Replace workflow PAT usage with GitHub App authentication

.github/actions/github-app-token/action.yml

@@ -0,0 +1,116 @@
+name: Get GitHub automation token
+description: Creates a GitHub App installation token with a temporary PAT fallback
+
+inputs:
+  mode:
+    description: Authentication mode (app, app-with-fallback, or pat)
+    required: false
+    default: app-with-fallback
+  azure-client-id:
+    description: Client ID of the Azure workload identity
+    required: false
+  azure-tenant-id:
+    description: Azure tenant ID
+    required: false
+  azure-subscription-id:
+    description: Azure subscription containing the Key Vault
+    required: false
+  key-vault-name:
+    description: Azure Key Vault name
+    required: false
+  key-name:
+    description: Key Vault key used to sign the GitHub App JWT
+    required: false
+  github-app-client-id:
+    description: GitHub App client ID
+    required: false
+  github-app-installation-id:
+    description: GitHub App installation ID
+    required: false
+  repository:
+    description: Repository to include in the installation token
+    required: false
+  permission-profile:
+    description: Permission profile (issues, pull-requests, or devflow)
+    required: true
+  fallback-token:
+    description: PAT used temporarily when app authentication is unavailable
+    required: false
+
+outputs:
+  token:
+    description: GitHub App installation token or fallback PAT
+    value: ${{ steps.select-token.outputs.token }}
+  source:
+    description: Selected authentication source
+    value: ${{ steps.select-token.outputs.source }}
+
+runs:
+  using: composite
+  steps:
+    - name: Validate authentication mode
+      shell: bash
+      env:
+        AUTH_MODE: ${{ inputs.mode || 'app-with-fallback' }}
+      run: |
+        if [[ "$AUTH_MODE" != "app" && "$AUTH_MODE" != "app-with-fallback" && "$AUTH_MODE" != "pat" ]]; then
+          echo "::error::Unsupported GitHub authentication mode."
+          exit 1
+        fi
+
+    - name: Sign in to Azure
+      id: azure-login
+      if: ${{ (inputs.mode || 'app-with-fallback') != 'pat' }}
+      continue-on-error: true
+      uses: azure/login@7ddb5af1ef8758cf1353cf3b42f940aee27ba21c # v3.0.2
+      with:
+        client-id: ${{ inputs.azure-client-id }}
+        tenant-id: ${{ inputs.azure-tenant-id }}
+        subscription-id: ${{ inputs.azure-subscription-id }}
+
+    - name: Create GitHub App installation token
+      id: app-token
+      if: ${{ (inputs.mode || 'app-with-fallback') != 'pat' && steps.azure-login.outcome == 'success' }}
+      continue-on-error: true
+      shell: bash
+      env:
+        AZURE_SUBSCRIPTION_ID: ${{ inputs.azure-subscription-id }}
+        KEY_VAULT_NAME: ${{ inputs.key-vault-name }}
+        KEY_NAME: ${{ inputs.key-name }}
+        GITHUB_APP_CLIENT_ID: ${{ inputs.github-app-client-id }}
+        GITHUB_APP_INSTALLATION_ID: ${{ inputs.github-app-installation-id }}
+        TARGET_REPOSITORY: ${{ inputs.repository }}
+        PERMISSION_PROFILE: ${{ inputs.permission-profile }}
+      run: |
+        token="$(node "$GITHUB_ACTION_PATH/create-token.js")"
+        echo "::add-mask::$token"
+        echo "token=$token" >> "$GITHUB_OUTPUT"
+
+    - name: Select authentication token
+      id: select-token
+      shell: bash
+      env:
+        AUTH_MODE: ${{ inputs.mode || 'app-with-fallback' }}
+        APP_TOKEN: ${{ steps.app-token.outputs.token }}
+        FALLBACK_TOKEN: ${{ inputs.fallback-token }}
+      run: |
+        if [[ "$AUTH_MODE" != "pat" && -n "$APP_TOKEN" ]]; then
+          token="$APP_TOKEN"
+          source="app"
+          echo "::notice::GitHub authentication source: app"
+        elif [[ "$AUTH_MODE" == "app-with-fallback" && -n "$FALLBACK_TOKEN" ]]; then
+          token="$FALLBACK_TOKEN"
+          source="pat-fallback"
+          echo "::warning::GitHub authentication source: PAT fallback"
+        elif [[ "$AUTH_MODE" == "pat" && -n "$FALLBACK_TOKEN" ]]; then
+          token="$FALLBACK_TOKEN"
+          source="pat-forced"
+          echo "::warning::GitHub authentication source: PAT (forced rollout mode)"
+        else
+          echo "::error::GitHub App authentication is unavailable and no fallback PAT was provided."
+          exit 1
+        fi
+
+        echo "::add-mask::$token"
+        echo "token=$token" >> "$GITHUB_OUTPUT"
+        echo "source=$source" >> "$GITHUB_OUTPUT"

