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Security Flaw with Linux 2.4 Kernel and IPTables
Sc00ter writes "According to this security advisory from Tempest Security Technologies there is a security flaw in the Linux 2.4 kernel when using IPTables." In a nutshell: if you're using a 2.4 system as a firewall, you need to read this.
Firewalling is a well-known technique for "improving security". But
a lot of security folks just does not understand that a firewall isn't
the "brick wall" they tend to think it is. Any firewall can be
penetrated, often through the configured "holes" (i.e., TCP traffic at
port 80 etc.). The best practise is to consider your network just as
insecure with a firewall as without a firewall. Then, but only then,
does the firewall give you the protection you think it does.
Of course, this makes you wonder why a bug in the firewall
implementation in Linux makes this large headlines. You shouldn't rely
on you firewall anyway. And when gnome is included in the Linux
kernel, everything will be bloated and insecure. The survivors will be
those that didn't depend too heavily on their Linux firewalls.
the patch to fix is is right here diff -urN linux-2.4.3.orig/net/ipv4/netfilter/ip_conntrack_f tp.c linux/net/ipv4/netfilter/ip_conntrack_ftp.c
- --- linux-2.4.3.orig/net/ipv4/netfilter/ip_conntrack_f tp.c Fri Aug 11 05:35:15 2000
+++ linux/net/ipv4/netfilter/ip_conntrack_ftp.c Mon Apr 16 02:18:30 2001
@@ -187,7 +187,12 @@
(int)matchlen, data + matchoff,
matchlen, ntohl(tcph->seq) + matchoff);
- - /* Update the ftp info */
+ /*
+ * Update the ftp info only if the source address matches the address specified
+ * in the PORT or PASV command. Closes hole where packets could be dangerously
+ * marked as RELATED to bypass filtering rules. Thanks to Cristiano Lincoln
+ * Mattos for the report.
+ */
LOCK_BH(&ip_ftp_lock);
if (htonl((array[0] tuplehash[dir].tuple.src.ip) {
@@ -197,13 +202,8 @@
info->ftptype = dir;
info->port = array[4] tuplehash[dir].tuple.src.ip));
+ UNLOCK_BH(&ip_ftp_lock);
+ return NF_ACCEPT;
}
t = ((struct ip_conntrack_tuple)
found that sucker out on the net :-)
I realize that you could set up iptables to deny RELATED connections to reserved ports, but in the case of ip_conntrack_ftp specifically why add them at all? I haven't had a chance to brush up on RFC959, so I could be wrong, but it seems like we could reasonably assume that no legitimate connection will request a PORT or PASV connection to a reserved port, so why place them as RELATED?
Hi all,
Summary: if you have multiple networks behind your iptables packet filter (eg. a DMZ) and you use the ip_conntrack_ftp module (or you compiled it into your kernel) you should apply the simple patch, otherwise a breakin on one protected network can be used to allow probes on the other protected network.
OK, so what happened? The connection tracking code in 2.4 (ip_conntrack) can be extended to understand complex protocols, like FTP (ip_conntrack_ftp). When it sees an FTP server or client say "for the data, connect to 1.2.3.4 port 56" it remembers it, and when that connection comes in, it classifies the packet as "RELATED", not "NEW" (as packets creating new connections are normally marked). Most packet filter setups simply allow all RELATED packets.
Of course, my original code only set up this "expectation" if the ftp server gave its own IP address in that "connect here" message. However, one early user, Enrico Scholtz, had a setup where the FTP server REALLY DID use a different IP address for the data connection. After some thought, I allowed it: you can allow arbitrary connections to be marked RELATED, but only from inside a network already.
The problem is that if you are using a single box to connect your DMZ, your internal network, and the outside world, and someone breaks into the DMZ, they can use a machine there (if you allow any FTP to or from those machines) to tell the firewall to expect a connection from the outside to the internal network, giving you access to probe your internal network.
The patch provided (by James Morris) in the posting simply stops returns to ignoring an FTP server or client which says to connect on a different IP from the one it is on. In some form, it will be in 2.4.4.
