diff --git a/.htaccess b/.htaccess
index f4024c632ac6d117bfbaf3e816878574db8dc98c..4716fa12c8cb1f0caac10eebdb946c4dff28ce66 100644
--- a/.htaccess
+++ b/.htaccess
@@ -180,8 +180,10 @@ AddEncoding gzip svgz
   </IfModule>
 </IfModule>
 
-# Add headers to all responses.
+# Various header fixes.
 <IfModule mod_headers.c>
   # Disable content sniffing, since it's an attack vector.
   Header always set X-Content-Type-Options nosniff
+  # Disable Proxy header, since it's an attack vector.
+  RequestHeader unset Proxy
 </IfModule>
diff --git a/composer.lock b/composer.lock
index 90b375c0f5bec3f7262fcfb4f6cfe0a4ffde1c6e..132fba5188265a4c9295c7ca2bb8f50f5d09ecf4 100644
--- a/composer.lock
+++ b/composer.lock
@@ -678,32 +678,32 @@
         },
         {
             "name": "guzzlehttp/guzzle",
-            "version": "6.1.0",
+            "version": "6.2.1",
             "source": {
                 "type": "git",
                 "url": "https://github.com/guzzle/guzzle.git",
-                "reference": "66fd14b4d0b8f2389eaf37c5458608c7cb793a81"
+                "reference": "3f808fba627f2c5b69e2501217bf31af349c1427"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://api.github.com/repos/guzzle/guzzle/zipball/66fd14b4d0b8f2389eaf37c5458608c7cb793a81",
-                "reference": "66fd14b4d0b8f2389eaf37c5458608c7cb793a81",
+                "url": "https://api.github.com/repos/guzzle/guzzle/zipball/3f808fba627f2c5b69e2501217bf31af349c1427",
+                "reference": "3f808fba627f2c5b69e2501217bf31af349c1427",
                 "shasum": ""
             },
             "require": {
-                "guzzlehttp/promises": "~1.0",
-                "guzzlehttp/psr7": "~1.1",
-                "php": ">=5.5.0"
+                "guzzlehttp/promises": "^1.0",
+                "guzzlehttp/psr7": "^1.3.1",
+                "php": ">=5.5"
             },
             "require-dev": {
                 "ext-curl": "*",
-                "phpunit/phpunit": "~4.0",
-                "psr/log": "~1.0"
+                "phpunit/phpunit": "^4.0",
+                "psr/log": "^1.0"
             },
             "type": "library",
             "extra": {
                 "branch-alias": {
-                    "dev-master": "6.1-dev"
+                    "dev-master": "6.2-dev"
                 }
             },
             "autoload": {
@@ -736,20 +736,20 @@
                 "rest",
                 "web service"
             ],
-            "time": "2015-09-08 17:36:26"
+            "time": "2016-07-15 17:22:37"
         },
         {
             "name": "guzzlehttp/promises",
-            "version": "1.0.2",
+            "version": "1.2.0",
             "source": {
                 "type": "git",
                 "url": "https://github.com/guzzle/promises.git",
-                "reference": "97fe7210def29451ec74923b27e552238defd75a"
+                "reference": "c10d860e2a9595f8883527fa0021c7da9e65f579"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://api.github.com/repos/guzzle/promises/zipball/97fe7210def29451ec74923b27e552238defd75a",
-                "reference": "97fe7210def29451ec74923b27e552238defd75a",
+                "url": "https://api.github.com/repos/guzzle/promises/zipball/c10d860e2a9595f8883527fa0021c7da9e65f579",
+                "reference": "c10d860e2a9595f8883527fa0021c7da9e65f579",
                 "shasum": ""
             },
             "require": {
@@ -787,20 +787,20 @@
             "keywords": [
                 "promise"
             ],
-            "time": "2015-08-15 19:37:21"
+            "time": "2016-05-18 16:56:05"
         },
         {
             "name": "guzzlehttp/psr7",
-            "version": "1.2.0",
+            "version": "1.3.1",
             "source": {
                 "type": "git",
                 "url": "https://github.com/guzzle/psr7.git",
-                "reference": "4ef919b0cf3b1989523138b60163bbcb7ba1ff7e"
+                "reference": "5c6447c9df362e8f8093bda8f5d8873fe5c7f65b"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://api.github.com/repos/guzzle/psr7/zipball/4ef919b0cf3b1989523138b60163bbcb7ba1ff7e",
-                "reference": "4ef919b0cf3b1989523138b60163bbcb7ba1ff7e",
+                "url": "https://api.github.com/repos/guzzle/psr7/zipball/5c6447c9df362e8f8093bda8f5d8873fe5c7f65b",
+                "reference": "5c6447c9df362e8f8093bda8f5d8873fe5c7f65b",
                 "shasum": ""
             },
             "require": {
@@ -816,7 +816,7 @@
             "type": "library",
             "extra": {
                 "branch-alias": {
-                    "dev-master": "1.0-dev"
+                    "dev-master": "1.4-dev"
                 }
             },
             "autoload": {
@@ -845,7 +845,7 @@
                 "stream",
                 "uri"
             ],
-            "time": "2015-08-15 19:32:36"
+            "time": "2016-06-24 23:00:38"
         },
         {
             "name": "ircmaxell/password-compat",
diff --git a/core/composer.json b/core/composer.json
index 550c7813d73473a1118a9d7ac0719e209641c0aa..2d58d79e1ea9f9d2950b74486c8ac92b0c604fae 100644
--- a/core/composer.json
+++ b/core/composer.json
@@ -21,7 +21,7 @@
         "twig/twig": "^1.23.1",
         "doctrine/common": "2.5.*",
         "doctrine/annotations": "1.2.*",
-        "guzzlehttp/guzzle": "~6.1",
+        "guzzlehttp/guzzle": "~6.2",
         "symfony-cmf/routing": "~1.4",
         "easyrdf/easyrdf": "0.9.*",
         "zendframework/zend-feed": "~2.4",
diff --git a/core/lib/Drupal/Core/Http/ClientFactory.php b/core/lib/Drupal/Core/Http/ClientFactory.php
index a68f0851757f98132bd8976f1a77a489c45557a9..3dcf35374ebd2f81f6694f854a0ea3e1bc8c903c 100644
--- a/core/lib/Drupal/Core/Http/ClientFactory.php
+++ b/core/lib/Drupal/Core/Http/ClientFactory.php
@@ -52,6 +52,13 @@ public function fromOptions(array $config = []) {
         'User-Agent' => 'Drupal/' . \Drupal::VERSION . ' (+https://www.drupal.org/) ' . \GuzzleHttp\default_user_agent(),
       ],
       'handler' => $this->stack,
+      // Security consideration: prevent Guzzle from using environment variables
+      // to configure the outbound proxy.
+      'proxy' => [
+        'http' => NULL,
+        'https' => NULL,
+        'no' => [],
+      ]
     ];
 
