properly resolve app IDs

Signed-off-by: Fredrik Adelöw <freben@gmail.com>

.changeset/large-experts-sort.md

@@ -1,5 +1,7 @@
 ---
-'@backstage/plugin-events-backend-module-github': patch
+'@backstage/plugin-events-backend-module-github': minor
 ---
 
-Support webhook validation based on `integrations.github.[].apps.[].webhookSecret` too
+**BREAKING**: Removed the `createGithubSignatureValidator` export.
+
+Added support webhook validation based on `integrations.github.[].apps.[].webhookSecret`.

plugins/events-backend-module-github/package.json

@@ -47,7 +47,11 @@
     "@backstage/config": "workspace:^",
     "@backstage/integration": "workspace:^",
     "@backstage/plugin-events-node": "workspace:^",
-    "@octokit/webhooks-methods": "^3.0.0"
+    "@backstage/types": "workspace:^",
+    "@octokit/auth-callback": "^5.0.0",
+    "@octokit/webhooks-methods": "^3.0.0",
+    "lodash": "^4.17.21",
+    "octokit": "^3.0.0"
   },
   "devDependencies": {
     "@backstage/backend-test-utils": "workspace:^",

plugins/events-backend-module-github/report.api.md

@@ -4,17 +4,10 @@
 
 ```ts
 import { BackendFeature } from '@backstage/backend-plugin-api';
-import { Config } from '@backstage/config';
 import { EventParams } from '@backstage/plugin-events-node';
 import { EventsService } from '@backstage/plugin-events-node';
-import { RequestValidator } from '@backstage/plugin-events-node';
 import { SubTopicEventRouter } from '@backstage/plugin-events-node';
 
-// @public
-export function createGithubSignatureValidator(
-  config: Config,
-): RequestValidator | undefined;
-
 // @public (undocumented)
 const _default: BackendFeature;
 export default _default;

plugins/events-backend-module-github/src/http/createGithubSignatureValidator.test.ts

@@ -22,6 +22,7 @@ import {
 } from '@backstage/plugin-events-node';
 import { sign } from '@octokit/webhooks-methods';
 import { createGithubSignatureValidator } from './createGithubSignatureValidator';
+import { OctokitProviderService } from '../util/octokitProviderService';
 
 class TestContext implements RequestValidationContext {
   #details?: Partial<RequestRejectionDetails>;
@@ -35,6 +36,10 @@ class TestContext implements RequestValidationContext {
   }
 }
 
+const octokitProvider = {
+  getOctokit: jest.fn(),
+} satisfies OctokitProviderService;
+
 describe('createGithubSignatureValidator', () => {
   const secret = 'valid-secret';
   const configWithoutSecret = new ConfigReader({});
@@ -54,7 +59,7 @@ describe('createGithubSignatureValidator', () => {
           host: 'github.com',
           apps: [
             {
-              appId: 1,
+              appId: 7,
               privateKey: 'a',
               clientId: 'b',
               clientSecret: 'c',
@@ -65,8 +70,13 @@ describe('createGithubSignatureValidator', () => {
       ],
     },
   });
-  const payloadString = '{"test": "payload", "score": 5.0}';
-  const payload = JSON.parse(payloadString);
+  const payload = {
+    test: 'payload',
+    score: 5.0,
+    repository: { html_url: 'https://github.com/backstage/backstage' },
+    installation: { id: 70 },
+  };
+  const payloadString = JSON.stringify(payload);
   const payloadBuffer = Buffer.from(payloadString);
   const validSignature = sign({ secret, algorithm: 'sha256' }, payloadString);
 
@@ -84,16 +94,19 @@ describe('createGithubSignatureValidator', () => {
   };
 
   it('should return undefined if no secret is configured', async () => {
-    expect(createGithubSignatureValidator(configWithoutSecret)).toEqual(
-      undefined,
-    );
+    expect(
+      createGithubSignatureValidator(configWithoutSecret, octokitProvider),
+    ).toEqual(undefined);
   });
 
   it('secret configured, reject request without signature', async () => {
     const request = await requestWithSignature(undefined);
     const context = new TestContext();
 
