diff --git a/src/cmap/auth/mongo_credentials.ts b/src/cmap/auth/mongo_credentials.ts
index b11f6fa4b5..4e2ab95ea4 100644
--- a/src/cmap/auth/mongo_credentials.ts
+++ b/src/cmap/auth/mongo_credentials.ts
@@ -2,7 +2,7 @@
 
 import type { Document } from '../../bson';
 import { MongoAPIError, MongoMissingCredentialsError } from '../../error';
-import { AuthMechanism } from './providers';
+import { AUTH_MECHS_AUTH_SRC_EXTERNAL, AuthMechanism } from './providers';
 
 // https://github.com/mongodb/specifications/blob/master/source/auth/auth.rst
 function getDefaultAuthMechanism(ismaster?: Document): AuthMechanism {
@@ -136,11 +136,7 @@ export class MongoCredentials {
       throw new MongoMissingCredentialsError(`Username required for mechanism '${this.mechanism}'`);
     }
 
-    if (
-      this.mechanism === AuthMechanism.MONGODB_GSSAPI ||
-      this.mechanism === AuthMechanism.MONGODB_AWS ||
-      this.mechanism === AuthMechanism.MONGODB_X509
-    ) {
+    if (AUTH_MECHS_AUTH_SRC_EXTERNAL.has(this.mechanism)) {
       if (this.source != null && this.source !== '$external') {
         // TODO(NODE-3485): Replace this with a MongoAuthValidationError
         throw new MongoAPIError(
diff --git a/src/cmap/auth/providers.ts b/src/cmap/auth/providers.ts
index 07436baff7..9c7b1f4b82 100644
--- a/src/cmap/auth/providers.ts
+++ b/src/cmap/auth/providers.ts
@@ -12,3 +12,10 @@ export const AuthMechanism = Object.freeze({
 
 /** @public */
 export type AuthMechanism = typeof AuthMechanism[keyof typeof AuthMechanism];
+
+/** @internal */
+export const AUTH_MECHS_AUTH_SRC_EXTERNAL = new Set<AuthMechanism>([
+  AuthMechanism.MONGODB_GSSAPI,
+  AuthMechanism.MONGODB_AWS,
+  AuthMechanism.MONGODB_X509
+]);
diff --git a/src/connection_string.ts b/src/connection_string.ts
index a2f3764364..77f412b71c 100644
--- a/src/connection_string.ts
+++ b/src/connection_string.ts
@@ -5,7 +5,7 @@ import { URLSearchParams } from 'url';
 
 import type { Document } from './bson';
 import { MongoCredentials } from './cmap/auth/mongo_credentials';
-import { AuthMechanism } from './cmap/auth/providers';
+import { AUTH_MECHS_AUTH_SRC_EXTERNAL, AuthMechanism } from './cmap/auth/providers';
 import { Compressor, CompressorName } from './cmap/wire_protocol/compression';
 import { Encrypter } from './encrypter';
 import { MongoAPIError, MongoInvalidArgumentError, MongoParseError } from './error';
@@ -125,7 +125,12 @@ export function resolveSRVRecord(options: MongoOptions, callback: Callback<HostA
         const replicaSet = txtRecordOptions.get('replicaSet') ?? undefined;
         const loadBalanced = txtRecordOptions.get('loadBalanced') ?? undefined;
 
-        if (!options.userSpecifiedAuthSource && source) {
+        if (
+          !options.userSpecifiedAuthSource &&
+          source &&
+          options.credentials &&
+          !AUTH_MECHS_AUTH_SRC_EXTERNAL.has(options.credentials.mechanism)
+        ) {
           options.credentials = MongoCredentials.merge(options.credentials, { source });
         }
 
