Re: [Int-area] Adam Roach's Discuss on draft-ietf-intarea-provisioning-domains-10: (with DISCUSS and COMMENT)

[Tommy Pauly <[e-mail]>]

Hi Adam,

Thanks again for bringing this up! I've updated our text to include mitigations for this attack. It can be found here (https://github.com/IPv6-mPvD/mpvd-ietf-drafts/pull/25), but here's an overview of the proposed text:

In Section 4.1, I've added two new paragraphs. The first describes time limits on fetching PvD info:

In addition to adding a random delay when fetching Additional Information, hosts
MUST enforce a minimum time between requesting Additional Information
for a given PvD on the same network. This minimum time is RECOMMENDED
to be 10 seconds, in order to avoid hosts causing a denial-of-service on the
PvD server. Hosts also MUST limit the number of requests that are made to
different PvD Additional Information servers on the same network within a short
period of time. A RECOMMENDED value is to issue no more than five PvD
Additional Information requests in total on a given network within 10 seconds.
For more discussion, see {{security}}.

The second also makes clear the behavior to take in case of failure, which will be the case for non-PvD web servers:

If the request for PvD Additional Information fails due to a TLS error,
an HTTP error, or because the retrieved file does not contain valid PvD JSON,
hosts MUST close any connection used to fetch the PvD Additional Information,
and MUST NOT request the information for that PvD ID again for the duration
of the local network attachment. For more discussion, see {{security}}.

In addition, I added text to the Security Considerations:

An attacker generating RAs on a local network can use the H-flag and the PvD ID
to cause hosts on the network to make requests for PvD Additional Information
from servers. This can become a denial-of-service attack if not mitigated. To mitigate
this attack, hosts MUST limit the rate at which they fetch a particular PvD's
Additional Information, limit the rate at which they fetch any PvD Additional
Information on a given local network, and stop making requests to any PvD ID
that does not respond with valid JSON. Details are provided in {{retr}}. This attack
can be targeted at generic web servers, in which case the host behavior of stopping
requesting for any server that doesn't behave like a PvD Additional Information server
is critical. For cases in which an attacker is pointing hosts at a valid PvD Additional
Information server (but one that is not actually associated with the local network),
the server SHOULD reject any requests that do not originate from the expected IPv6
prefix as described in {{serverop}}.

The existing text referenced here about server behavior is:

The server providing the JSON files SHOULD also check whether the
client address is contained by the prefixes listed in the additional
information, and SHOULD return a 403 response code if there is no
match.

Let me know if this addresses your concerns!

Best,
Tommy

> On Jan 21, 2020, at 9:26 PM, Adam Roach via Datatracker <[e-mail]> wrote:
> 
> ----------------------------------------------------------------------
> DISCUSS:
> ----------------------------------------------------------------------
> 
> Thanks to the authors and working group for their work on this document.  I
> have one major concern about the ability for this mechanism to be abused to
> form DDoS attacks, described below. Unfortunately, while I have identified the
> attack, I don't have an easy solution to propose that mitigates it satisfactorily.
> 
> I also have a handful of mostly editorial comments on the document.
> 
> ---------------------------------------------------------------------------
> 
> §6:
> 
> I was expecting to see a discussion of the DDoS attack that may result from a
> large network (or a rogue host on such a network) sending out a PvD ID
> containing the hostname of a victim machine, and setting the "H" flag.
> 
> Since the messages used to trigger these HTTP connections are extremely
> lightweight, unauthenticated UDP messages, and the resulting HTTP connections
> require the exchange of a significant number of packets in addition to a
> number of cryptographic operations, this is a very high ratio amplification
> attack, both in terms of network and CPU resources.
> 
> Given that the delay setting comes from the network instead of being
> independently computed by the host, such an attack could be honed to be
> particularly devastating.  Although it isn't a complete mitigation, one
> approach to consider would be moving computation of the delay upper bound to
> the host, or specifying a minimum upper bound of several minutes (where a
> smaller value will cause the host to use this minimum upper bound).
> 
> Regardless of how this is ultimately handled, I think this is a pretty severe
> risk that needs addressing in the document prior to publication.
>