-    const validator = createGithubSignatureValidator(configWithSecret);
+    const validator = createGithubSignatureValidator(
+      configWithSecret,
+      octokitProvider,
+    );
     await validator!(request, context);
 
     expect(context.details).not.toBeUndefined();
@@ -105,7 +118,10 @@ describe('createGithubSignatureValidator', () => {
     const request = await requestWithSignature('invalid signature');
     const context = new TestContext();
 
-    const validator = createGithubSignatureValidator(configWithSecret);
+    const validator = createGithubSignatureValidator(
+      configWithSecret,
+      octokitProvider,
+    );
     await validator!(request, context);
 
     expect(context.details).not.toBeUndefined();
@@ -117,7 +133,10 @@ describe('createGithubSignatureValidator', () => {
     const request = await requestWithSignature(await validSignature);
     const context = new TestContext();
 
-    const validator = createGithubSignatureValidator(configWithSecret);
+    const validator = createGithubSignatureValidator(
+      configWithSecret,
+      octokitProvider,
+    );
     await validator!(request, context);
 
     expect(context.details).toBeUndefined();
@@ -126,8 +145,20 @@ describe('createGithubSignatureValidator', () => {
   it('secret configured, accept request with valid signature defined in integrations', async () => {
     const request = await requestWithSignature(await validSignature);
     const context = new TestContext();
+    octokitProvider.getOctokit.mockResolvedValue({
+      rest: {
+        apps: {
+          getInstallation: async () => ({
+            data: { app_id: 7 },
+          }),
+        },
+      },
+    });
 
-    const validator = createGithubSignatureValidator(configWithAppSecret);
+    const validator = createGithubSignatureValidator(
+      configWithAppSecret,
+      octokitProvider,
+    );
     await validator!(request, context);
 
     expect(context.details).toBeUndefined();

plugins/events-backend-module-github/src/http/createGithubSignatureValidator.ts

@@ -22,6 +22,8 @@ import {
   RequestValidator,
 } from '@backstage/plugin-events-node';
 import { verify } from '@octokit/webhooks-methods';
+import { createAppIdResolver } from '../util/createAppIdResolver';
+import { OctokitProviderService } from '../util/octokitProviderService';
 
 /**
  * Validates that the request received is the expected GitHub request
@@ -36,29 +38,31 @@ import { verify } from '@octokit/webhooks-methods';
  */
 export function createGithubSignatureValidator(
   config: Config,
+  octokitProvider: OctokitProviderService,
 ): RequestValidator | undefined {
-  const webhookSecrets = new Set<string>();
-
   const integrations = ScmIntegrations.fromConfig(config);
+
+  // GitHub App installation ID to secret
+  const githubAppSecrets = new Map<number, string>();
   for (const integration of integrations.github.list()) {
-    for (const app of integration.config.apps ?? []) {
-      if (app.webhookSecret) {
-        webhookSecrets.add(app.webhookSecret);
+    for (const { appId, webhookSecret } of integration.config.apps ?? []) {
+      if (appId && webhookSecret) {
+        githubAppSecrets.set(appId, webhookSecret);
       }
     }
   }
 
-  const moduleSecret = config.getOptionalString(
+  // A single optional secret for all GitHub events
+  const genericSecret = config.getOptionalString(
     'events.modules.github.webhookSecret',
   );
-  if (moduleSecret) {
-    webhookSecrets.add(moduleSecret);
-  }
 
-  if (webhookSecrets.size === 0) {
+  if (!genericSecret && githubAppSecrets.size === 0) {
     return undefined;
   }
 
+  const appIdResolver = createAppIdResolver(octokitProvider);
+
   return async (
     request: RequestDetails,
     context: RequestValidationContext,
@@ -69,9 +73,19 @@ export function createGithubSignatureValidator(
 
     if (signature) {
