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TCP Vulnerability Published
Bob Slidell writes "According to Yahoo!, there is a critical flaw in TCP that affects everyone and everything. The article is scant on details and long on fear, hopefully someone will post more details on this." The advisory has more information, and is long on details but only moderate on fear.
This just hit the misc@openbsd mail list:It sounds (again) like proactive security auditting saves the day!
http://www.osvdb.org/displayvuln.php?osvdb_id=4030
...
TCP Reset Spoofing
OSVDB ID: 4030
Rating: TBD
Disclosure Date: Apr 20, 2004
Description:
The TCP stack implementation of numerous vendors contains a flaw that may allow a remote denial of service. The issue is triggered when spoofed TCP Reset packets are received by the targeted TCP stack, and will result in loss of availability for the the attacked TCP services.
Move along, little to see here.
John.
Yep, that's the issue. I submitted too, but :(.
Anyway, the way I read it you basically run the TCP attack against a BGP peering router, causing it to drop one or more of it's peering relationships. Do that enough and you can cause the routes being advertised by that router (and also TO that router from the peering connections you're breaking) to be 'dampened' - a protective mechanism in BGP to prevent a flapping route from making all the peers recalculate their routes nonstop.
It's kind of like one peer putting the other one's routes in "time-out" until he plays nice.
As frightening as this "vulnerability" sounds, this is nothing really new; other TCP weaknesses are syn floods (not quite the same thing, but somewhat similar -- in fact, this vulnerability might as well be called a "RST flood"), connection hijacking (by sniffing packets and sending spoofed packets with the correct sequence numbers), and so on. It's also an implementation issue that is largely caused by implementations having loose checking of TCP sequence and ack numbers, or accepting too large of a window of sequence numbers.
I wouldn't say TCP is broken or that some other solution would be much better; it would be tough to design a transport protocol that is still simple (and doesnt use CPU burning hashing/encryption techniques) that wouldn't have these sorts of vulnerabilities (especially since it's so easy to spoof IP packets); calling this vulnerability severe is like screaming that highways are fundamentally unsafe because someone could point their car the wrong way and start smashing into oncoming traffic.
-fren
"Where are we going, and why am I in this handbasket?"
Service providers on the other hands, must protect their routers because the BGP protocol used to distribute Internet routes between them, massively uses TCP. And when routes go missing, it is hundreds if not thousands of routes to your favourite places that go unreacheable.
The problem in the case of BGP is made worse by dampening, i.e. keeping the flapping routes out of the routing table for a certain amount of time (up to several hours). BGP routes dampening is not always configured. A determined attacker with this knowlege would be able to knock large portions of the Internet offline for hours.
I happen to work for a large, nationwide backbone. We've known about this for about a week now. BGP, configured without an MD5 key (as is usually the case) is extremely vulnerable to this exploit. This is the reason for the top-secret effort in the past week to MD5 all peering sessions, both internal and external on most major networks worldwide. Without this, it's trivial to exploit, in fact we already have source code provided by the NCISS. Input a few IPs and BGP's TCP port number, and wham you take down a peering session. For those that don't understand what this means, prior to the security changes that have been implemented in the last week, the global internet was largely susceptible to this flaw in such a way that major portions could have been taken offline easily. A priority was put on this within the intra-NOC communications channels that exist that has never been seen before to lock this down before the public knew about it. We were embargoed by DHS to not release the information until tomorrow.
3.2.1.4. TCP RST/FIN/FIN-ACK
For more information about what TCP resets are and why they can be harmful, see RFC3360.
Please correct me if I got my facts wrong.
The article is being presented at CanSecWest, and is called "Slipping in the Window" by Paul A. Watson. I have two friends at CanSecWest, I've asked them to attend and report back what the feeling is.
NANOG members are talking about it, and several regional Tier-1 players have already issued customer notifications.
