Re: [Last-Call] [tsvwg] Secdir last call review of draft-ietf-tsvwg-ecn-l4s-id-26

["Valery Smyslov" <valery@xxxxxxxxxxx>]

HI Bob,

 

please, see inline.

 

From: Bob Briscoe [mailto:in@xxxxxxxxxxxxxx]
Sent: Wednesday, July 20, 2022 8:08 PM
To: Valery Smyslov; secdir@xxxxxxxx
Cc: draft-ietf-tsvwg-ecn-l4s-id.all@xxxxxxxx; last-call@xxxxxxxx; tsvwg@xxxxxxxx
Subject: Re: [tsvwg] Secdir last call review of draft-ietf-tsvwg-ecn-l4s-id-26

 

Valery,

Thank you for putting in all the work to review this. See [BB] inline.

On 19/07/2022 07:56, Valery Smyslov via Datatracker wrote:

Reviewer: Valery Smyslov

Review result: Has Issues

I have reviewed this document as part of the security directorate's ongoing

effort to review all IETF documents being processed by the IESG.  These

comments were written primarily for the benefit of the security area directors.

 Document editors and WG chairs should treat these comments just like any other

last call comments.

The draft proposes an experimental Explicit Congestion Notification protocol

for low latency, low loss and scalable throughput (L4S) networks. The draft

also proposes a way for L4S and classic traffic to co-exist in the same network

by marking the L4S traffic with a value ECT(1) in IP header.

The draft contains a Security Considerations section that refers to the

security considerations in the architecture document for L4S networks

(draft-ietf-tsvwg-l4s-arch). It also references to Appendix C.1 for discussions

of the methods used to ensure integrity of congestion notification signals and

also discusses issues arising when traffic containing different DSCP values is

encapsulated in a single VPN tunnel - the replay protection mechanism can make

low priority traffic unable to pass through.

I think that the Security Considerations section lacks discussion of what can

happen with this protocol if an attacker actively manipulates the signals on

the path. Discussion in Appendix C.1 looks insufficient to me (and it mostly

concerned with misbehaved peers and not with active attacker on the path). 

[BB] Once an attacker has broken a network operator's system or physical access control to be able to mount on-path attacks, it is surely common knowledge that the operator's service is toast. Most of the fields in the IP header have little if any protection against on-path attacks {Note 1}.

This draft defines an experimental use of one codepoint in a pre-existing field in the IP header. I don't believe that warrants mentioning the possibility that the access control of a network operator's systems could be breached. For instance, as far as I can see, there was not even discussion of on-path attacks in RFC791 or RFC2460 when IPv4 and IPv6 were defined, nor in any of their updates. And none in RFC2474 when the DSCP was defined. And none in RFC3168 when ECN was defined.

 

          Well, RFC791 doesn’t even have a security considerations section, so what? :-)

 

We could write a sentence into the draft saying that this codepoint is in the IP header, and everything in the IP header is vulnerable to on-path attack if system or physical access control is compromised. But I would rather not, because it wouldn't be specifically relevant to this draft (although I would not fight hard against it if the sec area  director insisted).

          My point was that it’s worth to mention what on-path attacker can specifically do with this protocol, not to add a sentence that such an attacker can do any kind of evil (which is generally right). For example:

 

                      An attacker capable to block packets or to modify them on the path can disrupt this protocol in the following ways:

-        re-classify L4S traffic as classic and vise-versa

-        manipulate ECN signals that may cause buffers overrun or underrun

-        ...

 

                      An attacker capable to only inject new packets into the network can disrupt this protocol in the following ways:

-        ?

________
{Note 1}: For instance, an on-path attacker can delete some or all passing packets. It can send them out of the wrong interface and increment the hop limit, creating a routing loop with no hop limit. It can change their source and/or destination addresses (appropriately changing their TCP checksums - or not for that matter). It can change their (outer) DSCP. It can reduce the hop limit (or TTL in v4) so that someone else downstream looks as if they are dropping packets before they reach their destination, and so on.

          Note also, that attacker’s capabilities may be limited to only inject new packets and not to block or modify the existing packets.

          I think it’s worth to clarify how such an attacker can influence operations of this protocol (see above).

The

conclusion made in C.1, that integrity protection of ECT(1) is not needed, must

be backed up in my opinion. 

