linear-webhooks

Linear Webhooks

When to Use This Skill

• Setting up Linear webhook handlers
• Debugging Linear signature verification failures
• Validating the Linear-Signature HMAC-SHA256 header
• Handling Linear Issue, Comment, Project, Cycle, IssueLabel, or IssueSLA events
• Reacting to create, update, and remove actions on Linear entities
• Rejecting stale webhook deliveries via the webhookTimestamp field

Essential Code (USE THIS)

Linear Signature Verification (JavaScript)

Linear signs each webhook with HMAC-SHA256 over the raw request body, hex-encoded, sent in the Linear-Signature header. Linear has no first-party Node SDK helper for verifying webhooks, so manual verification is the recommended approach.

const crypto = require('crypto');

function verifyLinearWebhook(rawBody, signatureHeader, secret) {
  if (!signatureHeader || !secret) return false;

  // HMAC-SHA256(rawBody, secret) → hex
  const expected = crypto
    .createHmac('sha256', secret)
    .update(rawBody)
    .digest('hex');

  try {
    return crypto.timingSafeEqual(
      Buffer.from(signatureHeader, 'hex'),
      Buffer.from(expected, 'hex')
    );
  } catch {
    return false;
  }
}

// Reject deliveries older than 1 minute (replay protection)
function isFreshTimestamp(webhookTimestamp) {
  if (typeof webhookTimestamp !== 'number') return false;
  const skewMs = Math.abs(Date.now() - webhookTimestamp);
  return skewMs <= 60 * 1000;
}

Express Webhook Handler

const express = require('express');
const app = express();

// CRITICAL: Use express.raw() - Linear signs the raw body
app.post('/webhooks/linear',
  express.raw({ type: 'application/json' }),
  (req, res) => {
    const signature = req.headers['linear-signature'];
    const event = req.headers['linear-event'];      // e.g. "Issue", "Comment"
    const delivery = req.headers['linear-delivery']; // UUID for idempotency

    if (!verifyLinearWebhook(req.body, signature, process.env.LINEAR_WEBHOOK_SECRET)) {
      return res.status(400).send('Invalid signature');
    }

    const payload = JSON.parse(req.body.toString());

    // Linear requires rejecting deliveries older than 1 minute
    if (!isFreshTimestamp(payload.webhookTimestamp)) {
      return res.status(400).send('Stale webhook');
    }

    console.log(`Linear ${event} ${payload.action} (delivery: ${delivery})`);

    switch (event) {
      case 'Issue':
        console.log(`Issue ${payload.action}:`, payload.data?.title);
        break;
      case 'Comment':
        console.log(`Comment ${payload.action} on issue ${payload.data?.issueId}`);
        break;
      case 'Project':
        console.log(`Project ${payload.action}:`, payload.data?.name);
        break;
      case 'IssueSLA':
        console.log(`SLA event on issue ${payload.issueData?.id}`);
        break;
      default:
        console.log(`Unhandled Linear event: ${event}`);
    }

    res.status(200).send('OK');
  }
);