Re: Re(2) (ichimunki): Security issues when allowing file upload via CGI

[nufsaid]

I like the idea, but don't you have to be careful of this line?

my $file_cmd_output = `file $fn`;

$fn is tainted and doesn't this give them the chance to sneak a command in via $fn? Need to make sure $fn is clean.

Joe.

[dmmiller2k]

Sigh.

Yes, you're right about $fn being tainted, but I was speaking more to the principle of using the standard *NIX command, "file" (or reasonable facsimile, such as the perl module File::MMagic, for example) to check the type of the file by examining the first several bytes (or more).

Certainly, by the time I intended this code to run, a file would have been uploaded to some directory somewhere. My expectation was that $fn would, at the time of the call, contain the full pathname to this file, and would not necessarily have been entered directly by a web-page user.

But then, I DID fail to document these expectations (read: preconditions, using programming-by-contract terminology), so I deserve to be chastised for it.

(takes 40 lashes with a wet noodle)

dmm

You can give a man a fish and feed him for a day ...
Or, you can teach him to fish and feed him for a lifetime