@@ -570,9 +575,7 @@ export const OPTIONS = {
       let source = options.credentials?.source;
       if (
         mechanism === AuthMechanism.MONGODB_PLAIN ||
-        mechanism === AuthMechanism.MONGODB_GSSAPI ||
-        mechanism === AuthMechanism.MONGODB_AWS ||
-        mechanism === AuthMechanism.MONGODB_X509
+        AUTH_MECHS_AUTH_SRC_EXTERNAL.has(mechanism)
       ) {
         // some mechanisms have '$external' as the Auth Source
         source = '$external';
diff --git a/test/unit/connection_string.test.js b/test/unit/connection_string.test.ts
similarity index 69%
rename from test/unit/connection_string.test.js
rename to test/unit/connection_string.test.ts
index d4bda2cfb2..a5152b526b 100644
--- a/test/unit/connection_string.test.js
+++ b/test/unit/connection_string.test.ts
@@ -1,10 +1,14 @@
-'use strict';
-
-const { MongoParseError, MongoDriverError, MongoInvalidArgumentError } = require('../../src/error');
-const { loadSpecTests } = require('../spec');
-const { parseOptions } = require('../../src/connection_string');
-const { AuthMechanism } = require('../../src/cmap/auth/providers');
-const { expect } = require('chai');
+import { expect } from 'chai';
+import * as dns from 'dns';
+import * as sinon from 'sinon';
+import { promisify } from 'util';
+
+import { MongoCredentials } from '../../src/cmap/auth/mongo_credentials';
+import { AUTH_MECHS_AUTH_SRC_EXTERNAL, AuthMechanism } from '../../src/cmap/auth/providers';
+import { parseOptions, resolveSRVRecord } from '../../src/connection_string';
+import { MongoDriverError, MongoInvalidArgumentError, MongoParseError } from '../../src/error';
+import { MongoOptions } from '../../src/mongo_client';
+import { loadSpecTests } from '../spec';
 
 // NOTE: These are cases we could never check for unless we write our own
 //       url parser. The node parser simply won't let these through, so we
@@ -30,7 +34,9 @@ describe('Connection String', function () {
       auth: { user: 'testing', password: 'llamas' }
     };
 
-    expect(() => parseOptions('mongodb://localhost', optionsWithUser)).to.throw(MongoParseError);
+    expect(() => parseOptions('mongodb://localhost', optionsWithUser as any)).to.throw(
+      MongoParseError
+    );
   });
 
   it('should support auth passed with username', function () {
@@ -38,7 +44,7 @@ describe('Connection String', function () {
       authMechanism: 'SCRAM-SHA-1',
       auth: { username: 'testing', password: 'llamas' }
     };
-    const options = parseOptions('mongodb://localhost', optionsWithUsername);
+    const options = parseOptions('mongodb://localhost', optionsWithUsername as any);
     expect(options.credentials).to.containSubset({
       source: 'admin',
       username: 'testing',
@@ -94,6 +100,13 @@ describe('Connection String', function () {
     expect(options.credentials.mechanism).to.equal(AuthMechanism.MONGODB_GSSAPI);
   });
 
+  it('should provide default authSource when valid AuthMechanism provided', function () {
+    const options = parseOptions(
+      'mongodb+srv://jira-sync.pw0q4.mongodb.net/testDB?authMechanism=MONGODB-AWS&retryWrites=true&w=majority'
+    );
+    expect(options.credentials.source).to.equal('$external');
+  });
+
   it('should parse a numeric authSource with variable width', function () {
     const options = parseOptions('mongodb://test@localhost/?authSource=0001');
     expect(options.credentials.source).to.equal('0001');
@@ -127,7 +140,7 @@ describe('Connection String', function () {
           });
 
           it('does not set the ssl option', function () {
-            expect(options.ssl).to.be.undefined;
+            expect(options).to.not.have.property('ssl');
           });
         });
 
@@ -139,7 +152,7 @@ describe('Connection String', function () {
           });
 
           it('does not set the ssl option', function () {
-            expect(options.ssl).to.be.undefined;
+            expect(options).to.not.have.property('ssl');
           });
         });
       });
@@ -163,7 +176,7 @@ describe('Connection String', function () {
           });
 
           it('does not set the ssl option', function () {
-            expect(options.ssl).to.be.undefined;
+            expect(options).to.not.have.property('ssl');
           });
         });
 
@@ -176,7 +189,7 @@ describe('Connection String', function () {
             });
 
             it('does not set the ssl option', function () {
-              expect(options.ssl).to.be.undefined;
+              expect(options).to.not.have.property('ssl');
             });
           });
 
