name: "Security: Fork PR Alert"

on:
  pull_request:
    types: [opened, synchronize, closed, reopened]

permissions:
  pull-requests: write
  contents: read

jobs:
  fork-pr-monitor:
    if: github.event.pull_request.head.repo.full_name != github.repository
    runs-on: ubuntu-latest
    steps:
      - name: Check for suspicious patterns
        uses: actions/github-script@3a2844b7e9c422d3c10d287c895573f7108da1b3 # v9
        with:
          script: |
            const pr = context.payload.pull_request;
            const alerts = [];

            // 1. Check for [skip ci] in commit messages from fork PRs
            if (context.payload.action === 'opened' || context.payload.action === 'synchronize') {
              const commits = await github.rest.pulls.listCommits({
                owner: context.repo.owner,
                repo: context.repo.repo,
                pull_number: pr.number,
                per_page: 100
              });

              const skipCiCommits = commits.data.filter(c =>
                /\[skip ci\]|\[ci skip\]|\[no ci\]/i.test(c.commit.message)
              );

              if (skipCiCommits.length > 0) {
                alerts.push(`⚠️ **[skip ci] detected in fork PR** — ${skipCiCommits.length} commit(s) contain CI skip directives. Commits: ${skipCiCommits.map(c => c.sha.substring(0, 7)).join(', ')}`);
              }
            }

            // 2. Check for force-push that reduces changed files to 0 (evidence cleanup)
            if (context.payload.action === 'synchronize') {
              const prDetails = await github.rest.pulls.get({
                owner: context.repo.owner,
                repo: context.repo.repo,
                pull_number: pr.number
              });

              if (prDetails.data.changed_files === 0) {
                alerts.push(`🚨 **Zero-file fork PR after force-push** — PR was force-pushed to show 0 changed files. This matches the TanStack attack cleanup pattern.`);
              }
            }

            // 3. Check for rapid open-then-close (PR used only to trigger CI)
            if (context.payload.action === 'closed' && !pr.merged) {
              const created = new Date(pr.created_at);
              const closed = new Date(pr.closed_at);
              const minutesOpen = (closed - created) / (1000 * 60);

              if (minutesOpen < 30) {
                alerts.push(`🚨 **Fork PR closed rapidly** — opened and closed within ${Math.round(minutesOpen)} minutes without merging. May indicate a CI-trigger-only attack.`);
              }
            }

            // 4. Check for large bundled files (>5000 lines) added by the PR
            if (context.payload.action === 'opened' || context.payload.action === 'synchronize') {
              const files = await github.rest.pulls.listFiles({
                owner: context.repo.owner,
                repo: context.repo.repo,
                pull_number: pr.number,
                per_page: 100
              });

              const largeNewFiles = files.data.filter(f =>
                f.status === 'added' && f.additions > 5000
              );

              if (largeNewFiles.length > 0) {
                alerts.push(`⚠️ **Large files added by fork PR** — ${largeNewFiles.map(f => '`' + f.filename + '` (' + f.additions + ' lines)').join(', ')}. Bundled payloads are a common supply-chain attack vector.`);
              }
            }

            // Report alerts
            if (alerts.length > 0) {
              const body = [
                '## 🔒 Supply Chain Security Alert',
                '',
                'This fork PR triggered the following security alerts:',
                '',
                alerts.join('\n\n'),
                '',
                '---',
                '_Automated by supply-chain security monitor. See [TanStack incident](https://socket.dev/blog/tanstack-npm-packages-compromised-mini-shai-hulud-supply-chain-attack) for context._'
              ].join('\n');

              // Post as PR comment
              await github.rest.issues.createComment({
                owner: context.repo.owner,
                repo: context.repo.repo,
                issue_number: pr.number,
                body: body
              });

              // Also set the action as failed annotation
              core.warning(alerts.join(' | '));
            }