Re: os.tmpname security risk?

[Patrick Donnelly <batrick@...>]

On Wed, Jul 13, 2011 at 8:17 PM, Steve Litt <slitt@troubleshooters.com> wrote:
> On Saturday, July 02, 2011 06:29:52 AM HyperHacker wrote:
>> The 5.1 manual states, for os.tmpname():
>> > On some systems (POSIX), this function also creates a file with
>> > that name, to avoid security risks. (Someone else might create
>> > the file with wrong permissions in the time between getting the
>> > name and creating the file.)
>>
>> But what stops someone from removing that file and creating their
>> own, or changing its permissions? It doesn't seem like this really
>> mitigates the risk at all.
>
> I think the assumption is you'll use the file very quickly and then
> erase it, making it very hard for someone to switch out the file during
> the short time you'll be using it.
>
> I rolled my own temp file maker that in my opinion is better than those
> offered by Lua's distribution:
>
> http://www.troubleshooters.com/codecorn/lua/luaos.htm#_Temporary_Files
>
> Be careful -- I think the "%%%" in the error message should be "@@@".

Your file maker is not better. There is a race condition between the
test to check for the existence of the desired file and the opening of
the file for writing (which creates it).

This is why Lua and everyone else uses mkstemp [1].

[1] http://pubs.opengroup.org/onlinepubs/009695399/functions/mkstemp.html

-- 
- Patrick Donnelly