This exploit goes up against TCP connections that have been established for long periods of time. i.e. not web connections. The most prevalent would be BGP peer connections, which can be up for days on end easily. Without having read details, or the paper itself, by forging packets of BGP peers with adjusted window sizes, you can cause a router to reset (possibly hang, depending on IOS or JunoOS version, not sure about this) it's BGP peer connection. If you were doing eBGP and had your own AS, a directed attack against your gateway routers could force flapping, which would cause route dampening, and lead to denial of service.
What you need to do, is contact your ISP if you are an enterprise network admin, and establish MD5 authentication on your BGP sessions. Check with Cisco or Juniper and find out if your code will drop non-MD5 BGP packets directed at it. An ACL won't do, the attacker would forge the src-ip of a known peer.
This is a completely non-trivial attack to coordinate. You need to know the IP address of the BGP peer of a customer, or the route reflector, and then get the IP address right in an attempt to bypass ACLs and get the BGP session to hang. eBGP multihop means that IP could be any number of routers, and unless you have inside info, you don't know what it is.
Potentially, looking glasses could be used to mount attacks at NAPs or other peering points, but again, I think the major players will be ready for it very shortly, and will spend most of today (if they are any good) coordinating with legal teams to slam the shit out of any forged sessions they see, and start cooperating to run traces with other providers.
If I could editorialize one moment, none of this would be an issue if providers took better care to implement anti-spoofing techniques. Forged src-ip addresses are the bane of security. Most of these attacks don't care about 2-way communcations, they just want to reset connections. Spoofed src-ip lets them do that. Rant off.
- It mainly affects the Border Gateway Protocol (BGP) that occurs at a high level in the net. Few computers are involved.
- The issue was first publicized about a month ago at CanSecWest, so those in the know have had a month to work on this.
- The steps to mitigate the problem are a matter of tweaking settings (like window size) or setting up protocols (like encryption). This is not a matter of rewriting the entire protocol.
The article is interesting in that it shows how the bits and pieces of the Internet fit together, but I won't be losing any sleep over this one.===== Murphy's Law is recursive. =====
Real quote.
God Bless America. Why? Did it sneeze?
The NISCC articles explains things clearly. Nobody needs any more information. Besides, this "new" attack was already known. People just didn't realize how easy it is.
Here's a link to US-CERT's TA04-111A on this topic.
As they state, there is a simple solution: TCP MD5 Signature Option with BGP. Any ISP worth their salt will already be doing it. The rest will learn the hard way.
.
This has been supported in Cisco IOS way back since ~1998 in IOS 11.2
Read the BGP "Bible": Internet Routing Architectures or look at any best-practices guides which will state that TCP MD5 sigs should always be used with BGP.
Or search CCO:
router bgp 109
neighbor 145.2.2.2 password v61ne0qkel33&
It's just a single line that has to be added to both peer sides.
The issue described in this advisory is the practicability of resetting an established TCP connection by sending suitable TCP packets with the RST (Reset) or SYN (Synchronise) flags set.
The packets need to have source and destination IP addresses that match the established connection as well as the same source and destination TCP ports.
The fact that TCP sessions can be reset by sending suitable RST and SYN packets is a design feature of TCP according to RFC 793, but a reset attack is only possible at all because the source IP address and TCP port can be forged or "spoofed".
Although denial of service using crafted TCP packets is a well known weakness of TCP, until recently it was believed that a successful denial of service attack was not achievable in practice. The reason for this is that the receiving TCP implementation checks the sequence number of the RST or SYN packet, which is a 32 bit number, giving a probability of 1/232 of guessing the sequence number correctly (assuming a random distribution).
The discoverer of the practicability of the RST attack was Paul A. Watson, who describes his research in his paper "Slipping In The Window: TCP Reset Attacks", presented at the CanSecWest 2004 conference. He noticed that the probability of guessing an acceptable sequence number is much higher than 1/232 because the receiving TCP implementation will accept any sequence number in a certain range (or "window") of the expected sequence number. The window makes TCP reset attacks practicable.
Any application protocol which relies on long term TCP connections and for which the source and destination IP addresses and TCP ports are known or can be easily guessed will be vulnerable to at least denial of service attacks
Saying Java is nice because it works on all OS's is like saying that anal sex is nice because it works on all genders.
