diff --git a/Doc/howto/urllib2.rst b/Doc/howto/urllib2.rst
index 24a4156..cea5403 100644
--- a/Doc/howto/urllib2.rst
+++ b/Doc/howto/urllib2.rst
@@ -538,6 +538,15 @@ setting up a `Basic Authentication`_ handler: ::
     through a proxy.  However, this can be enabled by extending urllib.request as
     shown in the recipe [#]_.
 
+.. note::
+
+    If the environment variable ``REQUEST_METHOD`` is set, which usually
+    indicates your script is running in a CGI environment, the environment
+    variable ``HTTP_PROXY`` will be ignored. This is because that variable can
+    be injected by a client using the "Proxy:" HTTP header. If you need to use
+    an HTTP proxy in a CGI environment, either use ``ProxyHandler`` explicitly,
+    or make sure the variable name is in lowercase (``http_proxy``).
+
 
 Sockets and Layers
 ==================
diff --git a/Doc/library/urllib.request.rst b/Doc/library/urllib.request.rst
index 6c1bfb8..42c599f 100644
--- a/Doc/library/urllib.request.rst
+++ b/Doc/library/urllib.request.rst
@@ -173,6 +173,14 @@ The :mod:`urllib.request` module defines the following functions:
    If both lowercase and uppercase environment variables exist (and disagree),
    lowercase is preferred.
 
+   Note that if the environment variable ``REQUEST_METHOD`` is set, which
+   usually indicates your script is running in a CGI environment, the
+   environment variable ``HTTP_PROXY`` (uppercase) will be ignored. This is
+   because that variable can be injected by a client using the "Proxy:" HTTP
+   header. If you need to use an HTTP proxy in a CGI environment, either use
+   ``ProxyHandler`` explicitly, or make sure the variable name is in lowercase
+   (``http_proxy``)
+
 
 The following classes are provided:
 
@@ -280,6 +288,14 @@ The following classes are provided:
    list of hostname suffixes, optionally with ``:port`` appended, for example
    ``cern.ch,ncsa.uiuc.edu,some.host:8080``.
 
+    Note that if the environment variable ``REQUEST_METHOD`` is set, which
+    usually indicates your script is running in a CGI environment, the
+    environment variable ``HTTP_PROXY`` (uppercase) will be ignored. This is
+    because that variable can be injected by a client using the "Proxy:" HTTP
+    header. If you need to use an HTTP proxy in a CGI environment, either use
+    ``ProxyHandler`` explicitly, or make sure the variable name is in lowercase
+    (``http_proxy``).
+
 
 .. class:: HTTPPasswordMgr()
 
diff --git a/Lib/test/test_urllib.py b/Lib/test/test_urllib.py
index 5d05f8d..c26c52a 100644
--- a/Lib/test/test_urllib.py
+++ b/Lib/test/test_urllib.py
@@ -232,6 +232,18 @@ class ProxyTests(unittest.TestCase):
         self.assertTrue(urllib.request.proxy_bypass_environment('anotherdomain.com:8888'))
         self.assertTrue(urllib.request.proxy_bypass_environment('newdomain.com:1234'))
 
+    def test_proxy_cgi_ignore(self):
+        try:
+            self.env.set('HTTP_PROXY', 'http://somewhere:3128')
+            proxies = urllib.request.getproxies_environment()
+            self.assertEqual('http://somewhere:3128', proxies['http'])
+            self.env.set('REQUEST_METHOD', 'GET')
+            proxies = urllib.request.getproxies_environment()
+            self.assertNotIn('http', proxies)
+        finally:
+            self.env.unset('REQUEST_METHOD')
+            self.env.unset('HTTP_PROXY')
+
     def test_proxy_bypass_environment_host_match(self):
         bypass = urllib.request.proxy_bypass_environment
         self.env.set('NO_PROXY',
diff --git a/Lib/urllib/request.py b/Lib/urllib/request.py
index 1731fe3..c157dac 100644
--- a/Lib/urllib/request.py
+++ b/Lib/urllib/request.py
@@ -2412,6 +2412,11 @@ def getproxies_environment():
         name = name.lower()
         if value and name[-6:] == '_proxy':
             proxies[name[:-6]] = value
+    # If we are running as CGI script, forget HTTP_PROXY (uppercase) as it
+    # may be set from the web server by a "Proxy:" header from the client
+    # If lowercase, it will still be used thanks to the next block
+    if 'REQUEST_METHOD' in os.environ:
+        proxies.pop('http', None)
     for name, value in os.environ.items():
         if name[-6:] == '_proxy':
             name = name.lower()