Security Flaw with Linux 2.4 Kernel and IPTables 74
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.
Re:2.4 as a production firewall? (Score:1)
Re:Just goes to show (Score:1)
No, that's wrong. (Score:1)
IPTables and firewalls (Score:2)
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 (Score:2)
Re:Can we do something (Score:1)
Why allow reserved ports? (Score:2)
Re:Extent of the vulnerability (Score:1)
> anything, but this was hardly a subtle or complicated hole. "we want to introduce a
> way for the firewall to open and close ports on-the-fly, and in response to client
> actions, and requiring coordination between the firewall and various apps and
> daemons... hmmm what could go wrong with this?" And, what could go wrong is, the
> client might lie. omigod! who coulda thought of that!...
Here's what you missed: the client can already connect to machines behind the firewall, because it, too, is behind the firewall.
So, it can enable outside machines to connect directly. There's little added danger in that. But it's this "behind the firewall" thinking that lead to the error in the first place.
> so, my point is, if this one snuck through, do we have any sense that more subtle errors
> have not been made?
I guarantee there are other errors (20,000 lines of kernel code; you don't need to be a genius to figure out that there are bugs). A small subset of these will be exploitable in some way. A subset of these will expose *you* to something you didn't want.
Rusty.
Extent of the vulnerability (Score:5)
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.
So nobody shoule ever buy anything ? (Score:2)
Sorry, but the world simply doesn't work like that ..
--
Why pay for drugs when you can get Linux for free ?
Re:Another reason to dump FTP (Score:1)
Mirror links (Score:3)
Since the site is slashdotted, here are the links to the advisories on the netfilter sites:
Re:2.4 as a production firewall? (Score:1)
i presumed they were referring to the entire packet filtering architecture in the *bsd kernels, not the tools themselves. am i totally off in the weeds here?
Peter
Re:Just goes to show (Score:1)
Re:Just goes to show (Score:1)
Re:Can we do something (Score:2)
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Re:Just goes to show (Score:1)
I think everyone using windows has been saying "give me a nice secure copy of windows" for quite a while
Re:Can we do something (Score:2)
/.esque rant deluxe (Score:1)
Somebody pour me some hot grits.
Re:ftp (Score:3)
maybe that's why... (Score:1)
BTW I highly recommend O&M to the hackish crowd =)
"Firewall" is not the correct word, people (Score:2)
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Re:Kernel Focus - Drop the desktop focus (Score:1)
yep, macs will now help linux get software (Score:1)
Also, one an app is ported to Linux Os 10 can follow ! this is going to be good, I hope. I think there will be a bit of Linux>0S 10 ports, not only the other direction. Linux currently is accruing much more crediblity in the business world than Macintoshes have had, because of the unix background for linux, and its stability and no nosense attitude. Also, having a user base that exceeds Apple helps now, also...
Re:So how different would the response be... (Score:2)
At least we know it has been fixed in Linux.
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Perhaps I read the patch wrong (Score:2)
James
Re:2.4 as a production firewall? (Score:1)
IPfilter was ported to Linux some time ago, but it is not maintained.
With the new Netfilter framework, it should be much simpler to port IPF to Linux and easier to maintain. This would certainly be a good thing.
Open source (Score:2)
Hey guys, this is open source: there was a security issue with 2.4 - it is fixed now.
MODERATORS! (Score:2)
That was not "Overrated".
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Re:"Firewall" is not the correct word, people (Score:1)
Re:Another reason to dump FTP (Score:1)
Stick it on your web page somewhere, and then you've got a client wherever you go!
Re:Can we do something (Score:1)
treke
Fame is a vapor; popularity an accident; the only earthly certainty is oblivion.
Re:12 posts and its slashdotted already :) (Score:1)
Re:2.4 as a production firewall? (Score:2)
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
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Re:2.4 as a production firewall? (Score:5)
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
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Re:12 posts and its slashdotted already :) (Score:1)
Not as big as it might have been (Score:3)
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....
Re:ftp (Score:3)
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.
Nice FUD, champy. (Score:1)
You could say that the problem is both acknowledged [microsoft.com] and fixed [microsoft.com].
Why do I say FUD? Because there's a world of difference between a DoS and a DDoS.
Unless there's some mechanism in this overflow that allows one to elevate privelege level which the report and MS both neglected to mention?
P.S.: Linux' firewall product has a hole in it that allows attackers to use it to mount a DDoS. Like Microsoft's hole, it's patched. Wasn't this my original point?
Mandrake Firewall (Score:1)
Mike Roberto
- GAIM: MicroBerto
Re:2.4 as a production firewall? (Score:2)
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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Rundown of the article (Score:5)
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.
alternatives to ftp? (Score:1)
Re:2.4 as a production firewall? (Score:1)
If you need 4+ 9's uptime, then yes, I would definitely be sticking to the 2.2 series kernels. You Know It's Going To Work. But when you can afford a little downtime, the benefits of using 2.4 + IPTables certainly outweighs the risks. (I won't list the advantages here, as they are numerous, and we all know what they are.)
signature smigmature
Not that recent (Score:5)
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.
but (Score:1)
Re:Phew! (Score:2)
A good Win32 firewall product is WinRoute Pro [tinysoftware.com]. 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.
