fix(NODE-5945): make AWS session token optional (#4006)

Co-authored-by: Neal Beeken <neal.beeken@mongodb.com>

src/cmap/auth/mongodb_aws.ts

@@ -116,6 +116,7 @@ export class MongoDBAWS extends AuthProvider {
 
     const accessKeyId = credentials.username;
     const secretAccessKey = credentials.password;
+    // Allow the user to specify an AWS session token for authentication with temporary credentials.
     const sessionToken = credentials.mechanismProperties.AWS_SESSION_TOKEN;
 
     // If all three defined, include sessionToken, else include username and pass, else no credentials
@@ -129,6 +130,8 @@ export class MongoDBAWS extends AuthProvider {
     const db = credentials.source;
     const nonce = await randomBytes(32);
 
+    // All messages between MongoDB clients and servers are sent as BSON objects
+    // in the payload field of saslStart and saslContinue.
     const saslStart = {
       saslStart: 1,
       mechanism: 'MONGODB-AWS',
@@ -212,7 +215,8 @@ async function makeTempCredentials(
   provider?: () => Promise<AWSCredentials>
 ): Promise<MongoCredentials> {
   function makeMongoCredentialsFromAWSTemp(creds: AWSTempCredentials) {
-    if (!creds.AccessKeyId || !creds.SecretAccessKey || !creds.Token) {
+    // The AWS session token (creds.Token) may or may not be set.
+    if (!creds.AccessKeyId || !creds.SecretAccessKey) {
       throw new MongoMissingCredentialsError('Could not obtain temporary MONGODB-AWS credentials');
     }
 

test/integration/auth/mongodb_aws.test.ts

@@ -5,7 +5,13 @@ import * as http from 'http';
 import { performance } from 'perf_hooks';
 import * as sinon from 'sinon';
 
-import { MongoAWSError, type MongoClient, MongoDBAWS, MongoServerError } from '../../mongodb';
+import {
+  MongoAWSError,
+  type MongoClient,
+  MongoDBAWS,
+  MongoMissingCredentialsError,
+  MongoServerError
+} from '../../mongodb';
 
 function awsSdk() {
   try {
@@ -81,6 +87,37 @@ describe('MONGODB-AWS', function () {
     expect(provider).to.be.instanceOf(MongoDBAWS);
   });
 
+  describe('with missing aws token', () => {
+    let awsSessionToken: string | undefined;
+
+    beforeEach(() => {
+      awsSessionToken = process.env.AWS_SESSION_TOKEN;
+      delete process.env.AWS_SESSION_TOKEN;
+    });
+
+    afterEach(() => {
+      if (awsSessionToken != null) {
+        process.env.AWS_SESSION_TOKEN = awsSessionToken;
+      }
+    });
+
+    it('should not throw an exception when aws token is missing', async function () {
+      client = this.configuration.newClient(process.env.MONGODB_URI);
+
+      const result = await client
+        .db('aws')
+        .collection('aws_test')
+        .estimatedDocumentCount()
+        .catch(error => error);
+
+      // We check only for the MongoMissingCredentialsError
+      // and do check for the MongoServerError as the error or numeric result
+      // that can be returned depending on different types of environments
+      // getting credentials from different sources.
+      expect(result).to.not.be.instanceOf(MongoMissingCredentialsError);
+    });
+  });
+
   describe('EC2 with missing credentials', () => {
     let client;