Re: shell script cgi

[Brian Hatch <vuln-dev () ifokr org>]

> thought the wrong way in my last message.
>
> Try the following string for $HTTP_USER_AGENT:
>
> "`cat /etc/passwd`"

The HTTP_USER_AGENT string is set by Apache (etc)
as an environment variable.  Let's make the unradical
assumption that your browser is not a shell script,
and thus is setting it correctly without any shell
expansion problems possible.  So HTTP_USER_AGENT is
set to

        `cat /etc/passwd`

To emulate this, let's set it ourselves in a normal shell:

        bash$ export VAR='`cat /etc/passwd`'
        bash$ echo $VAR
        `cat /etc/passwd`

I use the single quotes here to make sure no expansion
tricks are possible, which is how your web browser will
work.  The var is set exactly.  Excellent.  Can we abuse
it?

        bash$ echo $VAR
        `cat /etc/passwd`
        bash$ echo "$VAR"
        `cat /etc/passwd`

Nope.  How 'bout if we have:

        bash$ export VAR='blahblahblah *'
        bash$ echo $VAR
        blahblahblah file1.cgi file2.cgi file3.cgi
        bash$ echo "$VAR"
        blahblahblah *

See that with no quotes you can get a file listing through
shell expansion.



--
Brian Hatch                  I used to be a lumberjack,
   Systems and                but I just couldn't hack
   Security Engineer          it, so they gave me the ax.
http://www.ifokr.org/bri/

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