Hi Bernard,
I (re-)read the papers you send.
- "Extensible Authentication Protocol Vulnerabilities and Improvements
Improvements"
This paper talks attacks on availability by spoofing messages. It looks into
a small amount of ways where spoofed messages causes the TLS connection to
fail, especially it focus on alert messages.
TLS and EAP-TLS is trivial for an on-path attacker to shut down. Basically
any small error is fatal in TLS. Focusing on alert messages does not seem
correct. An attacker can e.g. at any time send a small record with length 2^15,
which causes TLS to terminate the connection. I think the only thing that can
be achieved without rewriting TLS is that availability attacks does not cause
long-term damage, i.e. the nodes can retry the handshake.
- "An Initial Security Analysis of the IEEE 802.1X Standard"
This paper discusses attacks on 801.11. The weaknesses described seems to
come from 802.11 / EAP interactions.
The discussions around IETF 102 was that the uncertainty (NewSessionTicket or
not) in TLS 1.3 was "inconvinient" and that it would be convient to get rid of
the uncertainty. Bernard then pointed out that any message changing the state
need to be authenticated to that it is not possible to spoof. I think that both
the application layer commit message as well as close_notify fulfill these old
requirements.
I think your analysis is correct.
1. What constitutes an "alternative success" indication in the EAP-TLS 1.3
protocol?
The -13 commitment message verifies that both sides are in possession of a
derived key. But the server finished already does that. As you state, the -13
commitment message is not an alternative success indication. I think it would
be easy to make it an alternative success indication be mandating that the
EAP-TLS server must only send it after verifying the client finished. I do not
think defining a new exporter is needed.
The -14 close_notify is also not an alternative success indication and can not
be made into one either.
If the requirement is that an alternative authenticated success indication is
needed. Then I think:
- We need to have an extra roundtrip.
- close_notify does not work and cannot be made to work
- Application data commit message could work as an alternative success
indication. TLS would have to be profiled so the alternative success is only
send be EAP-TLS server after client finished processed successfully.
2. At what point should keys be pushed down to the lower layer?
What is the exact requirement for eapKeyAvailable = TRUE to be set?
My understanding reading RFC 4137 (correct me if I am wrong) is that it is not
enough that the other party is authenticated, but that an alternative success
indication has been sent/received. If that is correct the EAP-TLS server would
set eapKeyAvailable = TRUE after sendign the alternative success indication and
the EAP-TLS client would set eapKeyAvailable = TRUE after receiving the
alternative success indication.
3. What constitutes an "alternative failure" indication in EAP-TLS 1.3?
Yes, I agree, receipt of TLS Alert messages should be considered an alternative
failure mechanism.
4. What is the purpose of the commitment message?
The -01 to -13 purpose was to indicate in an authenticated way that the EAP-TLS
method would not continue and that only success or failure could follow.
If an alternative success indication is required. Which it seems to be
according to your mail. I think the alternative success indication would
replace the old commitment message.
I think
Cheers,
John
From: Emu <emu-boun...@ietf.org> on behalf of Bernard Aboba
<bernard.ab...@gmail.com>
Date: Tuesday, 2 February 2021 at 16:25
To: "emu@ietf.org" <emu@ietf.org>
Subject: [Emu] Underspecification of EAP-TLS 1.3 State Machine
The EAP TLS 1.3 specification does not currently specify how EAP TLS 1.3
interacts with the EAP state machine as specified in RFC 4137. It appears to
me that this under-specification is at the root of the questions about the
commitment message.
Historically, under-specification of the EAP-TLS state machine has been a
source of potential security vulnerabilities by enabling packet injection
attacks [1][2], including attacks involving the injection of EAP-Success and
EAP-Failure mechanisms.
RFC 4137 Sections 4.1.1 and 4.1.2 define several variables:
altAccept (boolean)
Alternate indication of success, as described in
[RFC3748<https://tools.ietf.org/html/rfc3748>].
altReject (boolean)
Alternate indication of failure, as described in
[RFC3748<https://tools.ietf.org/html/rfc3748>].
eapKeyData (EAP key)
Set in peer state machine when keying material becomes available.
Set during the METHOD state. Note that this document does not
define the structure of the type "EAP key". We expect that it
will be defined in
[Keying<https://tools.ietf.org/html/rfc4137#ref-Keying>].
eapKeyAvailable (boolean)
Set to TRUE in the SUCCESS state if keying material is available.
The actual key is stored in eapKeyData.
The issue in the EAP-TLS 1.3 specification is that the interlock with these
variables is not defined.
For example, it appears to me that the commitment message verifies that both
sides are in possession of a derived key,
allowing the eapKeyData variables to be set. However, it does not appear to me
that the successful validation of the commitment message is
sufficient to allow keys to be pushed down to the lower layer, allowing data to
flow.
Therefore the eapKeyAvailable variable should not yet be set to TRUE.
Also, the commitment message does not constitute an alternative success
indication because it is possible for an
alternative failure indication (e.g. a TLS alert) to be sent after the
commitment message.
If the commitment message did constitute an alternative success mechanism, then
an attacker injecting an
EAP-Success message would be able to cause the peer to believe that
authentication
had succeeded even though it had actually failed (e.g. TLS alert sent after the
commitment message).
Given these issues, I believe the specification needs to answer several
questions:
1. What constitutes an "alternative success" indication in the EAP-TLS 1.3
protocol?
2. At what point should keys be pushed down to the lower layer?
3. What constitutes an "alternative failure" indication in EAP-TLS 1.3?
4. What is the purpose of the commitment message?
Only question 3 looks straight forward to me: receipt of TLS Alert messages
should be considered an alternative failure mechanism,
although perhaps some caveats need to be applied to address the injection
attacks described in [1].
[1] EAP Vulnerabilities and Improvements, Extensible Authentication Protocol
Vulnerabilities and Improvements
(sjsu.edu)<https://scholarworks.sjsu.edu/cgi/viewcontent.cgi?article=1431&context=etd_projects>
[2] An Analysis of the IEEE 802.1X Standard, An Initial Security Analysis of
the IEEE 802.1X Standard | Request PDF
(researchgate.net)<https://www.researchgate.net/publication/2562682_An_Initial_Security_Analysis_of_the_IEEE_8021X_Standard>
_______________________________________________
Emu mailing list
Emu@ietf.org
https://www.ietf.org/mailman/listinfo/emu