       const body = request.raw.body.toString(request.raw.encoding);
-      for (const secret of webhookSecrets) {
-        if (await verify(secret, body, signature)) {
-          return; // OK
+
+      if (githubAppSecrets.size) {
+        const appId = await appIdResolver(request);
+        if (appId && githubAppSecrets.has(appId)) {
+          if (await verify(githubAppSecrets.get(appId)!, body, signature)) {
+            return;
+          }
+        }
+      }
+
+      if (genericSecret) {
+        if (await verify(genericSecret, body, signature)) {
+          return;
         }
       }
     }

plugins/events-backend-module-github/src/index.ts

@@ -20,6 +20,7 @@
  *
  * @packageDocumentation
  */
+
 import { createBackendFeatureLoader } from '@backstage/backend-plugin-api';
 
 export default createBackendFeatureLoader({
@@ -31,5 +32,8 @@ export default createBackendFeatureLoader({
   },
 });
 
-export { createGithubSignatureValidator } from './http/createGithubSignatureValidator';
+// TODO(freben): This is not exported at the moment since it depends on the octokit provider.
+// Until we have made that a core thing in integrations, we can't export it
+// export { createGithubSignatureValidator } from './http/createGithubSignatureValidator';
+
 export { GithubEventRouter } from './router/GithubEventRouter';

plugins/events-backend-module-github/src/service/eventsModuleGithubWebhook.ts

@@ -20,6 +20,7 @@ import {
 } from '@backstage/backend-plugin-api';
 import { eventsExtensionPoint } from '@backstage/plugin-events-node/alpha';
 import { createGithubSignatureValidator } from '../http/createGithubSignatureValidator';
+import { octokitProviderServiceRef } from '../util/octokitProviderService';
 
 /**
  * Module for the events-backend plugin,
@@ -36,9 +37,13 @@ export default createBackendModule({
       deps: {
         config: coreServices.rootConfig,
         events: eventsExtensionPoint,
+        octokitProvider: octokitProviderServiceRef,
       },
-      async init({ config, events }) {
-        const validator = createGithubSignatureValidator(config);
+      async init({ config, events, octokitProvider }) {
+        const validator = createGithubSignatureValidator(
+          config,
+          octokitProvider,
+        );
         if (validator) {
           events.addHttpPostIngress({
             topic: 'github',

plugins/events-backend-module-github/src/util/createAppIdResolver.ts

@@ -0,0 +1,66 @@
+/*
+ * Copyright 2025 The Backstage Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { RequestDetails } from '@backstage/plugin-events-node';
+import { OctokitProviderService } from './octokitProviderService';
+import lodash from 'lodash';
+
+export type AppIdResolver = (
+  request: RequestDetails,
+) => Promise<number | undefined>;
+
+/**
+ * Helps with resolving what app ID (if any) that sent a webhook event.
+ */
+export function createAppIdResolver(
+  octokitProvider: OctokitProviderService,
+): AppIdResolver {
+  const installationIdToAppId = new Map<number, Promise<number | undefined>>();
+
+  return async (request: RequestDetails) => {
+    const installationId = lodash.get(
+      request.body,
+      'installation.id',
+    ) as unknown;
+
+    if (!installationId || typeof installationId !== 'number') {
+      return undefined;
+    }
+
+    let appIdPromsie = installationIdToAppId.get(installationId);
+    if (appIdPromsie) {
+      return await appIdPromsie;
+    }
+
+    const repositoryUrl = lodash.get(
+      request.body,
+      'repository.html_url',
+    ) as unknown;
+
+    if (!repositoryUrl || typeof repositoryUrl !== 'string') {
+      return undefined;
+    }
+
+    const octokit = await octokitProvider.getOctokit(repositoryUrl);
+    appIdPromsie = octokit.rest.apps
+      .getInstallation({ installation_id: installationId })
+      .then(response => Number(response.data.app_id))
