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

by thraxil (Prior)
on Dec 06, 2001 at 20:48 UTC ( #129983=note: print w/ replies, xml ) Need Help??


in reply to (ichimunki) Re: Security issues when allowing file upload via CGI
in thread Security issues when allowing file upload via CGI

never underestimate the stupidity of IE and outlook.

if you take an html file, name it foo.jpg and send it with a mime-type of image/jpeg, IE 5 on the mac and IE 4 on windows will happily parse and render it as html. (probably some versions of outlook exhibit this broken behavior too).

this technique was once used in a hotmail exploit. email someone a "jpg" and it could grab their password cookie and submit it to another site.

if securityfocus hadn't changed the structure of their bugtraq archives and broken my bookmarks, i could give you a link...

i don't think it's quite dumb enough to run an .exe the same way but there's still a lot of mischief that can be done with html+javascript/vbscript

anders pearson


Comment on Re: (ichimunki) Re: Security issues when allowing file upload via CGI
Re(2) (ichimunki): Security issues when allowing file upload via CGI
by dmmiller2k (Chaplain) on Dec 06, 2001 at 23:31 UTC

    Roger that!!

    Had it not been for Micro$oft's feature-laden behemoths, (and their commensurate security patches, and security-patch patches, and so on, new ones of which seem to be required almost daily), nevermind OS-related issues, there might never have erupted as pervasive an anti-virus cottage industry as we have (which has since become a full-fledged industry).

    I think you should quarantine uploaded files and run the shell command,

    file {upload filename}

    on them to confirm they are what they purport to be.

    use File::Basename; sub validate_image _file { my $fn = shift; my %file_types = ( jpg => 'JPEG file', jpeg => 'JPEG file', gif => 'GIF file, v8[79]' ); my $ext = lc (fileparse $fn )[-1]; # get suffix return 0 unless exists $file_types{$ext}; my $file_cmd_output = `file $fn`; chomp $file_cmd_output; return 0 unless $file_cmd_output =~ /^$file_types{$ext}$/; # OK, we probably have what we think we have # go ahead and make it accessible, etc. accept_file( $fn ); # ... or whatever return 1; }

    Update: The expectation here is that before this subroutine is called, a file has already been uploaded (ostensibly one whose name ends in .jpg, .jpeg, or .gif) and "quarantined" -- that is, stored somewhere "safe", out of harm's way -- and that the parameter, $fn, to the sub is the full path to this file.

    (Thanks, nufsaid, for bringing up the issue of the tainted-ness of $fn)

    dmm

    Just call me the Anti-Gates ...
      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.

        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
        

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