Subject: [OPSAWG] Roman Danyliw's Discuss on draft-ietf-opsawg-tacacs-13: (with DISCUSS and COMMENT)

From: Roman Danyliw via Datatracker <[email-masked]>
Date: Wed, 15 May 2019 12:12:50 -0700

Roman Danyliw has entered the following ballot position for
draft-ietf-opsawg-tacacs-13: Discuss

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DISCUSS:
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(1) I appreciate the deliberate and thoughtful attempt in this section to
enumerate the possible risks/attacks and mitigations of the protocol as is.  In
addition to the top-level risks in Section 10.1, I can see the value of
maintaining symmetry between Sections 5+10.2; 6+10.3 and 7+10.4.  In the spirit
of the middle ground this draft is trying to realize (document the as-is, but
highlight the issues), I have the following feedback:

(a) Section 10.1.  I recommend replacing the first three paragraphs of Section
10.1 (“TACACS+ protocol does not …”, “While the protocol …”, and “Even though
…”) with the following text synthesized from Joe Salowey’s LC review
(https://mailarchive.ietf.org/arch/msg/secdir/rsqrNbVEKph1RdWh836Ard73pHs) and
the current introduction:

TACACS+ protocol does not include a security mechanism that would meet
modern-day requirements.  These security mechanisms would be best referred to
as “obfuscation” and not “encryption” since they provide no meaningful
integrity, privacy or replay protection.  An attacker with access to the data
stream should be assumed to be able to read and modify all TACACS+ packets.
Without mitigation, a range of risks such as the following are possible:

Accounting information may be modified by the man-in-the-middle attacker,
making such logs unsuitable and untrustable for auditing purposes.

Invalid or misleading values may be inserted by the man-in-the-middle attacker
in various fields at known offsets to try and circumvent the authentication or
authorization checks even inside the obfuscated body.

(b) I recommend finding an alternative home and strengthening the text “For
this reason, deployments SHOULD NOT use connections with
TAC_PLUS_UNENCRYPTED_FLAG, as mentioned in the Best Practices section (Section
10.5)”.  It seemed odd to mix deployment guidance in a list of risks as
currently written.  I take Andrej Ota’s point from
https://mailarchive.ietf.org/arch/msg/secdir/UgtsSfh1RaauNoMRi87FRqtI0YI that
there is no harm in requiring the obfuscation, such as it is.  Furthermore, why
couldn’t this be MUST NOT use?

(c) Section 10.5.3.  I concur with the SECDIR recommendation and the follow-up
discussion with Andrej Ota per
https://mailarchive.ietf.org/arch/msg/secdir/UgtsSfh1RaauNoMRi87FRqtI0YI which
would: s/stronger authentication/less weak/

(2) Section 10.2.  I’m confused by the deprecation of
TAC_PLUS_AUTHEN_STATUS_FOLLOW but a seemingly weaker “SHOULD NOT be used in
modern deployments”.  I was expecting a MUST NOT.

(3) Section 10.4.  Why shouldn’t accounting sessions also use secure transport
per 10.5 (like 10.3 and 10.4) given the risks outlined in the text?  I was
expecting to see this section open with “Accounting Session SHOULD be used via
a secure transport (see Best Practices section (Section 10.5))".

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COMMENT:
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(1) Editorial Nits:

** Section 10.5.3.  Typo.  s/administraots/administrators/
** Global.  Various places in the document have an extra space between the end
of a reference and the closing period.  Recommend: s/] ./]./g

(2) I endorse Mirja and Deborah’s point that strong text is needed in Section 1
to state that this document is describing the current deployment of the
protocol which has serious security issues.