.github/actions/github-app-token/create-token.js

@@ -0,0 +1,149 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+const crypto = require('node:crypto');
+const { execFileSync } = require('node:child_process');
+
+const PERMISSION_PROFILES = Object.freeze({
+  issues: Object.freeze({
+    issues: 'write',
+  }),
+  'pull-requests': Object.freeze({
+    contents: 'read',
+    pull_requests: 'write',
+  }),
+  devflow: Object.freeze({
+    contents: 'read',
+    issues: 'write',
+    pull_requests: 'write',
+  }),
+});
+
+function base64Url(value) {
+  return Buffer.from(value).toString('base64url');
+}
+
+function base64ToBase64Url(value) {
+  return Buffer.from(value, 'base64').toString('base64url');
+}
+
+function createJwtSigningInput(clientId, nowSeconds) {
+  const header = base64Url(JSON.stringify({ alg: 'RS256', typ: 'JWT' }));
+  const payload = base64Url(JSON.stringify({
+    iat: nowSeconds - 60,
+    exp: nowSeconds + 540,
+    iss: clientId,
+  }));
+  return `${header}.${payload}`;
+}
+
+function signJwt(signingInput, config, execute = execFileSync) {
+  const digest = crypto.createHash('sha256').update(signingInput).digest('base64');
+  const signature = execute(
+    'az',
+    [
+      'keyvault', 'key', 'sign',
+      '--subscription', config.azureSubscriptionId,
+      '--vault-name', config.keyVaultName,
+      '--name', config.keyName,
+      '--algorithm', 'RS256',
+      '--digest', digest,
+      '--query', 'signature',
+      '--output', 'tsv',
+      '--only-show-errors',
+    ],
+    { encoding: 'utf8' },
+  ).trim();
+
+  if (!signature) {
+    throw new Error('Key Vault returned an empty signature.');
+  }
+
+  return `${signingInput}.${base64ToBase64Url(signature)}`;
+}
+
+async function createInstallationToken(config, dependencies = {}) {
+  const execute = dependencies.execute ?? execFileSync;
+  const request = dependencies.fetch ?? fetch;
+  const nowSeconds = dependencies.nowSeconds ?? Math.floor(Date.now() / 1000);
+  const repositoryParts = config.targetRepository.split('/');
+
+  if (repositoryParts.length !== 2 || repositoryParts.some((part) => part.length === 0)) {
+    throw new Error('TARGET_REPOSITORY must use the owner/repository format.');
+  }
+  if (!Object.hasOwn(PERMISSION_PROFILES, config.permissionProfile)) {
+    throw new Error('PERMISSION_PROFILE must be issues, pull-requests, or devflow.');
+  }
+  const permissions = PERMISSION_PROFILES[config.permissionProfile];
+
+  const [, repository] = repositoryParts;
+  const signingInput = createJwtSigningInput(config.githubAppClientId, nowSeconds);
+  const jwt = signJwt(signingInput, config, execute);
+
+  const response = await request(
+    `https://api.github.com/app/installations/${config.githubAppInstallationId}/access_tokens`,
+    {
+      method: 'POST',
+      headers: {
+        Accept: 'application/vnd.github+json',
+        Authorization: `Bearer ${jwt}`,
+        'X-GitHub-Api-Version': '2022-11-28',
+      },
+      body: JSON.stringify({
+        repositories: [repository],
+        permissions,
+      }),
+    },
+  );
+
+  if (!response.ok) {
+    throw new Error(`GitHub installation token request failed with HTTP ${response.status}.`);
+  }
+
+  const result = await response.json();
+  if (typeof result.token !== 'string' || result.token.length === 0) {
+    throw new Error('GitHub returned an empty installation token.');
+  }
+
+  return result.token;
+}
+
+function readConfig(environment) {
+  const config = {
+    azureSubscriptionId: environment.AZURE_SUBSCRIPTION_ID,
+    keyVaultName: environment.KEY_VAULT_NAME,
+    keyName: environment.KEY_NAME,
+    githubAppClientId: environment.GITHUB_APP_CLIENT_ID,
+    githubAppInstallationId: environment.GITHUB_APP_INSTALLATION_ID,
+    targetRepository: environment.TARGET_REPOSITORY,
+    permissionProfile: environment.PERMISSION_PROFILE,
+  };
+
+  if (Object.values(config).some((value) => !value)) {
+    throw new Error('Required GitHub App authentication configuration is missing.');
+  }
+
+  return config;
+}
+
+async function main() {
+  try {
+    const token = await createInstallationToken(readConfig(process.env));
+    process.stdout.write(token);
+  } catch {
+    console.error('GitHub App token generation failed.');
+    process.exitCode = 1;
+  }
+}
+
+if (require.main === module) {
+  void main();
+}
+
+module.exports = {
+  base64ToBase64Url,
+  createInstallationToken,
+  createJwtSigningInput,
+  readConfig,
+  signJwt,
+  PERMISSION_PROFILES,
+};