FYI, I was travelling while this broke, and the Netfilter Core Team handled the issue with great aplomb. Kudos to them!
Hope that clarifies,
Rusty.
Sorry, but the world simply doesn't work like that ..
--
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echo '[q]sa[ln0=aln80~Psnlbx]16isb572CCB9AE9DB03273snlbxq' |dc
Since the site is slashdotted, here are the links to the advisories on the netfilter sites:
This is your sig. There are thousands more, but this one is yours.
You forgot "Your Rights Online," which is colored similar to what the $CHILD_PROCESS produces when she barfs up her strained peas.
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Slashdot will not change their logo to blue until Kmart agrees to a sufficently large payment. Then it will be flashing blue for ten minutes at selected times during the day.
It really only affect Active FTP sessions. Passive sessions really work without a problem with firewalls, because the connections and transfers are all done in band. If you want to save your self the trouble, just set all your FTP servers to passive only. No problem then at all.
--
How do you know it's not? How do you know Windows doesn't have the same problem in its Internet Connection Sharing?
At least we know it has been fixed in Linux.
--
Patrick Doyle
I mod down every jackass who puts his moderation policy in his sig. Oh, wait a sec....
Perhaps I am reading the patch wrong, however it seems to me that the patch only protects other hosts, you out still add related connections to the same host, you were ftping to.....
James
Hey guys, this is open source: there was a security issue with 2.4 - it is fixed now.
Tell your friends about xenu.net
He's right. OpenBSD doesn't run any daemons by default, making it secure by default. However, if you run apache on OpenBSD, and there's a hole in apache, your OpenBSD system is no longer secure. Tbus "OpenBSD is secure until you use it for anything practical."
That was not "Overrated".
------
Quite simply because I'm more familiar with Linux and I wasn't aware a BSD solution existed. I needed a software solution with full NAT and firewall, and I knew it was available in linux 2.4. And, frankly, we haven't had any problems with it at all. No crashes, unexplained slowdowns, or weird security problems. This issue actually doesn't affect me at all, since I don't use conntrack_ftp on this firewall (no FTP servers behind it), but I'm patching it anyways in case we use it in the future.
Now that I know a BSD solution exists, I'll make a note of it. But since we're not having problems with this setup, I'm not inclined to move it to BSD. From the department of redundancy department, if it ain't broke, don't fix it.
-Todd
---
"The details of my life are quite inconsequential..."
As you said, if it ain't broke, don't fix it. And conversely, if it is broke, fix it as soon as possible.
I run a couple 2.4 production firewalls because we needed features available in IpTables that aren't available in IpChains, such as full NAT. There are reasons to run a bleeding edge firewall like a 2.4 system in production, you just have to balance the benefits with the cost of running something that is essentially beta.
In my case, the features outweigh the risk. I can deal with a little downtime, if neccessary, and I have other firewalls that give me rudimentary protection if my 2.4 box fails. Sure, it's not for you, and that's great. But it doesn't mean it's not for everyone.
-Todd
---
"The details of my life are quite inconsequential..."
The only way to exploit this is if the FTP client is under the attacker's control, and passing through the firewall. This breaks down into two possible catagories:
Case 1 is the most scare from a sysad standpoint: if I had set up a world-accessible FTP server protected by a Linux firewall, the world could punch holes in my firewall.
Case 2 would be more the case of a company that chose to limit employees' access to the Internet, and the employees could use this to punch holes in the firewall. This is not as much a security risk (if you have employees who are security risks, you need to identify them and fire them.) After all, if one of your employees controls a server outside the firewall, he can always set up a proxy server on port 80 and do whatever he wants.
Still, I'm glad this sort of thing was caught and corrected. This is why peer review is important for security....
www.eFax.com are spammers
It really only affect Active FTP sessions. Passive sessions really work without a problem with firewalls, because the connections and transfers are all done in band. If you want to save your self the trouble, just set all your FTP servers to passive only. No problem then at all.
