Re: Attacking networks using DHCP, DNS - probably kills DNSSEC
In message <5.1.1.6.2.20030628124252.033e5600@idiom.com>, Bill Stewart writes:
Somebody did an interesting attack on a cable network's customers. They cracked the cable company's DHCP server, got it to provide a "Connection-specific DNS suffic" pointing to a machine they owned, and also told it to use their DNS server. This meant that when your machine wanted to look up yahoo.com, it would look up yahoo.com.attackersdomain.com instead.
This looks like it has the ability to work around DNSSEC. Somebody trying to verify that they'd correctly reached yahoo.com would instead verify that they'd correctly reached yahoo.com.attackersdomain.com, which can provide all the signatures it needs to make this convincing.
So if you're depending on DNSSEC to secure your IPSEC connection, do make sure your DNS server doesn't have a suffix of echelon.nsa.gov...
No, that's just not true of DNSsec. DNSsec doesn't depend on the integrity of the connection to your DNS server; rather, the RRsets are digitally signed. In other words, it works a lot like certificates, with a trust chain going back to a magic root key. I'm not saying that there can't be problems with that model, but compromised DNS servers (and poisoned DNS caches) are among the major threat models it was designed to deal with. If nothing else, the existence of caching DNS servers, which are not authoritative for the information they hand out, makes a transmission-based solution pretty useless. --Steve Bellovin, http://www.research.att.com/~smb (me) http://www.wilyhacker.com (2nd edition of "Firewalls" book) --------------------------------------------------------------------- The Cryptography Mailing List Unsubscribe by sending "unsubscribe cryptography" to majordomo@metzdowd.com
At 11:15 PM 06/28/2003 -0400, Steven M. Bellovin wrote:
In message <5.1.1.6.2.20030628124252.033e5600@idiom.com>, Bill Stewart writes:
This looks like it has the ability to work around DNSSEC. Somebody trying to verify that they'd correctly reached yahoo.com would instead verify that they'd correctly reached yahoo.com.attackersdomain.com, which can provide all the signatures it needs to make this convincing.
So if you're depending on DNSSEC to secure your IPSEC connection, do make sure your DNS server doesn't have a suffix of echelon.nsa.gov...
No, that's just not true of DNSsec. DNSsec doesn't depend on the integrity of the connection to your DNS server; rather, the RRsets are digitally signed. In other words, it works a lot like certificates, with a trust chain going back to a magic root key.
I thought about that, and I think this is an exception, because this attack tricks your machine into using the trust chain yahoo.com.attackersdomain.com., which it controls, instead of the trust chain yahoo.com., which DNSSEC protects adequately. So you're getting a trustable answer to the wrong query. I'm less sure of the implementation issues of the "Connection-specific DNS suffix", and I've seen conflicting documentation. If the resolver looks up "domain.suffix" before "domain", then the attacker's DNS doesn't need to control the DNS access, and only needs to provide the attacker's certificates, but if the resolver looks up "domain" before "domain.suffix", then the attacker also needs to make sure that the lookup of "domain" fails, which is most easily done by telling the DHCP client to use the attacker's DNS server along with telling it the suffix. (That doesn't add any extra work to the attack, but does make it a bit easier to trace the attacker after the fact; if you're not replacing the attacker's DNS server entry, then all you need is a legitimate-looking server for "*.attackersdomain.com". In either case, somebody who can pull off this kind of an attack probably uses a compromised machine to run the DNS server on anyway.)
I'm not saying that there can't be problems with that model, but compromised DNS servers (and poisoned DNS caches) are among the major threat models it was designed to deal with. If nothing else, the existence of caching DNS servers, which are not authoritative for the information they hand out, makes a transmission-based solution pretty useless.
DNSSEC seems to do a pretty thorough job of making sure that if you look up the correct domain name, you'll get the correct answer, in spite of attackers trying to prevent it. But this attack tricks you into looking up the wrong domain name, and DNSSEC makes sure that you get the correct answer for the wrong name, which isn't the result you want.
Bill Stewart <bill.stewart@pobox.com> writes:
At 11:15 PM 06/28/2003 -0400, Steven M. Bellovin wrote:
In message <5.1.1.6.2.20030628124252.033e5600@idiom.com>, Bill Stewart writes:
This looks like it has the ability to work around DNSSEC. Somebody trying to verify that they'd correctly reached yahoo.com would instead verify that they'd correctly reached yahoo.com.attackersdomain.com, which can provide all the signatures it needs to make this convincing.
So if you're depending on DNSSEC to secure your IPSEC connection, do make sure your DNS server doesn't have a suffix of echelon.nsa.gov...
No, that's just not true of DNSsec. DNSsec doesn't depend on the integrity of the connection to your DNS server; rather, the RRsets are digitally signed. In other words, it works a lot like certificates, with a trust chain going back to a magic root key.
I thought about that, and I think this is an exception, because this attack tricks your machine into using the trust chain yahoo.com.attackersdomain.com., which it controls, instead of the trust chain yahoo.com., which DNSSEC protects adequately. So you're getting a trustable answer to the wrong query.
