From ed0fb617ef629a39d27441336951c098b13324d0 Mon Sep 17 00:00:00 2001
From: Martin Kraft <martinkraft@gmail.com>
Date: Fri, 24 Aug 2018 08:49:31 -0400
Subject: MM-11786: Adds API endpoint to retrieve redirect locations. (#9284)

---
 api4/system.go      | 32 ++++++++++++++++++++++++++++++++
 api4/system_test.go | 34 ++++++++++++++++++++++++++++++++++
 model/client4.go    | 15 +++++++++++++++
 3 files changed, 81 insertions(+)

diff --git a/api4/system.go b/api4/system.go
index 9177b8940..65d3b424b 100644
--- a/api4/system.go
+++ b/api4/system.go
@@ -40,6 +40,8 @@ func (api *API) InitSystem() {
 	api.BaseRoutes.ApiRoot.Handle("/logs", api.ApiHandler(postLog)).Methods("POST")
 
 	api.BaseRoutes.ApiRoot.Handle("/analytics/old", api.ApiSessionRequired(getAnalytics)).Methods("GET")
+
+	api.BaseRoutes.ApiRoot.Handle("/redirect_location", api.ApiSessionRequiredTrustRequester(getRedirectLocation)).Methods("GET")
 }
 
 func getSystemPing(c *Context, w http.ResponseWriter, r *http.Request) {
@@ -447,3 +449,33 @@ func testS3(c *Context, w http.ResponseWriter, r *http.Request) {
 
 	ReturnStatusOK(w)
 }
+
+func getRedirectLocation(c *Context, w http.ResponseWriter, r *http.Request) {
+	url := r.URL.Query().Get("url")
+	if len(url) == 0 {
+		c.SetInvalidParam("url")
+		return
+	}
+
+	client := &http.Client{
+		CheckRedirect: func(req *http.Request, via []*http.Request) error {
+			return http.ErrUseLastResponse
+		},
+	}
+
+	m := make(map[string]string)
+	m["location"] = ""
+
+	res, err := client.Head(url)
+	if err != nil {
+		// Always return a success status and a JSON string to limit the amount of information returned to a
+		// hacker attempting to use Mattermost to probe a private network.
+		w.Write([]byte(model.MapToJson(m)))
+		return
+	}
+
+	m["location"] = res.Header.Get("Location")
+
+	w.Write([]byte(model.MapToJson(m)))
+	return
+}
diff --git a/api4/system_test.go b/api4/system_test.go
index 205572cf6..c66e08976 100644
--- a/api4/system_test.go
+++ b/api4/system_test.go
@@ -3,6 +3,7 @@ package api4
 import (
 	"fmt"
 	"net/http"
+	"net/http/httptest"
 	"os"
 	"strings"
 	"testing"
@@ -700,3 +701,36 @@ func TestSupportedTimezones(t *testing.T) {
 	CheckNoError(t, resp)
 	assert.Equal(t, supportedTimezonesFromConfig, supportedTimezones)
 }
+
+func TestRedirectLocation(t *testing.T) {
+	expected := "https://mattermost.com/wp-content/themes/mattermostv2/img/logo-light.svg"
+
+	testServer := httptest.NewServer(http.HandlerFunc(func(res http.ResponseWriter, req *http.Request) {
+		res.Header().Set("Location", expected)
+		res.WriteHeader(http.StatusFound)
+		res.Write([]byte("body"))
+	}))
+	defer func() { testServer.Close() }()
+
+	mockBitlyLink := testServer.URL
+
+	th := Setup().InitBasic().InitSystemAdmin()
+	defer th.TearDown()
+	Client := th.Client
+
+	_, resp := th.SystemAdminClient.GetRedirectLocation("https://mattermost.com/", "")
+	CheckNoError(t, resp)
+
+	_, resp = th.SystemAdminClient.GetRedirectLocation("", "")
+	CheckBadRequestStatus(t, resp)
+
+	actual, resp := th.SystemAdminClient.GetRedirectLocation(mockBitlyLink, "")
+	CheckNoError(t, resp)
+	if actual != expected {
+		t.Errorf("Expected %v but got %v.", expected, actual)
+	}
+
+	Client.Logout()
+	_, resp = Client.GetRedirectLocation("", "")
+	CheckUnauthorizedStatus(t, resp)
+}
diff --git a/model/client4.go b/model/client4.go
index 48627c4b0..4f651b976 100644
--- a/model/client4.go
+++ b/model/client4.go
@@ -397,6 +397,10 @@ func (c *Client4) GetTotalUsersStatsRoute() string {
 	return fmt.Sprintf(c.GetUsersRoute() + "/stats")
 }
 
+func (c *Client4) GetRedirectLocationRoute() string {
+	return fmt.Sprintf("/redirect_location")
+}
+
 func (c *Client4) DoApiGet(url string, etag string) (*http.Response, *AppError) {
 	return c.DoApiRequest(http.MethodGet, c.ApiUrl+url, "", etag)
 }
@@ -3771,3 +3775,14 @@ func (c *Client4) UpdateTeamScheme(teamId, schemeId string) (bool, *Response) {
 		return CheckStatusOK(r), BuildResponse(r)
 	}
 }
+
+// GetRedirectLocation retrieves the value of the 'Location' header of an HTTP response for a given URL.
+func (c *Client4) GetRedirectLocation(urlParam, etag string) (string, *Response) {
+	url := fmt.Sprintf("%s?url=%s", c.GetRedirectLocationRoute(), url.QueryEscape(urlParam))
+	if r, err := c.DoApiGet(url, etag); err != nil {
+		return "", BuildErrorResponse(r, err)
+	} else {
+		defer closeBody(r)
+		return MapFromJson(r.Body)["location"], BuildResponse(r)
+	}
+}
-- 
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