Um, that's not how passive mode ftp works. You still have two sockets for the FTP, whether active or passive. What changes is who will initiate the connection for the data.
Without recently developed stateful firewall monitors, FTP is very difficult to arrange from host A behind firewall f(A) to host B behind firewall f(B). Active mode butts up against f(A), and passive mode butts up against f(B).
FTP is a broken protocol.
[
Just out of curiosity, why didn't you use a BSD solution, which has full NAT capability (and many more) while also being very well tested?
Jeremy
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The article's slow to load... so, as far as I can see, the problem is/was:
E feel free to correct me if I've got it wrong.
GROGGS: alive and well and living in
As you can see in the relevant bugtraq post, this was made public about 4 days ago.
The fix is already in the archives (a check that ensures that 'RELATED' connection have the same source address as the initiating original connection), and works fine.
A good Win32 firewall product is WinRoute Pro. It does all the communication and takes the OS out of the equation. Of course don't go with the default install but it can be made reasonably secure. I have installed it in a small installation in the past and considering they were going to use nothing, I felt better about using this. I'm glad I was just consulting at the time and didn't have to deal with it 24/7. Personally my own firewall uses IP chains.
At my current job, we use a commercial firewall product but use OpenBSD bridging firewalls internally to protect the internal network segments. You security better not be all in your firewall because all firewalls can be broken given enough time and skill. Lock down the routers, switches and for the love of God, lock the server room door(s). Remove every modem in the network and use a modem pool if there is some reason to have one at all. Put a firewall between the modem pool and the network and lock it pretty hard. Never underestimate the power of users (or sysadmins) to circumvent the security with postit notes, leaving critical systems logged on, leaving doors open to the public, etc.
Just because its _supposedly_ very well tested, doesn't mean security is guaranteed.
:).
;).
For example IPfilter also recently had a problem - it was passing fragments through, even when not supposed to.
And IP filters was supposed to be quite well tested.
It didn't affect the site I set up because I don't allow packets through at all - everything is proxied. I do run IPfilter but it is just to log attack attempts, and help make sure things are safe even if accidents happen
The router in front is also configured to do packet filtering. Most of the kiddies don't even get through the router filters, hardly see any attacks on the ipfilter logs.
All these precautions have been justified by the recent IPfilter flaw.
So if you really want to be safe layer your firewalls and don't use the same stuff. Try iptables if you want, but have something else behind it. And make sure the ISP-facing router in front is secured and also doing some filtering.
And don't run BIND or Sendmail. Both have been very well "tested" by people over the years
Cheerio,
Link.
Did this patch make it into redhat 7.1?
Daniel
Dunno, if Microsoft were to immediately acknowledge the existance of a security vulnerability and offer a patch that worked within a few hours or days of the alert...
I would still bitch and moan!
---
There is much cruelty in the universe, John.
Yeah, we seem to have the tour map.
Looks like it's time to go back to ZoneAlarm.
(since the site seems almost slashdotted a copy is made below)
/proc/net/ip_conntrack), of the following form:
/proc/net/ip_conntrack), of the following form:
:)
:)
:) but this attack bypasses the firewall rules by inserting an entry into the ruleset for RELATED connections -- for the attack to work, there must be a rule allowing the client to connect to an FTP server (through the firewall) in the first place, and the rule allowing the RELATED state for the specified connection. This is a very common setting, as most firewalls allow their clients to perform FTP, and the too-open RELATED rule is also very common -- i've seen it an lots of IPTables FAQs, guides, lists, etc.
f tp.c linux/net/ipv4/netfilter/ip_conntrack_ftp.c
f tp.c Fri Aug 11 05:35:15 2000
/* Update the ftp info */
/*
Security flaw in Linux 2.4 IPTables using FTP PORT
Tempest Security Technologies
a business unit of CESAR - Centro de Estudos e Sistemas Avançados do Recife
Author: Cristiano Lincoln Mattos, CISSP, SSCP
Recife, Pernambuco, Brazil
Table of Contents
Overview
Detailed description
Solutions
Demonstration tool
Download
Acknowledgements
Text version
Overview
Systems affected: Firewalls using Linux Kernel 2.4.x with IPTables
Release date: 16 April 2001
Platforms: Linux Kernel 2.4.x
Impact: If an attacker can establish an FTP connection passing through a Linux 2.4.x IPTables firewall with the state options allowing "related" connections (almost 100% do), he can insert entries into the firewall's RELATED ruleset table allowing the FTP Server to connect to any host and port protected by the firewalls rules, including the firewall itself.