A recently published I-D (here) claims 200 seconds is sufficient time for a broadband sub to successfully attack a TCP session, provided their ISP doesn't use egress filtering (and way too few do so).
This is rather serious. Whether you personally aren't susceptible is irrelevant if your upstreams are.
Maybe I missed something in the advisory, but this sounds like a good old TCP reset attack...
You did.
The news here is that you no longer require the sniffer, because you don't have to know the exact sequence number. Now you just have to be kind of close. How close apparently varies in size amongst vendor implementations.
LOL OK I happen to setup BGP router as part of my living so I might be able to shed some light on the subject. Standard security settings allready in place will stop this attack. OK Lets go for starters, BGP packets unless multihop should have a TTL of exactly 1 and come through a point to point interface. Basic anti-spoofing on each side will stop any packets that cleam to be from the other end of that interface from comming in any other interface same for any packets with the routers own address from comming in any interface. Thats basic router security. BGP does support preshared keys as well though I'm not sure if that will stop this attack as it's more to prevent session hijacking. I dont see a 'fix' for this comming out besides normal security settings.
This exploit along with many others could be made significatly harder to do if more ISP would just turn on anti spoofing rules. I cant think of a router that dosent support at least manual anti spoofing setups. The only time antispoofing might be a problem would be people that are utilizing multiple connections and trying to agrigate them together for outgoing bandwith.
No sir I dont like it.
Note: that's 1/2^32 (substitute your own favorite exponential syntax), not one in two hundred thirty-two.
Oh, say does that Star-Spangled Banner entwine / The myrtle of Venus with Bacchus's vine?
This came up on the linux-kernel mailing list two years ago during a thread on TCP MD5 stuff to avoid this very problem. Why is it just now making a splash?
This post from Alan Cox covers it nicely. Also see the rest of the thread.
Guessing a port and IP is fairly easy. Guessing the sequence number is not. This is why making sure that TCP initial sequence numbers are random is important.
This is old news.
For the insanely paranoid use a hardware entropy generator (TRNG) for choosing ISN's.
There are all sorts of attacks against network protocols when poor random number sources are used. This is but one example...
Title: Juniper NetScreen Advisory 58784
t ao/co ntact.html
Date: 21 April 2004
Version: 1
Impact:
A design flaw in the RFC specification of TCP may allow a blind attacker to successfully close a TCP connection.
Affected Products:
Juniper NetScreen Firewalls (all versions)
NISCC Reference: Vul/236929
http://www.uniras.gov.uk/vuls/2004/236929/tcp.htm
Max Risk: Medium
Summary:
A blind attacker with limited knowledge of a TCP connection may be able to successfully brute force the TCP Sequence number space and thereby cause a connection endpoint or firewall stateful filter to process a spoofed RST packet and close the connection.
Details:
The TCP Sequence number is one of the mechanisms that TCP uses to prevent a third party from inserting forged packets into the data-stream between two other hosts. While such an attack has always been known to be theoretically possible against TCP, it is was believed that the range of over four billion possible TCP Sequence numbers was large enough to prevent a successful attack. However, recently such an attack has been proven to be feasible in certain situations.
Specifically, if two hosts are known to talk to each other on a regular basis and/or for long periods of time over known port(s) then it may be possible for an attacker to brute force the TCP Sequence number space and successfully inject a forged packet into the connection and possibly disrupt communications. Certain connections, such as BGP4, which are long lived between two devices are especially vulnerable.
While all TCP connections are potentially vulnerable, NetScreen believes that NetScreen firewalls running BGP4 or with TCP Syn-Check enabled are likely to be vulnerable in practice. Other protocols such as SSH, HTTP and SMTP which usually have shorter connection times are less vulnerable.
Recommended Actions:
NetScreen firewall customers should do one or more of the following:
1) Configure Anti-Spoof protection as appropriate.
2) Use secure protocols such as ssh, HTTPS, BGP4 w/ MD5 Authentication
and IPSec which are more resistant to attack.
