Add flag to AwsLambdaReceiver to enable/disable signature verification (#2107)

* Add flag to `AwsLambdaReceiver` to enable/disable signature verification

* Address PR comments

* Add blurb to `signingSecret` JSDoc saying that the value of it doesn't matter if sig verification is disabled

Author: noah-guillory

src/receivers/AwsLambdaReceiver.spec.ts

@@ -542,6 +542,42 @@ describe('AwsLambdaReceiver', function () {
     );
     assert.equal(response.statusCode, 401);
   });
+
+  it('does not perform signature verification if signature verification flag is set to false', async () => {
+    const awsReceiver = new AwsLambdaReceiver({
+      signingSecret: '',
+      signatureVerification: false,
+      logger: noopLogger,
+    });
+    const handler = awsReceiver.toHandler();
+    const awsEvent = {
+      resource: '/slack/events',
+      path: '/slack/events',
+      httpMethod: 'POST',
+      headers: {
+        Accept: 'application/json,*/*',
+        'Content-Type': 'application/json',
+        Host: 'xxx.execute-api.ap-northeast-1.amazonaws.com',
+        'User-Agent': 'Slackbot 1.0 (+https://api.slack.com/robots)',
+        'X-Slack-Request-Timestamp': 'far back dude',
+        'X-Slack-Signature': 'very much invalid',
+      },
+      multiValueHeaders: {},
+      queryStringParameters: null,
+      multiValueQueryStringParameters: null,
+      pathParameters: null,
+      stageVariables: null,
+      requestContext: {},
+      body: urlVerificationBody,
+      isBase64Encoded: false,
+    };
+    const response = await handler(
+      awsEvent,
+      {},
+      (_error, _result) => {},
+    );
+    assert.equal(response.statusCode, 200);
+  });
 });
 
 // Composable overrides

src/receivers/AwsLambdaReceiver.ts

@@ -40,9 +40,41 @@ export interface AwsResponse {
 export type AwsHandler = (event: AwsEvent, context: any, callback: AwsCallback) => Promise<AwsResponse>;
 
 export interface AwsLambdaReceiverOptions {
+  /**
+   * The Slack Signing secret to be used as an input to signature verification to ensure that requests are coming from
+   * Slack.
+   *
+   * If the {@link signatureVerification} flag is set to `false`, this can be set to any value as signature verification
+   * using this secret will not be performed.
+   *
+   * @see {@link https://api.slack.com/authentication/verifying-requests-from-slack#about} for details about signing secrets
+   */
   signingSecret: string;
+  /**
+   * The {@link Logger} for the receiver
+   *
+   * @default ConsoleLogger
+   */
   logger?: Logger;
+  /**
+   * The {@link LogLevel} to be used for the logger.
+   *
+   * @default LogLevel.INFO
+   */
   logLevel?: LogLevel;
+  /**
+   * Flag that determines whether Bolt should {@link https://api.slack.com/authentication/verifying-requests-from-slack|verify Slack's signature on incoming requests}.
+   *
+   * @default true
+   */
+  signatureVerification?: boolean;
+  /**
+   * Optional `function` that can extract custom properties from an incoming receiver event
+   * @param request The API Gateway event {@link AwsEvent}
+   * @returns An object containing custom properties
+   *
+   * @default noop
+   */
   customPropertiesExtractor?: (request: AwsEvent) => StringIndexed;
 }
 
@@ -59,16 +91,20 @@ export default class AwsLambdaReceiver implements Receiver {
 
   private logger: Logger;
 
+  private signatureVerification: boolean;
+
   private customPropertiesExtractor: (request: AwsEvent) => StringIndexed;
 
   public constructor({
     signingSecret,
     logger = undefined,
     logLevel = LogLevel.INFO,
+    signatureVerification = true,
     customPropertiesExtractor = (_) => ({}),
   }: AwsLambdaReceiverOptions) {
     // Initialize instance variables, substituting defaults for each value
     this.signingSecret = signingSecret;
+    this.signatureVerification = signatureVerification;
     this.logger = logger ??
       (() => {
         const defaultLogger = new ConsoleLogger();
@@ -130,12 +166,14 @@ export default class AwsLambdaReceiver implements Receiver {
         return Promise.resolve({ statusCode: 200, body: '' });
       }
 
-      // request signature verification
-      const signature = this.getHeaderValue(awsEvent.headers, 'X-Slack-Signature') as string;
-      const ts = Number(this.getHeaderValue(awsEvent.headers, 'X-Slack-Request-Timestamp'));
-      if (!this.isValidRequestSignature(this.signingSecret, rawBody, signature, ts)) {
-        this.logger.info(`Invalid request signature detected (X-Slack-Signature: ${signature}, X-Slack-Request-Timestamp: ${ts})`);
-        return Promise.resolve({ statusCode: 401, body: '' });
+      if (this.signatureVerification) {
+        // request signature verification
+        const signature = this.getHeaderValue(awsEvent.headers, 'X-Slack-Signature') as string;
+        const ts = Number(this.getHeaderValue(awsEvent.headers, 'X-Slack-Request-Timestamp'));
+        if (!this.isValidRequestSignature(this.signingSecret, rawBody, signature, ts)) {
+          this.logger.info(`Invalid request signature detected (X-Slack-Signature: ${signature}, X-Slack-Request-Timestamp: ${ts})`);
+          return Promise.resolve({ statusCode: 401, body: '' });
+        }
       }
 
       // url_verification (Events API)