diff --git a/Foundation/NSURLSession/NSURLSessionTask.swift b/Foundation/NSURLSession/NSURLSessionTask.swift
index e44fb9da9f..7b6959af35 100644
--- a/Foundation/NSURLSession/NSURLSessionTask.swift
+++ b/Foundation/NSURLSession/NSURLSessionTask.swift
@@ -554,9 +554,31 @@ fileprivate extension URLSessionTask {
         
         // HTTP Options:
         easyHandle.set(followLocation: false)
+        
+        // The httpAdditionalHeaders from session configuration has to be added to the request.
+        // The request.allHTTPHeaders can override the httpAdditionalHeaders elements. Add the
+        // httpAdditionalHeaders from session configuration first and then append/update the
+        // request.allHTTPHeaders so that request.allHTTPHeaders can override httpAdditionalHeaders.
+        
+        let httpSession = session as! URLSession
+        var httpHeaders: [AnyHashable : Any]?
+        
+        if let hh = httpSession.configuration.httpAdditionalHeaders {
+            httpHeaders = hh
+        }
+        
+        if let hh = currentRequest?.allHTTPHeaderFields {
+            if httpHeaders == nil {
+                httpHeaders = hh
+            } else {
+                hh.forEach {
+                    httpHeaders![$0] = $1
+                }
+            }
+        }
 
         let customHeaders: [String]
-        let headersForRequest = curlHeaders(for: request)
+        let headersForRequest = curlHeaders(for: httpHeaders)
         if ((request.httpMethod == "POST") && (request.value(forHTTPHeaderField: "Content-Type") == nil)) {
             customHeaders = headersForRequest + ["Content-Type:application/x-www-form-urlencoded"]
         } else {
@@ -570,8 +592,7 @@ fileprivate extension URLSessionTask {
 
         //set the request timeout
         //TODO: the timeout value needs to be reset on every data transfer
-        let s = session as! URLSession
-        let timeoutInterval = Int(s.configuration.timeoutIntervalForRequest) * 1000
+        let timeoutInterval = Int(httpSession.configuration.timeoutIntervalForRequest) * 1000
         let timeoutHandler = DispatchWorkItem { [weak self] in
             guard let currentTask = self else { fatalError("Timeout on a task that doesn't exist") } //this guard must always pass
             currentTask.internalState = .transferFailed
@@ -597,10 +618,11 @@ fileprivate extension URLSessionTask {
     /// expects.
     ///
     /// - SeeAlso: https://curl.haxx.se/libcurl/c/CURLOPT_HTTPHEADER.html
-    func curlHeaders(for request: URLRequest) -> [String] {
+    func curlHeaders(for httpHeaders: [AnyHashable : Any]?) -> [String] {
         var result: [String] = []
         var names = Set<String>()
-        if let hh = currentRequest?.allHTTPHeaderFields {
+        if httpHeaders != nil {
+            let hh = httpHeaders as! [String:String]
             hh.forEach {
                 let name = $0.0.lowercased()
                 guard !names.contains(name) else { return }
diff --git a/TestFoundation/TestNSURLSession.swift b/TestFoundation/TestNSURLSession.swift
index 022c601577..2b5e6da134 100644
--- a/TestFoundation/TestNSURLSession.swift
+++ b/TestFoundation/TestNSURLSession.swift
@@ -35,6 +35,7 @@ class TestURLSession : XCTestCase {
             ("test_cancelTask", test_cancelTask),
             ("test_taskTimeout", test_taskTimeout),
             ("test_verifyRequestHeaders", test_verifyRequestHeaders),
+            ("test_verifyHttpAdditionalHeaders", test_verifyHttpAdditionalHeaders),
         ]
     }
 
@@ -349,6 +350,41 @@ class TestURLSession : XCTestCase {
         waitForExpectations(timeout: 30)
     }
 
+    // Verify httpAdditionalHeaders from session configuration are added to the request
+    // and whether it is overriden by Request.allHTTPHeaderFields.
+    
+    func test_verifyHttpAdditionalHeaders() {
+        let serverReady = ServerSemaphore()
+        globalDispatchQueue.async {
+            do {
+                try self.runServer(with: serverReady)
+            } catch {
+                XCTAssertTrue(true)
+                return
+            }
+        }
+        serverReady.wait()
+        let config = URLSessionConfiguration.default
+        config.timeoutIntervalForRequest = 5
+        config.httpAdditionalHeaders = ["header2": "svalue2", "header3": "svalue3"]
+        let session = URLSession(configuration: config, delegate: nil, delegateQueue: nil)
+        var expect = expectation(description: "download task with handler")
+        var req = URLRequest(url: URL(string: "http://127.0.0.1:\(serverPort)/requestHeaders")!)
+        let headers = ["header1": "rvalue1", "header2": "rvalue2"]
+        req.httpMethod = "POST"
+        req.allHTTPHeaderFields = headers
+        var task = session.dataTask(with: req) { (data, _, error) -> Void in
+            defer { expect.fulfill() }
+            let headers = String(data: data!, encoding: String.Encoding.utf8)!
+            XCTAssertNotNil(headers.range(of: "header1: rvalue1"))
+            XCTAssertNotNil(headers.range(of: "header2: rvalue2"))
+            XCTAssertNotNil(headers.range(of: "header3: svalue3"))
+        }
+        task.resume()
+        
+        waitForExpectations(timeout: 30)
+    }
+
     func test_taskTimeout() {
         let serverReady = ServerSemaphore()
         globalDispatchQueue.async {