No, I believe only one of the following situations can occur: * Your laptop see and uses the name "yahoo.com", and the DNS server translate them into yahoo.com.attackersdomain.com. If your laptop knows the DNSSEC root key, the attacker cannot spoof yahoo.com since it doesn't know the yahoo.com key. This attack is essentially a man-in-the-middle attack between you and your recursive DNS server. * Your laptop see and uses the name "yahoo.com.attackersdomain.com". You may be able to verify this using your DNSSEC root key, if the attackersdomain.com people have set up DNSSEC for their spoofed entries, but unless you are using bad software or judgment, you will not confuse this for the real "yahoo.com". Of course, everything fails if you ALSO get your DNSSEC root key from the DHCP server, but in this case you shouldn't expect to be secure. I wouldn't be surprised if some people suggest pushing the DNSSEC root key via DHCP though, because alas, getting the right key into the laptop in the first place is a difficult problem.
At 11:49 PM 06/29/2003 +0200, Simon Josefsson wrote:
No, I believe only one of the following situations can occur:
* Your laptop see and uses the name "yahoo.com", and the DNS server translate them into yahoo.com.attackersdomain.com. If your laptop knows the DNSSEC root key, the attacker cannot spoof yahoo.com since it doesn't know the yahoo.com key. This attack is essentially a man-in-the-middle attack between you and your recursive DNS server.
That doesn't happen. (Well, it could, but as you point out, it's not a successful attack methodology, because DNSSEC was designed to correctly take care of this.)
* Your laptop see and uses the name "yahoo.com.attackersdomain.com". You may be able to verify this using your DNSSEC root key, if the attackersdomain.com people have set up DNSSEC for their spoofed entries, but unless you are using bad software or judgment, you will not confuse this for the real "yahoo.com".
The DNS suffix business is designed so that your laptop tries to use "yahoo.com.attackersdomain.com", either before "yahoo.com" or after unsuccessfully trying "yahoo.com", depending on implementation. It may be bad judgement, but it's designed to support intranet sites for domains that want their web browsers and email to let you refer to "marketing" as opposed to "marketing.webservers.example.com", and Netscape-derived browsers support it as well as IE.
Of course, everything fails if you ALSO get your DNSSEC root key from the DHCP server, but in this case you shouldn't expect to be secure. I wouldn't be surprised if some people suggest pushing the DNSSEC root key via DHCP though, because alas, getting the right key into the laptop in the first place is a difficult problem.
I agree with you and Steve that this would be a Really Bad Idea. The only way to make it secure is to use an authenticated DHCP, which means you have to put authentication keys in somehow, plus you need a reasonable response for handling authentication failures, which means you need a user interface as well. It's also the wrong scope, since the DNSSEC is global information, not connection-oriented information, so it's not really DHCP's job. --------------------------------------------------------------------- The Cryptography Mailing List Unsubscribe by sending "unsubscribe cryptography" to majordomo@metzdowd.com
Bill Stewart <bill.stewart@pobox.com> writes:
* Your laptop see and uses the name "yahoo.com.attackersdomain.com". You may be able to verify this using your DNSSEC root key, if the attackersdomain.com people have set up DNSSEC for their spoofed entries, but unless you are using bad software or judgment, you will not confuse this for the real "yahoo.com".
The DNS suffix business is designed so that your laptop tries to use "yahoo.com.attackersdomain.com", either before "yahoo.com" or after unsuccessfully trying "yahoo.com", depending on implementation. It may be bad judgement, but it's designed to support intranet sites for domains that want their web browsers and email to let you refer to "marketing" as opposed to "marketing.webservers.example.com", and Netscape-derived browsers support it as well as IE.
It can be a useful feature, but it does not circumvent DNSSEC in any way, that I can see. DNSSEC see yahoo.com.attackersdomain.com and can verify that the IP addresses for that host are the one that the owner of the y.c.a.c domain publishes, and that is what DNSSEC delivers. The bad judgement I referred to was if your software, after DNSSEC verification, confuses yahoo.com with yahoo.com.attackersdomain.com.
Of course, everything fails if you ALSO get your DNSSEC root key from the DHCP server, but in this case you shouldn't expect to be secure. I wouldn't be surprised if some people suggest pushing the DNSSEC root key via DHCP though, because alas, getting the right key into the laptop in the first place is a difficult problem.
I agree with you and Steve that this would be a Really Bad Idea. The only way to make it secure is to use an authenticated DHCP, which means you have to put authentication keys in somehow, plus you need a reasonable response for handling authentication failures, which means you need a user interface as well. It's also the wrong scope, since the DNSSEC is global information, not connection-oriented information, so it's not really DHCP's job.
I think it is simpler to have the DNSSEC root key installed with the DNSSEC software. If someone can replace the root key in that distribution channel, they could also modify your DNSSEC software, so you are no worse off.
participants (3)
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Bill Stewart -
Simon Josefsson -
Steven M. Bellovin