dnsop W. Hardaker
Internet-Draft Parsons, Inc.
Intended status: Standards Track W. Kumari
Expires: August 21, 2017 Google
February 17, 2017
Security Considerations for RFC5011 Publishers
draft-hardaker-rfc5011-security-considerations-04
Abstract
This document describes the math behind the minimum time-length that
a DNS zone publisher must wait before using a new DNSKEY to sign
records when supporting the RFC5011 rollover strategies.
Status of This Memo
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This Internet-Draft will expire on August 21, 2017.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements notation . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Timing associated with RFC5011 processing . . . . . . . . . . 3
5. Denial of Service Attack Considerations . . . . . . . . . . . 4
5.1. Enumerated Attack Example . . . . . . . . . . . . . . . . 4
5.1.1. Attack Timing Breakdown . . . . . . . . . . . . . . . 5
6. Minimum RFC5011 Timing Requirements . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8. Operational Considerations . . . . . . . . . . . . . . . . . 7
9. Security Considerations . . . . . . . . . . . . . . . . . . . 8
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
11. Normative References . . . . . . . . . . . . . . . . . . . . 8
Appendix A. Changes / Author Notes. . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
RFC5011 [RFC5011] defines a mechanism by which DNSSEC validators can
extend their list of trust anchors when they've seen a new key
published in a zone. However, RFC5011 [intentionally] provides no
guidance to the publishers of DNSKEYs about how long they must wait
before switching to the newly published key for signing records.
Because of this lack of guidance, zone publishers may derive
incorrect assumptions about safe usage of the RFC5011 DNSKEY
advertising and rolling process. This document describes the minimum
security requirements from a publishers point of view and is intended
to compliment the guidance offered in RFC5011 (which is written to
provide timing guidance solely to the Validating Resolvers point of
view).
To verify this lack of understanding is wide-spread, the authors
reached out to 5 DNSSEC experts to ask them how long they thought
they must wait before using a new KSK that was being rolled according
to the 5011 process. All 5 experts answered with an insecure value,
and thus we have determined that this lack of operational guidance is
causing security concerns today. We hope that this document will
rectify this understanding and provide better guidance to zone
publishers that wish to make use of the RFC5011 rollover process.
One important note about ICANN's upcoming 2017 KSK rollover plan for
the root zone: the timing values chosen for rolling the KSK in the
root zone appear completely safe, and are not in any way affected by
the timing concerns introduced by this draft
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1.1. Requirements notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Background
The RFC5011 process describes a process by which a Validating
Resolver may accept a newly published KSK as a trust anchor for
validating future DNSSEC signed records. This document augments that
information with additional constraints, as required from the DNSKEY
publication point of view. Note that it does not define any other
operational guidance or recommendations about the RFC5011 process
from a publication point of view and restricts itself to solely the
security and operational ramifications of switching to a new key too
soon. Failure of a DNSKEY publisher to follow the minimum
recommendations associated with this draft will result in potential
denial-of-service attack opportunities against validating resolvers.
3. Terminology
Trust Anchor Publisher The entity responsible for publishing a
DNSKEY that can be used as a trust anchor.
4. Timing associated with RFC5011 processing
RFC5011's process of safely publishing a new key and then making use
of that key falls into a number of high-level steps:
1. Publish a new DNSKEY in the zone but continue to sign with the
old one.
2. Wait a period of time.
3. Begin using the new DNSKEY to sign the appropriate resource
records.
4. Optionally mark the older DNSKEY as revoked and publish the
revoked key.
This document discusses step 2 of the above process. Some
interpretations of RFC5011 have erroneously determined that the wait
time is equal to RFC5011's "hold down time".
This document describes an attack based on this (common) erroneous
belief, which results in a denial of service attack against the zone
if that value is used.
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5. Denial of Service Attack Considerations
If an attacker is able to provide a RFC5011 validating engine with
past responses, such as when it is in-path or able to otherwise
perform any number of cache poisoning attacks, the attacker may be
able to leave the RFC5011-compliant validator without an appropriate
DNSKEY trust anchor. This scenario will remain until an
administrator manually fixes the situation.
The following timeline illustrates this situation.