Linux 2.4.x includes NetFilter, a raw framework for filtering and mangling packets. IPTables, used for firewalling, is set inside the NetFilter framework. One of the new features in this setting is connection tracking, known to some as "stateful inspection". The four possible states it can mantain are: ESTABLISHED, NEW, RELATED and INVALID. We are interested here in the RELATED state -- it includes, among other things, the FTP DATA connections, active (PORT command) and passive (PASV command).
The module ip_conntrack_ftp is responsible for analysing FTP connections that pass through the firewall, looking for PORT and PASV commands, and including entries for those connections in the firewall's connection table. There is a security flaw in the manner in which the PORT command is interpreted and processed. Essentially, you can pass any IP/port in an FTP PORT commmand, and the module will not validate these parameters, adding an entry to the RELATED ruleset allowing connections from the FTP server, any source port, to the specified destination IP and port. In most cases, people make stringent security rules and have lax firewall rules regarding RELATED connections, allowing the attacker to connect to anywhere.
This can be used, for example, for the FTP server to connect to any TCP port on the firewall, or any other node protected by the firewall. Even though there may be rules normally denying this type of traffic, it would pass through the firewall, because of the rule allowing RELATED.
The attacker does not even need to have a valid login in the FTP server, as the PORT command is interpreted by the module independently of any authentication procedures (USER and PASS).
This is a security flaw which can be exploited when an attacker is in a position behind your firewall, i.e., "protected". For example, if your firewall protects an FTP Server and the attacker has compromised it by other means, he can use this to connect to other protected networks. Or, if your attacker is behind your firewall as a client and connects to an FTP server on the Internet, he can use it to allow this FTP server to connect to other protected networks.
Detailed description
Most firewall setups using IPTables include the following rule, for allowing established and related connections through:
iptables -A FORWARD -m state --state ESTABLISHED, RELATED -j ACCEPT
The "related" state includes connections such as the FTP data transfer connections, both active and passive modes. If related connections and FTP are allowed through the firewall, then the system is most likely vulnerable.
The attack consists in connecting to the FTP server (passing through the firewall) and using the PORT commands with arbitrary IP and port parameters - the normal parameters should be the client's IP and a random port.
To explain the process in more details, we'll outline the following scenario:
Client IP: 200.249.243.12, an IP on the internet
Firewall: 200.249.137.1 (internet interface) 200.249.193.1 (DMZ interface)
FTP server: 200.249.193.2 (inside a DMZ network, protected by the firewall)
In a normal ftp data transfer, the client would emit the following command to initiate an active data transfer:
PORT 200,249,243,12,4,10
Which would insert an entry in the connection table (cat
EXPECTING: proto=6 src=200.249.193.2 dst=200.249.243.12 sport=0 dport=1034
Allowing a connection from the FTP server to the client in the specified
port. Since the module ip_conntrack_ftp doesn't check the passed IP and
ports, an attacker can pass the following parameters:
PORT 200,249,193,1,0,22
Which would insert an entry in the connection table (cat
EXPECTING: proto=6 src=200.249.193.2 dst=200.249.193.1 sport=0 dport=22
Allowing a connection from the FTP server to the firewall, on port 22, ie, the SSH port. This will work by inserting the rule into the RELATED ruleset, which as shown above is normally too open. The rule can be inserted to any destination IP and port.
Of course, the FTP server will probably not accept the command (if it has anti-bounce protection), saying "Illegal PORT command", but the firewall will have interpreted the commands and added an "expecting related" entry as described above to its connection table. The attacker will then have ten seconds to establish the connection, before the entry expires and is removed from the connection table.
