         NOT NULL DEFAULT '',
            user_name   TEXT         NOT NULL DEFAULT '',
            channel_id  TEXT         NOT NULL DEFAULT '',
            team_id     TEXT         NOT NULL DEFAULT '',
            created_at  TIMESTAMPTZ  NOT NULL DEFAULT NOW()
          );

      - name: CreateIndex
        database: main
        run_when_succeeded: [CreateTable]
        hide_data_on_success: true
        query: |
          CREATE INDEX IF NOT EXISTS idx_slack_commands_created_at
            ON slack_commands (created_at DESC);

      - name: Respond
        run_when_succeeded: [CreateIndex]
        json_output: |-
          { "status": "seeded" }

schema.sql

-- Slack slash-command event log.
CREATE TABLE IF NOT EXISTS slack_commands (
  id          TEXT         PRIMARY KEY,
  command     TEXT         NOT NULL,
  text        TEXT         NOT NULL DEFAULT '',
  user_name   TEXT         NOT NULL DEFAULT '',
  channel_id  TEXT         NOT NULL DEFAULT '',
  team_id     TEXT         NOT NULL DEFAULT '',
  created_at  TIMESTAMPTZ  NOT NULL DEFAULT NOW()
);

CREATE INDEX IF NOT EXISTS idx_slack_commands_created_at ON slack_commands (created_at DESC);