From: Markus Koschany <apo@debian.org>
Date: Wed, 19 Oct 2022 19:50:16 +0200
Subject: CVE-2021-4019

Origin: https://github.com/vim/vim/commit/bd228fd097b41a798f90944b5d1245eddd484142
---
 src/ex_cmds.c             |  3 +--
 src/testdir/test_help.vim | 10 ++++++++++
 2 files changed, 11 insertions(+), 2 deletions(-)

--- a/src/ex_cmds.c
+++ b/src/ex_cmds.c
@@ -6658,8 +6658,7 @@ find_help_tags(
 		    || (vim_strchr((char_u *)"%_z@", arg[1]) != NULL
 							   && arg[2] != NUL)))
 	{
-	    STRCPY(d, "/\\\\");
-	    STRCPY(d + 3, arg + 1);
+	vim_snprintf((char *)d, IOSIZE, "/\\\\%s", arg + 1);
 	    /* Check for "/\\_$", should be "/\\_\$" */
 	    if (d[3] == '_' && d[4] == '$')
 		STRCPY(d + 4, "\\$");
--- a/src/testdir/test_help.vim
+++ b/src/testdir/test_help.vim
@@ -49,3 +49,13 @@ func Test_help_local_additions()
   call delete('Xruntime', 'rf')
   let &rtp = rtp_save
 endfunc
+
+" CVE-2021-4019
+func Test_help_long_argument()
+  try
+    exe 'help \%' .. repeat('0', 1021)
+  catch
+    call assert_match("E15:", v:exception)
+  endtry
+endfunc
+