I was looking over the new Bash exploit and was looking at this post in particular.

Attack scenarios of the new Bash vulnerability

What I don't understand is how does including bash in the user-agent string cause it to be executed. It seems odd to me that the user agent would ever be executed on the server. I know this i a broad question but I am just looking for a general understanding of how this is allowed to happen if possible.


Doing some further research I read through this which kind of cleared some stuff up for me.


Which explains that CGI is setting some environmental variables and some other input via stdin. Why no guard is in place to keep this from simply being a string is beyond me or why you are able to pass a bash function in a header.


1 Answer 1


What you are refering to is the Shellshock Bug, see https://blog.cloudflare.com/inside-shellshock/

The Shellshock problem specifically occurs when an attacker modifies the origin HTTP request to contain the magic () { :; }; string discussed above.

Suppose the attacker change the User-Agent header above from Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_4) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/37.0.2062.124 Safari/537.36 to simply () { :; }; /bin/eject. This creates the following variable inside a web server:

HTTP_USER_AGENT=() { :; }; /bin/eject

If that variable gets passed into bash by the web server, the Shellshock problem occurs. This is because bash has special rules for handling a variable starting with () { :; };. Rather than treating the variable HTTP_USER_AGENT as a sequence of characters with no special meaning, bash will interpret it as a command that needs to be executed (I've omitted the deeply technical explanations of why () { :; }; makes bash behave like this for the sake of clarity in this essay.)

The problem is that HTTP_USER_AGENT came from the User-Agent header which is something an attacker controls because it comes into the web server in an HTTP request. And that's a recipe for disaster because an attacker can make a vulnerable server run any command it wants (see examples below).

The solution is to upgrade bash to a version that doesn't interpret () { :; }; in a special way.

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