diff -up lua-5.4.0/src/liolib.c.bug7 lua-5.4.0/src/liolib.c
--- lua-5.4.0/src/liolib.c.bug7	2020-07-31 10:53:20.857070633 -0400
+++ lua-5.4.0/src/liolib.c	2020-07-31 10:53:58.694421042 -0400
@@ -279,6 +279,8 @@ static int io_popen (lua_State *L) {
   const char *filename = luaL_checkstring(L, 1);
   const char *mode = luaL_optstring(L, 2, "r");
   LStream *p = newprefile(L);
+  luaL_argcheck(L, ((mode[0] == 'r' || mode[0] == 'w') && mode[1] == '\0'),
+                   2, "invalid mode");
   p->f = l_popen(L, filename, mode);
   p->closef = &io_pclose;
   return (p->f == NULL) ? luaL_fileresult(L, 0, filename) : 1;