diff --git a/src/nvim/insexpand.c b/src/nvim/insexpand.c
index ce676a8639..3828eafafb 100644
--- a/src/nvim/insexpand.c
+++ b/src/nvim/insexpand.c
@@ -3203,6 +3203,7 @@ static void expand_by_function(int type, char *base, Callback *cb)
   assert(curbuf != NULL);
 
   const bool is_cpt_function = (cb != NULL);
+  const bool use_sandbox = is_cpt_function && was_set_insecurely(curwin, kOptComplete, OPT_LOCAL);
   if (!is_cpt_function) {
     char *funcname = get_complete_funcname(type);
     if (*funcname == NUL) {
@@ -3224,9 +3225,16 @@ static void expand_by_function(int type, char *base, Callback *cb)
   // switching to another window, it should not be needed and may end up in
   // Insert mode in another buffer.
   textlock++;
+  if (use_sandbox) {
+    sandbox++;
+  }
 
   // Call a function, which returns a list or dict.
-  if (callback_call(cb, 2, args, &rettv)) {
+  const bool called = callback_call(cb, 2, args, &rettv);
+  if (use_sandbox) {
+    sandbox--;
+  }
+  if (called) {
     switch (rettv.v_type) {
     case VAR_LIST:
       matchlist = rettv.vval.v_list;
@@ -5796,6 +5804,7 @@ static int get_userdefined_compl_info(colnr_T curs_col, Callback *cb, int *start
   const int save_State = State;
 
   const bool is_cpt_function = (cb != NULL);
+  const bool use_sandbox = is_cpt_function && was_set_insecurely(curwin, kOptComplete, OPT_LOCAL);
   if (!is_cpt_function) {
     // Call 'completefunc' or 'omnifunc' or 'thesaurusfunc' and get pattern
     // length as a string
@@ -5816,7 +5825,13 @@ static int get_userdefined_compl_info(colnr_T curs_col, Callback *cb, int *start
 
   pos_T pos = curwin->w_cursor;
   textlock++;
+  if (use_sandbox) {
+    sandbox++;
+  }
   colnr_T col = (colnr_T)callback_call_retnr(cb, 2, args);
+  if (use_sandbox) {
+    sandbox--;
+  }
   textlock--;
 
   State = save_State;
diff --git a/test/old/testdir/test_modeline.vim b/test/old/testdir/test_modeline.vim
index 6ffcf4c717..3c6dd6bc85 100644
--- a/test/old/testdir/test_modeline.vim
+++ b/test/old/testdir/test_modeline.vim
@@ -289,6 +289,59 @@ func Test_modeline_fails_modelineexpr()
   call s:modeline_fails('titlestring', 'titlestring=Something()', 'E992:')
 endfunc
 
+func Test_modeline_complete_uses_sandbox()
+  let modeline = &modeline
+  let modelineexpr = &modelineexpr
+
+  func! ModelineCompletePwnFindstart(findstart, base)
+    if a:findstart
+      call writefile(['findstart'], 'Xmodeline_complete_proof')
+      return 0
+    endif
+    return ['match']
+  endfunc
+
+  func! ModelineCompletePwnMatches(findstart, base)
+    if a:findstart
+      return 0
+    endif
+    call writefile(['matches'], 'Xmodeline_complete_proof')
+    return ['match']
+  endfunc
+
+  try
+    set modeline modelineexpr
+
+    call writefile([
+          \ 'vim: set complete=FModelineCompletePwnFindstart :',
+          \ 'body',
+          \ ], 'Xmodeline_complete_attack', 'D')
+    call delete('Xmodeline_complete_proof')
+    edit Xmodeline_complete_attack
+    call cursor(2, 1)
+    call assert_fails('call feedkeys("i\<C-N>\<Esc>", "xt")', 'E48:')
+    call assert_false(filereadable('Xmodeline_complete_proof'))
+    bwipe!
+
+    call writefile([
+          \ 'vim: set complete=FModelineCompletePwnMatches :',
+          \ 'body',
+          \ ], 'Xmodeline_complete_attack', 'D')
+    call delete('Xmodeline_complete_proof')
+    edit Xmodeline_complete_attack
+    call cursor(2, 1)
+    call assert_fails('call feedkeys("i\<C-N>\<Esc>", "xt")', 'E48:')
+    call assert_false(filereadable('Xmodeline_complete_proof'))
+    bwipe!
+  finally
+    let &modeline = modeline
+    let &modelineexpr = modelineexpr
+    call delete('Xmodeline_complete_proof')
+    delfunc ModelineCompletePwnFindstart
+    delfunc ModelineCompletePwnMatches
+  endtry
+endfunc
+
 func Test_modeline_setoption_verbose()
   let modeline = &modeline
   set modeline