3) Limit management to dedicated and/or internal interfaces
4) Upgrade to ScreenOS 5.0r6 which enhances the stateful firewall
functionality to protect devices on the network.
Patch Availability:
ScreenOS version 5.0r6 is currently available for Juniper NetScreen firewalls.
How to get ScreenOS:
Customers with a valid product warranty or a support contract may download the software from the Juniper NetScreen CSO web portal:
http://www.juniper.net/support/
For all other customers, including those with expired support contracts, please call your regional Juniper NetScreen TAC center at one of the numbers listed in:
http://www.juniper.net/support/nscn_support/
Select option 2 from the telephone menu and be sure to select the correct product from the phone tree. Once connected with an engineer state that you are calling in regards to a Security Advisory and provide the title of this notice as evidence of your entitlement to the specified release.
As with any new software installation, NetScreen customers planning to upgrade to any version of ScreenOS should carefully read the release notes and other relevant documentation before beginning any upgrade.
Wrong. Very wrong. Are you anaware of this field called "banking"? What about "financial trading"? These firms have huge portfolios and run and re-run various models on them. At night, the systems have to run various "end-of-day" scripts and reports, which take CPU-hours to complete. Most of this things run on Unix...
There is also on-the-fly image manipulation, and the scene-rendering done by fleets of Unix machines. The more CPUs each such Unix machine can fit, the better.
Then come databases -- depending on the queries (with joins and orderings), DB-servers can be CPU bound and appreciate multiple processors when available.
What about PVM? What was it -- and similar packages both free and commercial -- written for? What about this proverbial "beowulf clusters"? Of course, it is much better to have several CPUs inside the box, rather than in separate machines.
Until which time, the OpenBSD zealots will continue to deny the issue exists or is of any importance. I see...
In Soviet Washington the swamp drains you.
Actually, this is just a variation on an old attack with sequence number guessing. Some OSes have a very poor generator for these numbers (even though the vulnerability is known for ages) and it is possible to exploit it. Probably the most famous attack of this sort was Mitnick's break-in into Shimomura's machine ten years ago.
In practice, these attacks are quite tricky to perform, because they require good timing and quite a bit of skills, so they are quite rare. It is far easier to just exploit some common buffer overflow than this.
The impact could be not only DOS (by resetting the connection), but you can do also packet injection with potential for total system compromise. However, if the protocol used is encrypted (in Shimomura's case it wasn't, if I am not mistaken, Mitnick attacked rsh or rlogin), then the DOS is probably the worst thing that could happen to you. The encryption will reject the bogus packets, if properly implemented.
So, keep calm - this was here since at least 80-ties and there isn't much that you can do against it, except to use encryption. You can only hope, that your system vendor will decide to make their TCP stack less vulnerable (not likely to happen, since many systems still ship with the crappy sequence number generator!). Full fix is not possible anyway, unless you want to break the protocol.
Regards, JanThere is RFC 2385 titled Protection of BGP Sessions via the TCP MD5 Signature Option. In the first paragraph of its Introduction section it says -
The primary motivation for this option is to allow BGP to protect itself against the introduction of spoofed TCP segments into the connection stream. Of particular concern are TCP resets.
The date of publishing - August 1998, which makes it about 6 years old.
However the proposed option was primarily meant as a quick BGP fixup, which should've got depricated as soon as IPsec got RFC'ed and deployed. It did went standard few months later, but IPsec implementations enjoyed full share of cross-vendor compatibility problems.
With time Authenticated Header (AH) IPsec protocol didn't see much use or acceptance and IPsec slowly evolved (and keeps evolving) into confidentiality rather than authentication layer.
Besides it's an IP security after all, while the RST spoofing is a TCP problem. And one can quite rightfully percieve authenticating TCP with IPsec as an overkill.
Anyhow, long story short - it's a known and rather old problem with handful of existing and equally unattractive solutions. Perhaps this time around it will be addressed better due to the amount of the publicity it's aimed to get.
3.243F6A8885A308D313