[BB] Ah! we didn't mean that. I admit it isn't very clear, so how about this rewording (HTML format email reader required):

CURRENT:

   It is highly unlikely that ECT(1) will be needed for integrity

   protection in future. The ECN Nonce RFC [RFC3540] as been

   reclassified as historic, partly because other ways have been

   developed to protect feedback integrity of TCP and other

   transports [RFC8311] that do not consume a codepoint in the IP

   header.  For instance:

PROPOSED:

   It is highly unlikely that ECT(1) will be needed as a nonce for integrity

   protection of congestion notifications in future.  The ECN Nonce RFC [RFC3540] has been

   reclassified as historic, partly because other ways (that do 

   not consume a codepoint in the IP header) have been

   developed to protect feedback integrity of TCP and other

   transports [RFC8311] that do not consume a codepoint in the IP

   header.  For instance:

Reason:
We didn't mean that integrity protection /of/ ECT(1) is not needed.
We meant use of ECT(1) /for/ integrity protection of the rest of the ECN field is not needed, because other integrity protection methods are available (without having to burn a scarce codepoint in the IP header).

          OK.

 

The alternative methods to protect feedback

integrity looks inefficient to me (I admit, that I may be missing something,

especially with ConEx which I have only looked over, but it looks to me that

active attacker can fool these methods). What about TCP-AO - it only protects

TCP packet, so an attacker is still able to manipulate DSCP field.

[BB] What is the actionable request here?

 

         I would suggest to add some text describing what kind of integrity protection is implied here and what an attacker is still able to do

         when these mechanisms are used.

As said earlier, this draft defines an experimental use of one codepoint in a pre-existing field in the IP header. It points to three ways that congestion notification and feedback in this pre-existing field can already be protected. This draft itself doesn't make any of these three any less or any more viable.

          I understand.

Paraphrasing, your comment says "I haven't really read or thought about these methods deeply, including the output of years of work of the ConEx IETF WG, but they're probably inefficient and vulnerable to active attack." I hope you agree that this isn't a particularly constructive or respectful comment.

 

          Didn’t mean to disrespect the ConEx IETF WG work. My point was that as a security person I believe that the term “integrity protection”, that you use, implies using some cryptographic mechanism, if we talk about active attackers and not about accidental network failures. As far as I understand, among the three only TCP-AO uses it, but only to protect the TCP header, so an attacker can still manipulate the IP header fields, e.g. to re-classify L4S traffic.

 

Regarding TCP AO, the bullet already says it only protects the feedback in the TCP header.

          You may also want to mention end-to-end IPsec, that will also protect transport headers (not limited to TCP).

Part of discussion about VPN tunnels in the Security Considerations section is

a repetition of what was discussed in Section 6.2. The problem of blocking low

priority traffic by VPN replay protection mechanism is not new, but in my

opinion it is partly an operation issue depending on a threat model (users

behind VPN are usually "trusted" to some extend, but I agree that one's mileage

may vary).

[BB] Are you asking us to change any text?
I think the text already states under which circumstances the vulnerability exists (the threat model).

          I’d suggest to remove the para starting with “If the anti-replay window of a VPN egress is too small..”

          and replace it with a short text referencing Section 6.2. But it’s up to you.

The Security Considerations section also mentions the ACK-splitting attack

claiming that this recommendations prevent it, but no details are given (except

for a reference). It would be great if this para is expanded a bit.

[BB] That sentence was from one of the co-authors of all the L4S drafts, and I wrote it into the draft without checking that it was actually correct. It's very unusual for me to trust someone else without checking for myself (!). But now that I look into it, I'm not at all sure it is true.

I've emailed my co-authors, and I'll get back to you on this one.

          OK, thank you.

 

          Regards,

          Valery.

 

Bob

Nit: to my eye the phrase "optional anti-replay is mandatory in both IPsec and

DTLS" looks like oxymoron: either it is optional or it is mandatory. Note also,

that in some conditions anti-replay protection in IPsec cannot be used (with

multicast traffic).

[BB] The intended meaning was:

    "it is mandatory to allow the anti-replay facility to be disabled in both IPsec and DTLS"

We'll change it to that.


Bob

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Bob Briscoe                               http://bobbriscoe.net/

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