From a519b86e8fb6ff6aebeb37ee90e853550635ab05 Mon Sep 17 00:00:00 2001
From: Ashish Bhate <ashish.bhate@mattermost.com>
Date: Wed, 29 Sep 2021 19:30:23 +0530
Subject: [PATCH] [MM-36786] Check channel permissions before thread
 follow/unfollow (#18460)

---
 api4/user.go      | 10 ++++++++++
 api4/user_test.go | 25 +++++++++++++++++++++++++
 2 files changed, 35 insertions(+)

diff --git a/api4/user.go b/api4/user.go
index 25054090cb..d576eb2703 100644
--- a/api4/user.go
+++ b/api4/user.go
@@ -3059,6 +3059,11 @@ func unfollowThreadByUser(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	if !c.App.SessionHasPermissionToChannelByPost(*c.AppContext.Session(), c.Params.ThreadId, model.PermissionReadChannel) {
+		c.SetPermissionError(model.PermissionReadChannel)
+		return
+	}
+
 	err := c.App.UpdateThreadFollowForUser(c.Params.UserId, c.Params.TeamId, c.Params.ThreadId, false)
 	if err != nil {
 		c.Err = err
@@ -3087,6 +3092,11 @@ func followThreadByUser(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	if !c.App.SessionHasPermissionToChannelByPost(*c.AppContext.Session(), c.Params.ThreadId, model.PermissionReadChannel) {
+		c.SetPermissionError(model.PermissionReadChannel)
+		return
+	}
+
 	err := c.App.UpdateThreadFollowForUser(c.Params.UserId, c.Params.TeamId, c.Params.ThreadId, true)
 	if err != nil {
 		c.Err = err
diff --git a/api4/user_test.go b/api4/user_test.go
index 8247afdb54..8ac7da8479 100644
--- a/api4/user_test.go
+++ b/api4/user_test.go
@@ -5870,6 +5870,31 @@ func TestFollowThreads(t *testing.T) {
 		require.GreaterOrEqual(t, uss.Threads[0].LastViewedAt, uss.Threads[0].LastReplyAt)
 
 	})
+
+	t.Run("No permission to channel", func(t *testing.T) {
+		// Add user1 to private channel
+		_, appErr := th.App.AddUserToChannel(th.BasicUser, th.BasicPrivateChannel2, false)
+		require.Nil(t, appErr)
+		defer th.App.RemoveUserFromChannel(th.Context, th.BasicUser.Id, "", th.BasicPrivateChannel2)
+
+		// create thread in private channel
+		rpost, resp, err := th.Client.CreatePost(&model.Post{ChannelId: th.BasicPrivateChannel2.Id, Message: "root post"})
+		require.NoError(t, err)
+		CheckCreatedStatus(t, resp)
+		_, resp, err = th.Client.CreatePost(&model.Post{ChannelId: th.BasicPrivateChannel2.Id, Message: "testReply", RootId: rpost.Id})
+		require.NoError(t, err)
+		CheckCreatedStatus(t, resp)
+
+		// Try to follow thread as other user who is not in the private channel
+		resp, err = th.Client.UpdateThreadFollowForUser(th.BasicUser2.Id, th.BasicTeam.Id, rpost.Id, true)
+		require.Error(t, err)
+		CheckForbiddenStatus(t, resp)
+
+		// Try to unfollow thread as other user who is not in the private channel
+		resp, err = th.Client.UpdateThreadFollowForUser(th.BasicUser2.Id, th.BasicTeam.Id, rpost.Id, false)
+		require.Error(t, err)
+		CheckForbiddenStatus(t, resp)
+	})
 }
 
 func checkThreadListReplies(t *testing.T, th *TestHelper, client *model.Client4, userId string, expectedReplies, expectedThreads int, options *model.GetUserThreadsOpts) (*model.Threads, *model.Response) {