Date: Tue, 25 Mar 2014 09:48:11 -0400 From: Olafur Gudmundsson <ogud@ogud.com> To: freebsd-security@freebsd.org Subject: Re: NTP security hole CVE-2013-5211? (Gary Palmer) Message-ID: <EAD9B42E-3A77-4254-B9C6-4B0FAFE4F246@ogud.com> In-Reply-To: <mailman.89.1395748802.54679.freebsd-security@freebsd.org> References: <mailman.89.1395748802.54679.freebsd-security@freebsd.org>
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On Mar 25, 2014, at 8:00 AM, freebsd-security-request@freebsd.org wrote: >=20 > Message: 1 > Date: Mon, 24 Mar 2014 11:02:08 -0400 > From: Gary Palmer <gpalmer@freebsd.org> > To: Brett Glass <brett@lariat.org> > Cc: "freebsd-security@freebsd.org" <freebsd-security@freebsd.org>, > Remko Lodder <remko@freebsd.org>, "Ronald F. Guilmette" > <rfg@tristatelogic.com> > Subject: Re: NTP security hole CVE-2013-5211? > Message-ID: <20140324150208.GA5238@in-addr.com> > Content-Type: text/plain; charset=3Dus-ascii >=20 > On Fri, Mar 21, 2014 at 06:13:10PM -0600, Brett Glass wrote: >> At 03:28 PM 3/21/2014, Remko Lodder wrote: >>=20 >>> Ofcourse the software should be well protected as well, and secteam@ = did his >>> best to offer the best solution possible. Though as mentioned by = Brett for >>> example we just cannot force the update of ntpd.conf on user = machines because >>> every admin could have legitimate reasons for having a configuration = in place >>> they decided to have. It's risky to change those things and = especially enforce >>> them on running machines. Most of his ideas were in the advisory = already >>> except for the 'disable monitor' part, which might be reason to = discuss >>> whether that makes sense or not. >>=20 >> I've suggested one other thing, and still think it would be a good = idea to >> thwart attacks: that we compile ntpd to source outgoing queries from = randomly >> selected ephemeral UDP ports rather than UDP port 123. (This was,=20 >> in fact, done >> in earlier releases of FreeBSD and I'm unsure why it was changed.) = This makes >> stateful firewalling less necessary and improves its performance if = it is done. >=20 >=20 > Could you please explain how randomising the source port of NTP = queries > would thwart NTP monitor amplification attacks? The attack works = because > the NTP control port on the server is always UDP/123, and I don't see = how > changing the source port would fix that. Unless I'm missing = something, you'd > need to change the port the daemon accepts queries on, not the port it = sources > outbound queries on. >=20 > Thanks, >=20 > Gary There are three problems=20 0. NTP can be tricked to give out big answer to forged addresses.=20 1. Some NTP servers listen on port 123 all address even when only = expecting to providing service on=20 "internal addresses", 2. NTP servers are easily discoverable due to the listening on fixed = port. =20 Moving the servers of port 123 will make the search for servers harder = but intrusion detection systems=20 will need to be modified to expect NTP traffic on any port.=20 IF and ONLY if people are willing to change how NTP servers are = discovered in DNS then servers can listen on any port.=20 Instead of loping up "A/AAAA" record for a name, the service discovery = should look up SRV record as that includes port number on each server. Even if everyone agrees to = make this change=20 there still has to be "temporarily" backwards hack to allow old software = to find the service.=20 (In the mail world MX (SRV equivalent) use is not universal after over = 25 years).=20 So while it is possible to move NTP servers off port 123 I do not think = it is worth the effort.=20 Olafur
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