Re: [saag] can an on-path attacker drop traffic?

Eric Rescorla <> Fri, 02 October 2020 14:03 UTC

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From: Eric Rescorla <>
Date: Fri, 2 Oct 2020 07:03:14 -0700
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To: Fernando Gont <>
Cc: Michael Richardson <>, IETF SAAG <>
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Subject: Re: [saag] can an on-path attacker drop traffic?
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As I think this discussion reveals, we don't have a really precise
description of MITM, in part because it's quite an old term from an
era where we had much less ability to analyze security protocols than
we do today. One result of improvements in analysis is the need for
precise definitions so that we can formalize the guarantees of systems
against those definitions. This process is rather further along in in
cryptography where there are very precise definitions for both the
assumptions that our primitives depend on (RO, CDH, etc.) and the
security guarantees they provide (IND-CCA2, etc.), but it is happening
in communications security as well.

In general, we need terms for both attacker capabilities and attacks.

For capabilities, our basic assumption is what is often called a
Dolev-Yao attacker, in which the attacker has complete control of the
channel (this is what 3552 describes as the Internet Threat model
[0]). However, it's also useful to try to consider more limited
attackers such as those who can only read from the wire and those who
cannot remove packets. To my knowledge, we don't have a consensus set
of precise definitions for these yet, though both 3552 and QUIC [1]
make a stab at this). Similarly, some attacks are well-defined
(reflection, identity misbinding, KCI, impersonation etc.) and some are

This brings us to MITM which is often used both for a set of
capabilities (usually roughly a Dolev-Yao attacker) and for a class of
active attacks including straight up impersonation to various kinds of
relay attacks). This broad a scope of usage isn't that useful and
rather than trying to give it a precise definition, it's probably time
to retire the term and replace it with a set of terms (perhaps
including some new terms) that do have precise definitions.



On Thu, Oct 1, 2020 at 8:01 PM Fernando Gont <> wrote:

> On 2/9/20 13:27, Michael Richardson wrote:
> [...]
> >
> > A firewall or router is a potential on-path attacker, but it can also
> drop packets.
> > What do we call this?
> > This was historically called a MITM, and it implied all the attributes of
> > on-path.  But it is unclear to me if MITM > on-path, or MITM == on-path.
> FWIW, I'd call this "man in the middle", and I'd say MITM > on-path.
> MITM implies the attacker receives the packets, and then it's up to the
> attacker to just inspect and pass on, or inspect & drop, or simply drop.
> Thanks,
> --
> Fernando Gont
> e-mail: ||
> PGP Fingerprint: 7809 84F5 322E 45C7 F1C9 3945 96EE A9EF D076 FFF1
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