5.1. Enumerated Attack Example
The following example settings are used in the example scenario
within this section:
TTL (all records) 1 day
DNSKEY RRSIG Signature Validity 10 days
Zone resigned every 1 day
Given these settings, the following sequence of events depicts how a
Trust Anchor Publisher that waits for only the RFC5011 hold time
timer length of 30 days subjects its users to a potential Denial of
Service attack. The timing schedule listed below is based on a new
trust anchor (a Key Signing Key (KSK)) being published at time T+0.
All numbers in this sequence refer to days before and after such an
event. Thus, T-1 is the day before the introduction of the new key,
and T+15 is the 15th day after the key was introduced into the
fictitious zone being discussed.
In this dialog, we consider two keys being published:
K_old The older KSK being replaced.
K_new The new KSK being transitioned into active use, using the
RFC5011 process.
In this dialog, the following actors are playing roles in this
situation:
Zone Signer The owner of a zone intending to publish a new Key-
Signing-Key (KSK) that will become a trust anchor by validators
following the RFC5011 process.
RFC5011 Validator A DNSSEC validator that is using the RFC5011
processes to track and update trust anchors.
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Attacker An attacker intent on foiling the RFC5011 Validator's
ability to successfully adopt the Zone Signer's K_new key as a new
trust anchor.
5.1.1. Attack Timing Breakdown
The following series of steps depicts the timeline in which an attack
occurs that foils the adoption of a new DNSKEY by a Trust Anchor
Publisher that revokes the old key too quickly.
T-1 The last signatures are published by the Zone Signer that signs
only K_old using K_old. The Attacker queries for, retrieves and
caches this keyset and corresponding signatures.
T-0 The Zone Signer adds K_new to his zone and signs the zone's key
set with K_old. The RFC5011 Validator retrieves the new key set
and corresponding signature set and notices the publication of
K_new. The RFC5011 Validator starts the (30-day) hold-down timer
for K_new.
T+5 The RFC5011 Validator queries for the zone's keyset per the
Active Refresh schedule, discussed in Section 2.3 of RFC5011.
Instead of receiving the intended published keyset, the Attacker
successfully replays the keyset and associated signatures that
they recorded at T-1. Because the signature lifetime is 10 days
(in this example), the replayed signature and keyset is accepted
as valid (being only 6 days old) and the RFC5011 Validator cancels
the hold-down timer for K_new.
T+10 The RFC5011 Validator queries for the zone's keyset and
discovers K_new again, signed by K_old (the attacker is unable to
replay the records cached at T-1, because they have now expired).
The RFC5011 Validator starts (anew) the hold-timer for K_new.
T+15,T+20, and T+25 The RFC5011 Validator continues checking the
zone's key set and lets the hold-down timer keep running without
resetting it.
T+30 The Zone Signer knows that this is the first time at which some
validators might accept K_new as a new trust anchor, since the
hold-down timer of a RFC5011 Validator not under attack that had
queried and retrieved K_new at T+0 would now have reached 30 days.
However, the hold-down timer of our attacked RFC5011 Validator is
only at 20 days.
T+35 The Zone Signer (mistakenly) believes that all validators
following the Active Refresh schedule (Section 2.3 of RFC5011)
should have accepted K_new as a the new trust anchor (since the
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hold down time of 30 days + 1/2 the signature validity period
would have passed). However, the hold-down timer of our attacked
RFC5011 Validator is only at 25 days; The replay attack at T+5
means its new hold-time timer actually started at T+10, and thus
at this time it's real hold-down timer is at T+35 - T+10 = 25
days, which is less than the RFC5011 required 30 days.
T+36 The Zone Signer, believing K_new is safe to use, switches their
active signing KSK to K_new and publishes a new DNSKEY set
signature signed with K_new. Non-attacked RFC5011 validators,
with a hold-down timer of at least 30 days, would have accepted
K_new into their set of trusted keys. But, because our attacked
RFC5011 Validator still has a hold-down timer for K_new at 26
days, it will fail to accept K_new as a trust anchor and since
K_old is no longer being used, all the KSK records from the zone
signed by K_new will be treated as invalid. Subsequently, all
keys in the key set are now unusable, invalidating all records in
the zone of any type and name.
6. Minimum RFC5011 Timing Requirements
Given the attack description in Section 5, the correct minimum length
of time required for the Zone Signer to wait before using K_new is:
waitTime = addHoldDownTime
+ (DNSKEY RRSIG Signature Validity)
+ MAX(MIN((DNSKEY RRSIG Signature Validity) / 2,
MAX(original TTL of K_old DNSKEY RRSet) / 2,
15 days),
1 hour)
+ 2 * MAX(TTL of all records)
The most confusing element of the above equation comes from the "3 *
(DNSKEY RRSIG Signature Validity) / 2" element, but is the most
critical to understand and get right.
