net/http, net/http/cgi: fix for CGI + HTTP_PROXY security issue

Because,

* The CGI spec defines that incoming request header "Foo: Bar" maps to
  environment variable HTTP_FOO == "Bar". (see RFC 3875 4.1.18)

* The HTTP_PROXY environment variable is conventionally used to configure
  the HTTP proxy for HTTP clients (and is respected by default for
  Go's net/http.Client and Transport)

That means Go programs running in a CGI environment (as a child
process under a CGI host) are vulnerable to an incoming request
containing "Proxy: attacker.com:1234", setting HTTP_PROXY, and
changing where Go by default proxies all outbound HTTP requests.

This is CVE-2016-5386, aka https://httpoxy.org/

Fixes #16405

Change-Id: I6f68ade85421b4807785799f6d98a8b077e871f0
Reviewed-on: https://go-review.googlesource.com/25010
Run-TryBot: Chris Broadfoot <cbro@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Chris Broadfoot <cbro@golang.org>

src/net/http/cgi/host.go

@@ -153,6 +153,10 @@ func (h *Handler) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 
 	for k, v := range req.Header {
 		k = strings.Map(upperCaseAndUnderscore, k)
+		if k == "PROXY" {
+			// See Issue 16405
+			continue
+		}
 		joinStr := ", "
 		if k == "COOKIE" {
 			joinStr = "; "

src/net/http/cgi/host_test.go

@@ -35,15 +35,18 @@ func newRequest(httpreq string) *http.Request {
 	return req
 }
 
-func runCgiTest(t *testing.T, h *Handler, httpreq string, expectedMap map[string]string) *httptest.ResponseRecorder {
+func runCgiTest(t *testing.T, h *Handler,
+	httpreq string,
+	expectedMap map[string]string, checks ...func(reqInfo map[string]string)) *httptest.ResponseRecorder {
 	rw := httptest.NewRecorder()
 	req := newRequest(httpreq)
 	h.ServeHTTP(rw, req)
-	runResponseChecks(t, rw, expectedMap)
+	runResponseChecks(t, rw, expectedMap, checks...)
 	return rw
 }
 
-func runResponseChecks(t *testing.T, rw *httptest.ResponseRecorder, expectedMap map[string]string) {
+func runResponseChecks(t *testing.T, rw *httptest.ResponseRecorder,
+	expectedMap map[string]string, checks ...func(reqInfo map[string]string)) {
 	// Make a map to hold the test map that the CGI returns.
 	m := make(map[string]string)
 	m["_body"] = rw.Body.String()
@@ -81,6 +84,9 @@ readlines:
 			t.Errorf("for key %q got %q; expected %q", key, got, expected)
 		}
 	}
+	for _, check := range checks {
+		check(m)
+	}
 }
 
 var cgiTested, cgiWorks bool
@@ -236,6 +242,31 @@ func TestDupHeaders(t *testing.T) {
 		expectedMap)
 }
 
+// Issue 16405: CGI+http.Transport differing uses of HTTP_PROXY.
+// Verify we don't set the HTTP_PROXY environment variable.
+// Hope nobody was depending on it. It's not a known header, though.
+func TestDropProxyHeader(t *testing.T) {
+	check(t)
+	h := &Handler{
+		Path: "testdata/test.cgi",
+	}
+	expectedMap := map[string]string{
+		"env-REQUEST_URI":     "/myscript/bar?a=b",
+		"env-SCRIPT_FILENAME": "testdata/test.cgi",
+		"env-HTTP_X_FOO":      "a",
+	}
+	runCgiTest(t, h, "GET /myscript/bar?a=b HTTP/1.0\n"+
+		"X-Foo: a\n"+
+		"Proxy: should_be_stripped\n"+
+		"Host: example.com\n\n",
+		expectedMap,
+		func(reqInfo map[string]string) {
+			if v, ok := reqInfo["env-HTTP_PROXY"]; ok {
+				t.Errorf("HTTP_PROXY = %q; should be absent", v)
+			}
+		})
+}
+
 func TestPathInfoNoRoot(t *testing.T) {
 	check(t)
 	h := &Handler{

src/net/http/transport.go

@@ -251,6 +251,9 @@ func ProxyFromEnvironment(req *Request) (*url.URL, error) {
 	}
 	if proxy == "" {
 		proxy = httpProxyEnv.Get()
+		if proxy != "" && os.Getenv("REQUEST_METHOD") != "" {
+			return nil, errors.New("net/http: refusing to use HTTP_PROXY value in CGI environment; see golang.org/s/cgihttpproxy")
+		}
 	}
 	if proxy == "" {
 		return nil, nil

src/net/http/transport_test.go

@@ -2060,7 +2060,8 @@ type proxyFromEnvTest struct {
 
 	env      string // HTTP_PROXY
 	httpsenv string // HTTPS_PROXY
-	noenv    string // NO_RPXY
+	noenv    string // NO_PROXY
+	reqmeth  string // REQUEST_METHOD
 
 	want    string
 	wanterr error
@@ -2084,6 +2085,10 @@ func (t proxyFromEnvTest) String() string {
 		space()
 		fmt.Fprintf(&buf, "no_proxy=%q", t.noenv)
 	}
+	if t.reqmeth != "" {
+		space()
+		fmt.Fprintf(&buf, "request_method=%q", t.reqmeth)
+	}
 	req := "http://example.com"
 	if t.req != "" {
 		req = t.req
@@ -2107,6 +2112,12 @@ var proxyFromEnvTests = []proxyFromEnvTest{
 	{req: "https://secure.tld/", env: "http.proxy.tld", httpsenv: "secure.proxy.tld", want: "http://secure.proxy.tld"},
 	{req: "https://secure.tld/", env: "http.proxy.tld", httpsenv: "https://secure.proxy.tld", want: "https://secure.proxy.tld"},
 
+	// Issue 16405: don't use HTTP_PROXY in a CGI environment,
+	// where HTTP_PROXY can be attacker-controlled.
+	{env: "http://10.1.2.3:8080", reqmeth: "POST",
+		want:    "<nil>",
+		wanterr: errors.New("net/http: refusing to use HTTP_PROXY value in CGI environment; see golang.org/s/cgihttpproxy")},
+
 	{want: "<nil>"},
 
 	{noenv: "example.com", req: "http://example.com/", env: "proxy", want: "<nil>"},
@@ -2122,6 +2133,7 @@ func TestProxyFromEnvironment(t *testing.T) {
 		os.Setenv("HTTP_PROXY", tt.env)
 		os.Setenv("HTTPS_PROXY", tt.httpsenv)
 		os.Setenv("NO_PROXY", tt.noenv)
+		os.Setenv("REQUEST_METHOD", tt.reqmeth)
 		ResetCachedEnvironment()
 		reqURL := tt.req
 		if reqURL == "" {