Re:ftp (Score:1)
I don't want to allow my internal clients to open connections to just about any port outside.
On the flip side if at my site I'm running an FTP server in passive mode, how do I protect it?
FTP is a bad protocol for firewalls. But RPC, RMI, H.323 are even worse
I don't know why they had to design FTP that way. Few (legit
Might as well switch to HTTP.
Cheerio,
Link.
"well tested"? (Score:2)
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.
Re:Kernel Focus - Drop the desktop focus (Score:1)
My point it, either do what apple did and put something out that is awesome that everyone will back, or give up. KDE and GNOME both suck. I liked afterstep or enlightenment better. As far as use ability and the way the GUI looks KDE and Gnome are worse than both MacOS and Windows. OSX rocks, it's just slow, but it will get it together but the main thing is it's good enough for corporate programs to back it and will put out commercial products that will be stable for OSX. Linux doesn't have much support for that.
Yes, linux has MAYA
So linux is riding off of OSX whether they believe it or not...
RedHat 7.1? (Score:2)
Re:RedHat 7.1? (Score:2)
Re:So how different would the response be... (Score:2)
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!
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Oh well... (Score:3)
Re:RedHat 7.1? (Score:1)
Linux distribution = osA for osB.
Bravo!
Re:Mandrake Firewall (Score:1)
Just about every unix service created has had some for of security problem, and it can happen years after it was originally created (bind/ftp/dns/yadayadayada).
Bugs won't show up until people are actually using things in production.
If everyone takes the 'wait and see' approach, then it will just take longer for the developers to find the bugs.
Look how many put the last vulnerable version of wu-ftpd into production. I don't think a 'wait and see' would have helped that situation out much.
slash . 'ed (Score:4)
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
Re:Open source (Score:2)
Linux is secure by de fault of de haxx0rs that hack de system and find de security holes.
Re:Can we do something (Score:1)
Re:Open source (Score:3)
Hey guys, this is open source: there was a security issue with 2.4 - it is fixed now.
Sure it may have been fixed, just like Microsoft has fixed tonnes of security faults with IIS, but that doesn't seem to stop the tens of thousands of vulnerable machines from waiting to be exploited. This is a big issue because something like a firewall tends to be on machines that you build and forget: My FreeBSD machine was configured, and I've almost forgotten how to configure it. It does the job and it does it well. So in other words even though the source tree somewhere may be fixed, that doesn't instantly fix all the machines out there.
On a similar note I'm still getting probed for the portmapper TCP 111 fault probably 4 times a day. Where there's smoke there's fire, and this indicates to me that there are a lot of machines out there with this vulnerability waiting to be ownzed.
So the moral of the story is this: It is largely irrelevant how quickly a patch comes out, because the sad reality is that a lot of people don't actively manage their machines (yes they should but we have to live in reality), so the mere existence of the fault is a serious problem.
Re:M$ isn't the only one with buggy code (Score:2)
2.4 as a production firewall? (Score:5)
ftp (Score:1)
If you're that concerned about security... (Score:1)
Re:alternatives to ftp? (Score:1)
it's in the latest openssh and commercial ssh packages.
Re:Why Slashdot? (Score:1)
http://netfilter.samba.org
Core Team: Harald Welte |James Morris |Marc Boucher | Rusty Russell
"Why didn't I join Microsoft? [LAUGHTER]"
I just wanted to post this link. (Score:1)
Re:M$ isn't the only one with buggy code (Score:1)
Re:12 posts and its slashdotted already :) (Score:1)
I was just about to post the same thing! Maybe someone's been messin' with their IP tables ;)
Phew! (Score:4)
Another reason to dump FTP (Score:1)
Re:Lame (Score:1)
FTP should have been implemented as a single-connection protocol, IMHO, but it's too late for that. There's always HTTP, but that seems a bit lacking in file transfer features for it to actually replace FTP. We'll just have to live with the problems of FTP, which includes, for Linux 2.4 firewall administrators, patching (and using) ip_conntrack_ftp.
Re:2.4 as a production firewall? (Score:1)
'This is not a help channel, if you need help visit #freebsdhelp'*
you wouldn't have been banned.
If you goto #freebsdhelp, the help is actually quite good. I have asked a few questions there in the past, and I've only had one question not be answered, which was kinda stupid since the answer I was looking for was in the manual pages. The question also was about programming, so I probably should have went to a programming channel anyway.
#freebsdhelp even has a website where they have tutorials, and other information about FreeBSD. I can't remember the URL, although if you visit #freebsdhelp (on EfNet), I'm sure they would give it to you. The last time I checked, the URL was in the channel topic.
The man pages in FreeBSD are very well written and complete; rarely would you need to ask someone a question becuase you can't find a man page on it. Doesn't this say atleast something to you?
I seriously don't think anyone would spend so much time writting docs and helping new users if they didn't care or want to help them.
* Ok, so thats not exactly what it says, but it's close enough
Re:Can we do something (Score:1)
Then you won't see post numbers either, so 'First Post' enthusiasts look like idiots.
Then add:
127.0.0.1 images.slashdot.org
127.0.0.1 ads.web.aol.com
To your host table and you'll not see banner ads or graphics, either.