MM-11786: Adds API endpoint to retrieve redirect locations. (#9284)

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
+}

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)
+}