Re: Port-Knocking vulnerabilities?

[Michael Rash <mbr () cipherdyne org>]

On Jan 01, 2008, Craig Wright wrote:

> First, you have mentioned SPA, and true this offers more then port knocking, you have also mixed port knocking and 
> SPA up in a couple of your comments.
>
> Now you make the comment that SPA can "encrypt" and store in the IP ID. IP ID is a 16 bit flag. That is there are a 
> max of 65535 values. Even with 4 IP packets it is functionally equivilant to a 3 character password. Given a standard 
> ADSL line and 68 byte packets I can send all combinations in just under 7.5 seconds (and this is not doing any 
> analysis on the IPID).
>
> That is SPA and not port knocking. That is the MORE secure of the two options.

Agreed.

> Yes this way will make a log entry, but are you sitting at the server 24x7 and monitoring ALL scans. Will you stop me 
> in less then 8 seconds?
>
> When did people consider a 3 character password safe?

That is why any implementation of SPA that just uses network or
transport layer headers to communicate information instead of
application layer data is inherently deficient and not worthy of the
SPA designation IMHO (even though technically, yes, it is just a single
packet).

In addition to the small key space that just using the 16 bit IP ID field
limits any such implementation to, there are additional limitations
including*:

- Such "SPA" packets are replayable against the target (unless some horrid
  S/KEY style iteration of a hash function is used or someone manually
  changes around the key for each SPA packet).  If the application layer
  were used (such as in fwknop: http://www.cipherdyne.org/fwknop) the
  replay problem is trivially solved by prepending every SPA packet with
  16 bytes of random data before it is encrypted.  This is possible
  because of the relatively large amount of data that can be transfered
  within the application layer payload of a single UDP packet.

- It is difficult to protect against MITM attacks because the source IP
  itself in the actual network layer header is allowed through the
  firewall policy on the server side.  Fwknop allows the source IP to be
  placed within the encrypted SPA payload, and is therefore not subject to
  alteration by an attacker that possesses an inline device that can stop
  an SPA packet and retransmit it with a different source IP.

- It is impossible to communicate client-side information that the
  server might find useful because the usage of packet headers is too
  restrictive.  A true SPA implementation can use 2048-bit GnuPG keys
  and do things like communicate crypt() passwords so the server can
  interface with additional authentication mechanisms.  Fwknop supports
  both Rijndael and GnuPG keys, crypt() passwords, and even the
  transmission of full shell commands.

Here is a list of features supported by fwknop - all of these are
possible because of the usage of payload data instead of just packet
headers for SPA communications:

http://www.cipherdyne.org/fwknop/docs/features.html

*Disclaimer: I developed fwknop so obviously I'm biased.  However, I
welcome any critique that can point out a non-obvious limitation in the
fwknop architecture or implementation.

--
Michael Rash
http://www.cipherdyne.org/
Key fingerprint = 53EA 13EA 472E 3771 894F  AC69 95D8 5D6B A742 839F



> Rgards,
> Dr Craig Wright (GSE-Compliance)
>
>
>
> Craig Wright
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> ________________________________________
>
> From: listbounce () securityfocus com [listbounce () securityfocus com] On Behalf Of Goldstein101 [goldstein101 () 
> gmail com]
> Sent: Tuesday, 1 January 2008 6:40 AM
> To: Robert Inder
> Cc: Ansgar -59cobalt- Wiechers; security-basics () securityfocus com
> Subject: Re: Port-Knocking vulnerabilities?
>
> I guess most of you haven't bothered to check what port knocking
> really is capable of 'cause I'm reading a lot of things that are not
> true.
>
> First of all, Port Knocking is just another layer of security. Think
> of it as the door of a room that contains a safe. You first have to
> break the port knocking daemon and then the safe. Not that easy,
> believe me.
>
> Second, Who said port knocking is like transmitting a password in
> cleartext? Most modern PK systems are encryption based. An attacker
> can sniff port numbers but packets usually contain other information
> that is used for authentication.
>
> For example I use Aldaba Knocking Suite (aldabaknocking.com) which
> provides Port Knocking and Single Packet Authorization.
>
> In Port knocking mode, basically the client sends this: [IP
> Address][Port Number][Open/Close Flag][Checksum].
>
> That information is encrypted and sent encoded in the IP-Id field of 4
> TCP-SYN packets. This way you have 2 forms of authentication: The
> first is simple: you need to know the exact port numbers to use when
> sending those TCP-Syn Packets. Second: you need to know a secret (the
> encryption key) used to encrypt the information. If you don't have it,
> you can send random data but when decrypted, it won't verify the
> checksum.
>
> However, Port Knocking has some disadvantages and vulnerabilities. A
> better and more elegant approach is SPA. Check it out. There are some
> papers out there.
>
>
> ..
>
>
>
>
>
>
>
>
>
>
>
> On Dec 31, 2007 7:27 PM, Robert Inder <robertinder () googlemail com> wrote:
> > On 29/12/2007, Ansgar -59cobalt- Wiechers <bugtraq () planetcobalt net> wrote:
> > > On 2007-12-28 Jay wrote:
> > > > Portknocking is a security mechanism as it is a type of
> > > > authentication. "Something you know" in this case the sequence of
> > > > ports to knock before a unstarted service or daemon begins listening
> > > > for connections.
> > >
> > > Since everything is transmitted in the clear port-knocking is as much of
> > > a security mechanism as cleartext passwords. Technically: maybe
> > > (depending on your definition). Realistically: no.
> >
> > I think your dismissal of port knocking (and, indeed, plain text
> > passwords) is unrealistic.
> >
> > If you can intercept my interaction with some remote server, you can
> > steal the relevant secrets (the password or the sequence of ports).
> >
> > But isn't that quite a substantial "if"?
> >
> > How are you going to do it?  Aren't you going to have to compromise
> > some other machine, either where I am, or where the server is (or, I
> > guess, where the relevant DNS records are), and then plant software to
> > deliberately wait and watch until a relevant interaction takes place?
> >
> > I'm not saying that's impossible.  But it would take considerable
> > knowledge, planning and effort.
> >
> > Why doesn't that make it a substantial defence against most kinds of
> > casual attack?
> >
> > Robert.
> >
> > --
> > Robert Inder        Interactive Information Ltd,            Registered
> > in Scotland
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> > louder than words
>
>
>
> --
> Goldstein.
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