     $config = NestedArray::mergeDeep($default_config, Settings::get('http_client_config', []), $config);
diff --git a/sites/default/default.settings.php b/sites/default/default.settings.php
index 770a3a79a84f0a72b5e4c99079a3639031da8e91..d6d130e319d76dd4b810a4768830ceba7e5c0b91 100644
--- a/sites/default/default.settings.php
+++ b/sites/default/default.settings.php
@@ -325,9 +325,6 @@
  *
  * You can also define an array of host names that can be accessed directly,
  * bypassing the proxy, in $settings['http_client_config']['proxy']['no'].
- *
- * If these settings are not configured, the system environment variables
- * HTTP_PROXY, HTTPS_PROXY, and NO_PROXY on the web server will be used instead.
  */
 # $settings['http_client_config']['proxy']['http'] = 'http://proxy_user:proxy_pass@example.com:8080';
 # $settings['http_client_config']['proxy']['https'] = 'http://proxy_user:proxy_pass@example.com:8080';
diff --git a/web.config b/web.config
index a0535a10db23e3245063b5390625b46cf349c594..562847125fa7e4b09d83d605af2684f9ab5ec5d7 100644
--- a/web.config
+++ b/web.config
@@ -34,6 +34,14 @@
           </conditions>
         </rule>
 
+        <rule name="Erase HTTP_PROXY" patternSyntax="Wildcard">
+          <match url="*.*" />
+          <serverVariables>
+            <set name="HTTP_PROXY" value="" />
+          </serverVariables>
+          <action type="None" />
+        </rule>
+
     <!-- To redirect all users to access the site WITH the 'www.' prefix,
      http://example.com/foo will be redirected to http://www.example.com/foo)
      adapt and uncomment the following:   -->