+      .catch(() => undefined);
+
+    installationIdToAppId.set(installationId, appIdPromsie);
+    return await appIdPromsie;
+  };
+}

plugins/events-backend-module-github/src/util/octokitProviderService.ts

@@ -0,0 +1,104 @@
+/*
+ * Copyright 2025 The Backstage Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import {
+  coreServices,
+  createServiceFactory,
+  createServiceRef,
+  RootConfigService,
+} from '@backstage/backend-plugin-api';
+import {
+  DefaultGithubCredentialsProvider,
+  GithubCredentialsProvider,
+  ScmIntegrationRegistry,
+  ScmIntegrations,
+} from '@backstage/integration';
+import { durationToMilliseconds, HumanDuration } from '@backstage/types';
+import { Octokit } from 'octokit';
+
+export interface OctokitProviderService {
+  getOctokit: (url: string) => Promise<Octokit>;
+}
+
+class OctokitProviderImpl implements OctokitProviderService {
+  readonly #integrations: ScmIntegrationRegistry;
+  readonly #githubCredentials: GithubCredentialsProvider;
+  readonly #octokitCache: Map<string, Octokit>;
+  readonly #octokitCacheTtl: HumanDuration;
+
+  constructor(config: RootConfigService) {
+    this.#integrations = ScmIntegrations.fromConfig(config);
+    this.#githubCredentials = DefaultGithubCredentialsProvider.fromIntegrations(
+      this.#integrations,
+    );
+    this.#octokitCache = new Map();
+    this.#octokitCacheTtl = { hours: 1 };
+  }
+
+  async getOctokit(url: string): Promise<Octokit> {
+    // TODO(freben): Be smart and cache these more granularly, e.g. by
+    // organization or even repo.
+    const integration = this.#integrations.github.byUrl(url);
+    if (!integration) {
+      throw new Error(`No integration found for url: ${url}`);
+    }
+    const key = integration.config.host;
+
+    if (this.#octokitCache.has(key)) {
+      return this.#octokitCache.get(key)!;
+    }
+
+    const { createCallbackAuth } = await import('@octokit/auth-callback');
+
+    const octokit = new Octokit({
+      baseUrl: integration.config.apiBaseUrl,
+      authStrategy: createCallbackAuth,
+      auth: {
+        callback: async () => {
+          try {
+            const credentials = await this.#githubCredentials.getCredentials({
+              url,
+            });
+            return credentials.token;
+          } catch {
+            return undefined;
+          }
+        },
+      },
+    });
+
+    this.#octokitCache.set(key, octokit);
+    setTimeout(() => {
+      this.#octokitCache.delete(key);
+    }, durationToMilliseconds(this.#octokitCacheTtl));
+
+    return octokit;
+  }
+}
+
+export const octokitProviderServiceRef =
+  createServiceRef<OctokitProviderService>({
+    id: 'octokitProvider',
+    scope: 'root',
+    defaultFactory: async service =>
+      createServiceFactory({
+        service,
+        deps: { config: coreServices.rootConfig },
+        async factory({ config }) {
+          return new OctokitProviderImpl(config);
+        },
+      }),
+  });

yarn.lock

@@ -6579,7 +6579,11 @@ __metadata:
     "@backstage/integration": "workspace:^"
     "@backstage/plugin-events-backend-test-utils": "workspace:^"
     "@backstage/plugin-events-node": "workspace:^"
+    "@backstage/types": "workspace:^"
+    "@octokit/auth-callback": "npm:^5.0.0"
     "@octokit/webhooks-methods": "npm:^3.0.0"
+    lodash: "npm:^4.17.21"
+    octokit: "npm:^3.0.0"
   languageName: unknown
   linkType: soft
 
@@ -12850,6 +12854,13 @@ __metadata:
   languageName: node
   linkType: hard
 
+"@octokit/auth-callback@npm:^5.0.0":
+  version: 5.0.1
+  resolution: "@octokit/auth-callback@npm:5.0.1"
+  checksum: 10/afa2fd71e1cc238c4fc09a1a8cc3b5c8d2f231aecdb8f8be2384857cde893aa4ac697f7c6aa0b61e4d5fa1f00d41fb349ceffb938977c02ee317d5432247f6ef
+  languageName: node
+  linkType: hard
+
 "@octokit/auth-oauth-app@npm:^5.0.0":
   version: 5.0.1
   resolution: "@octokit/auth-oauth-app@npm:5.0.1"