@@ -188,7 +201,7 @@ describe('Connection String', function () {
             });
 
             it('does not set the ssl option', function () {
-              expect(options.ssl).to.be.undefined;
+              expect(options).to.not.have.property('ssl');
             });
           });
         });
@@ -312,4 +325,110 @@ describe('Connection String', function () {
       expect(options.srvHost).to.equal('test1.test.build.10gen.cc');
     });
   });
+
+  describe('resolveSRVRecord()', () => {
+    const resolveSRVRecordAsync = promisify(resolveSRVRecord);
+
+    afterEach(async () => {
+      sinon.restore();
+    });
+
+    function makeStub(txtRecord: string) {
+      const mockAddress = [
+        {
+          name: 'localhost.test.mock.test.build.10gen.cc',
+          port: 2017,
+          weight: 0,
+          priority: 0
+        }
+      ];
+
+      const mockRecord: string[][] = [[txtRecord]];
+
+      // first call is for stubbing resolveSrv
+      // second call is for stubbing resolveTxt
+      sinon.stub(dns, 'resolveSrv').callsFake((address, callback) => {
+        return process.nextTick(callback, null, mockAddress);
+      });
+
+      sinon.stub(dns, 'resolveTxt').callsFake((address, whatWeTest) => {
+        whatWeTest(null, mockRecord);
+      });
+    }
+
+    for (const mechanism of AUTH_MECHS_AUTH_SRC_EXTERNAL) {
+      it(`should set authSource to $external for ${mechanism} external mechanism`, async function () {
+        makeStub('authSource=thisShouldNotBeAuthSource');
+        const credentials = new MongoCredentials({
+          source: '$external',
+          mechanism,
+          username: 'username',
+          password: mechanism === AuthMechanism.MONGODB_X509 ? undefined : 'password',
+          mechanismProperties: {}
+        });
+        credentials.validate();
+
+        const options = {
+          credentials,
+          srvHost: 'test.mock.test.build.10gen.cc',
+          srvServiceName: 'mongodb',
+          userSpecifiedAuthSource: false
+        } as MongoOptions;
+
+        await resolveSRVRecordAsync(options);
+        // check MongoCredentials instance (i.e. whether or not merge on options.credentials was called)
+        expect(options).property('credentials').to.equal(credentials);
+        expect(options).to.have.nested.property('credentials.source', '$external');
+      });
+    }
+
+    it('should set a default authSource for non-external mechanisms with no user-specified source', async function () {
+      makeStub('authSource=thisShouldBeAuthSource');
+
+      const credentials = new MongoCredentials({
+        source: 'admin',
+        mechanism: AuthMechanism.MONGODB_SCRAM_SHA256,
+        username: 'username',
+        password: 'password',
+        mechanismProperties: {}
+      });
+      credentials.validate();
+
+      const options = {
+        credentials,
+        srvHost: 'test.mock.test.build.10gen.cc',
+        srvServiceName: 'mongodb',
+        userSpecifiedAuthSource: false
+      } as MongoOptions;
+
+      await resolveSRVRecordAsync(options);
+      // check MongoCredentials instance (i.e. whether or not merge on options.credentials was called)
+      expect(options).property('credentials').to.not.equal(credentials);
+      expect(options).to.have.nested.property('credentials.source', 'thisShouldBeAuthSource');
+    });
+
+    it('should retain credentials for any mechanism with no user-sepcificed source and no source in DNS', async function () {
+      makeStub('');
+      const credentials = new MongoCredentials({
+        source: 'admin',
+        mechanism: AuthMechanism.MONGODB_SCRAM_SHA256,
+        username: 'username',
+        password: 'password',
+        mechanismProperties: {}
+      });
+      credentials.validate();
+
+      const options = {
+        credentials,
+        srvHost: 'test.mock.test.build.10gen.cc',
+        srvServiceName: 'mongodb',
+        userSpecifiedAuthSource: false
+      } as MongoOptions;
+
+      await resolveSRVRecordAsync(options as any);
+      // check MongoCredentials instance (i.e. whether or not merge on options.credentials was called)
+      expect(options).property('credentials').to.equal(credentials);
+      expect(options).to.have.nested.property('credentials.source', 'admin');
+    });
+  });
 });