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posted by n1 on Thursday September 25 2014, @01:59AM   Printer-friendly
from the well,-that's-not-ideal dept.

Ars reports that a new bug has been found in GNU Bash allows remote attackers to execute arbitrary code by setting the process trailing strings after function definitions in the values of environment variables.

This bug is reported to be present in RHEL (ver 4 through 7), Fedora, CentOS (ver 5 through 7), Ubuntu (ver 10.04 LTS, 12.04 LTS, and 14.04 LTS), Debian, and even OS X Mavericks.

This bug is exploitable through Apache servers with mod_cgi and mod_cgid loaded, OpenSSH, malicious DHCP servers in a compromised wireless access point through dhclient, as well as the CUPS printing system.

The Ars also includes a simple single liner that will test your setup for the newly found discovery:

env x='() { :;}; echo vulnerable' bash -c "echo this is a test"

A vulnerable system will output the following:

 this is a test

While a patched or unaffected system outputs:

bash: warning: x: ignoring function definition attempt
bash: error importing function definition for `x'
this is a test

A patch is already out, so administrators are advised to update Bash.

Editor's Update: Security Engineer Tavis Ormandy has said "The bash patch seems incomplete to me, function parsing is still brittle".

$ env X='() { (a)=>\' sh -c "echo date"; cat echo

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  • (Score: 1) by nishi.b on Thursday September 25 2014, @10:03PM

    by nishi.b (4243) on Thursday September 25 2014, @10:03PM (#98415)

    From what I understood, it is not necessary to have a bash shell at your disposal to trigger this.
    Here is the example I read :
    - a server runs Apache with CGI
    - it uses a CGI module (a perl program, a C program, java, whatever)
    - the URL parameters are transmitted to the program by Apache as environnement variables
    - let's say your CGI calls an external command, such as system("echo >> > /myCounter.txt")
    - An attacker goes to "" and sets its "referrer" string to a bash function definition
    - Your CGI script is executed, and runs the external command. This opens up a shell (bash) to run the provided command in the same environment, and *the env variables are interpreted by bash*.
    - This means that any CGI that uses a system() command for example (so executes a bash shell) can be used to run arbitrary commands on the server.

    I am not sure what happens with SSH, but I think it works like this :
    you can define env vars before running ssh to connect to a server, and SSH will try to copy the variables to the shell on the remote host (for example, to keep your language settings). If the shell is bash, the code in the vars will be executed even if the user does not type any command in his shell window.