The RFC5011 "Active Refresh" requirements state that:
A resolver that has been configured for an automatic update
of keys from a particular trust point MUST query that trust
point (e.g., do a lookup for the DNSKEY RRSet and related
RRSIG records) no less often than the lesser of 15 days, half
the original TTL for the DNSKEY RRSet, or half the RRSIG
expiration interval and no more often than once per hour.
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The important timing constraint that must be considered is the last
point at which a validating resolver may have received a replayed the
original DNSKEY set (K_old) without the new key. It's the next query
of the RFC5011 validator that the assured K_new will be seen. Thus,
the latest time a RFC5011 validator may begin their hold down timer
is an "Active Refresh" period after the last point that an attacker
can replay the K_old DNSKEY set.
The "Active Refresh" interval used by RFC5011 validator is determined
by the larger of (DNSKEY RRSIG Signature Validity) and (original TTL
for the DNSKEY RRSet). The Following text assumes that (DNSKEY RRSIG
Signature Validity) is larger of the two, which is operationally more
common today.
Thus, the worst case scenario of this attack is when the attacker can
replay K_old at just before (DNSKEY RRSIG Signature Validity). If a
RFC5011 validator picks up K_old at this this point, it will not have
a hold down timer started at all. It's not until the next "Active
Refresh" time that they'll pick up K_new with assurance, and thus
start their hold down timer. Thus, this is not at (DNSKEY RRSIG
Signature Validity) time past publication, but rather 3 * (DNSKEY
RRSIG Signature Validity) / 2.
The extra 2 * MAX(TTL of all records) is the standard added safety
margin when dealing with DNSSEC due to caching that can take place.
Because the 5011 steps require direct validation using the signature
validity, the authors aren't yet convinced it is needed in this
particular case.
For the parameters listed in Section 5.1, our example:
waitTime = 30
+ 10
+ 10 / 2
+ 2 * (1) (days)
waitTime = 47 (days)
This hold-down time of 47 days is 12 days longer than the frequently
perceived 35 days in T+35 above.
7. IANA Considerations
This document contains no IANA considerations.
8. Operational Considerations
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A companion document to RFC5011 was expected to be published that
describes the best operational practice considerations from the
perspective of a zone publisher and Trust Anchor Publisher. However,
this companion document was never written. The authors of this
document hope that it will at some point in the future, as RFC5011
timing can be tricky as we have shown. This document is intended
only to fill a single operational void that results in security
ramifications (specifically a denial of service attack against an
RFC5011 Validator). This document does not attempt to document any
other missing operational guidance for zone publishers.
9. Security Considerations
This document, is solely about the security considerations with
respect to the Trust Anchor Publisher of RFC5011 trust anchors /
keys. Thus the entire document is a discussion of Security
Considerations
10. Acknowledgements
The authors would like to especially thank to Michael StJohns for his
help and advice. We would also like to thank Bob Harold, Shane Kerr,
Matthijs Mekking, Duane Wessels, Petr Spaček, and everyone else
who assisted with this document.
11. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC5011] StJohns, M., "Automated Updates of DNS Security (DNSSEC)
Trust Anchors", STD 74, RFC 5011, DOI 10.17487/RFC5011,
September 2007, <http://www.rfc-editor.org/info/rfc5011>.
Appendix A. Changes / Author Notes.
From -00 to -02:
Additional background and clarifications in abstract.
Better separation in attack description between attacked and non-
attacked resolvers.
Some language cleanup.
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Clarified that this is maths ( and math is hard, let's go
shopping!)
Changed to " <?rfc include='reference....'?> " style references.
From -02 to -03:
Minor changes from Bob Harold
Clarified why 3/2 signature validity is needed
Changed min wait time math to include TTL value as well
From -03 to -04:
Fixed the waitTime equation to handle the difference between the
usage of the expiration time and the Active Refresh time.
More clarification text and text changes proposed by Petr
Spaček
Authors' Addresses
Wes Hardaker
Parsons, Inc.
P.O. Box 382
Davis, CA 95617
US
Email: ietf@hardakers.net
Warren Kumari
Google
1600 Amphitheatre Parkway
Mountain View, CA 94043
US
Email: warren@kumari.net
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