It is not even necessary to have logged in the FTP server since the module doesn't check for valid USER and PASS commands. All we have to do is trick the code into thinking we have established a connection (IP_CT_ESTABLISHED+IP_CT_IS_REPLY). To do that, it is only necessary to send any string to the FTP server, which should reply with "invalid command", and then we send the PORT command with our parameters... The FTP server will probably be complaining that a login has not been established yet, but the firewall will have done what we want it to:
220 tsunami FTP server ready.
xxxgarbagexxx
530 Please login with USER and PASS.
PORT 200,249,193,1,0,22
530 Please login with USER and PASS.
QUIT
221 Goodbye.
The implications should be obvious -- we outline two main scenarios of attack:
* The FTP server is protected by the firewall: in this case, the client (attacker) would be on the internet. If the FTP server is compromised by the attacker using other means, the attacker can insert rules allowing the FTP server to:
Connect to hosts on the internet, for downloading of trojans, tools, reverse tunnels, etc;
Connect to the firewall itself and exploit it from there onwards;
Connect to other hosts on networks protected by the firewall, such as an internal network, for example;
... use your imagination
* The client (attacker) is protected by the firewall: in this case, the client would connect to an FTP server that he controls on another network such as the internet (as long as the connection passes through the firewall). The attacker would insert rules allowing the FTP server that he controls to:
Connect to the firewall itself and attack it from there onwards;
Connect to other hosts on networks protected by the firewall, such as a DMZ or other networks for example;
... again, use your imagination
A few observations:
From my tests, the use of NAT (NAT of the FTP server, NAT of the client and NAT of the target) doesn't stop the attack in anyway. Of course, the attacker will only have to pay attention to which IP he is connecting to, but the entries are inserted into the connection table anyway.
By default, the ip_conntrack_ftp module only analyses FTP control connections on port 21, so this would only work on connections to FTP servers binding on port 21. Unless, obviously, the module were configured to listen on another port as well.
This should not need to be said
Solutions
First and foremost, you should tighten your firewall rules to limit the scope of this vulnerability, by only allowing RELATED connections to the hosts that really need them, and not to all connections.
The NetFilter core team was notified and quickly developed a patch. It is available at:
http://netfilter.samba.org/security-fix/
http://netfilter.gnumonks.org/security-fix/
http://netfilter.filewatcher.org/security-fix/
Since it is small, I've included it here:
diff -urN linux-2.4.3.orig/net/ipv4/netfilter/ip_conntrack_
--- linux-2.4.3.orig/net/ipv4/netfilter/ip_conntrack_
+++ linux/net/ipv4/netfilter/ip_conntrack_ftp.c Mon Apr 16 02:18:30 2001
@@ -187,7 +187,12 @@
(int)matchlen, data + matchoff,
matchlen, ntohl(tcph->seq) + matchoff);
-
+
+ * Update the ftp info only if the source address matches the address specified
+ * in the PORT or PASV command. Closes hole where packets could be dangerously
+ * marked as RELATED to bypass filtering rules. Thanks to Cris
I also noticed there is a patch for it on bugtrax already. How many of MS buggs are still wide open and well known?
The truth shall set you free!
Anybody using a 2.4 kernel for a production firewall needs to be LARTed, for exactly this reason. Whenever a major architecture changes, many bugs are bound to appear. I will stick with a 2.2 kernel on my firewall until 2.4 has matured a bit more. For production systems, the latest and greatest can be bad, mostly because of those early bugs. It's best to stay about a year or so behind when it comes to major upgrades. I'll probably start using 2.4 on my servers late this year. Another factor is "If it ain't broke, don't fix it." Unless a kernel is seriously broken, I won't be upgrading. I upgraded my kernel recently because of security concerns, but I won't be jumping to a new branch until it has been somewhat stressed.
First thing I thought was, "oh no not another problem with my win95 firewall" but it's a linux bug, Im safe!!