ACME Working Group R. Shoemaker
Internet-Draft ISRG
Intended status: Standards Track September 05, 2019
Expires: March 8, 2020
ACME TLS ALPN Challenge Extension
draft-ietf-acme-tls-alpn-06
Abstract
This document specifies a new challenge for the Automated Certificate
Management Environment (ACME) protocol which allows for domain
control validation using TLS.
Status of This Memo
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This Internet-Draft will expire on March 8, 2020.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. TLS with Application Layer Protocol Negotiation (TLS ALPN)
Challenge . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. acme-tls/1 Protocol Definition . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6.1. SMI Security for PKIX Certificate Extension OID . . . . . 6
6.2. ALPN Protocol ID . . . . . . . . . . . . . . . . . . . . 6
6.3. ACME Validation Method . . . . . . . . . . . . . . . . . 6
7. Appendix: Design Rationale . . . . . . . . . . . . . . . . . 7
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
9. Normative References . . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
The Automatic Certificate Management Environment (ACME) [RFC8555]
standard specifies methods for validating control of domain names via
HTTP and DNS. Deployment experience has shown it is also useful to
be able to validate domain control using the TLS layer alone. In
particular, this allows hosting providers, CDNs, and TLS-terminating
load balancers to validate domain control without modifying the HTTP
handling behavior of their backends. This separation of layers can
improve security and usability of ACME validation.
Early ACME drafts specified two TLS-based challenge types: TLS-SNI-01
and TLS-SNI-02. These methods were removed because they relied on
assumptions about the deployed base of HTTPS hosting providers that
proved to be incorrect. Those incorrect assumptions weakened the
security of those methods and are discussed in the "Design Rationale"
appendix.
This document specifies a new TLS-based challenge type, tls-alpn-01.
This challenge requires negotiating a new application-layer protocol
using the TLS Application-Layer Protocol Negotiation (ALPN) Extension
[RFC7301]. Because no existing software implements this protocol,
the ability to fulfill tls-alpn-01 challenges is effectively opt-in.
A service provider must proactively deploy new code in order to
implement tls-alpn-01, so we can specify stronger controls in that
code, resulting in a stronger validation method.
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2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. TLS with Application Layer Protocol Negotiation (TLS ALPN) Challenge
The TLS with Application Layer Protocol Negotiation (TLS ALPN)
validation method proves control over a domain name by requiring the
client to configure a TLS server to respond to specific connection
attempts utilizing the ALPN extension with identifying information.
The ACME server validates control of the domain name by connecting to
a TLS server at one of the addresses resolved for the domain name and
verifying that a certificate with specific content is presented.
The tls-alpn-01 ACME challenge object has the following format:
type (required, string): The string "tls-alpn-01"
token (required, string): A random value that uniquely identifies
the challenge. This value MUST have at least 128 bits of entropy.
It MUST NOT contain any characters outside the base64url alphabet
as described in [RFC4648] Section 5. Trailing'=' padding
characters MUST be stripped. See [RFC4086] for additional
information on randomness requirements.
The client prepares for validation by constructing a self-signed
certificate which MUST contain a acmeIdentifier extension and a
subjectAlternativeName extension [RFC5280]. The
subjectAlternativeName extension MUST contain a single dNSName entry
where the value is the domain name being validated. The
acmeIdentifier extension MUST contain the SHA-256 digest [FIPS180-4]
of the key authorization [RFC8555] for the challenge. The
acmeIdentifier extension MUST be critical so that the certificate
isn't inadvertently used by non-ACME software.
The acmeIdentifier extension is identified by the id-pe-
acmeIdentifier object identifier (OID) in the id-pe arc [RFC5280]:
id-pe-acmeIdentifier OBJECT IDENTIFIER ::= { id-pe 31 }
The extension has the following ASN.1 [X.680] format :
Authorization ::= OCTET STRING (SIZE (32))
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The extnValue of the id-pe-acmeIdentifier extension is the ASN.1 DER
encoding [X.690] of the Authorization structure, which contains the
SHA-256 digest of the key authorization for the challenge.
Once this certificate has been created it MUST be provisioned such
that it is returned during a TLS handshake where the "acme-tls/1"
application-layer protocol has been negotiated and a Server Name
Indication (SNI) extension [RFC6066] has been provided containing the
domain name being validated.
A client responds by POSTing an empty JSON object ({}) to the
challenge URL to acknowledge that the challenge is ready to be
validated by the server. The base64url encoding of the protected
headers and payload is described in [RFC8555] Section 6.1.
POST /acme/authz/1234/1
Host: example.com
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"kid": "https://example.com/acme/acct/1",
"nonce": "JHb54aT_KTXBWQOzGYkt9A",
"url": "https://example.com/acme/authz/1234/1"
}),
"payload": base64url({}),
"signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4"
}
On receiving this request from a client the server constructs and
stores the key authorization from the challenge "token" value and the
current client account key.
The server then verifies the client's control over the domain by
verifying that the TLS server was configured as expected using the
following steps:
1. The ACME server computes the expected SHA-256 digest of the key
authorization.
2. The ACME server resolves the domain name being validated and
chooses one of the IP addresses returned for validation (the
server MAY validate against multiple addresses if more than one
is returned).
3. The AMCE server initiates a TLS connection to the chosen IP
address, this connection MUST use TCP port 443. The ACME server
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MUST provide a ALPN extension with the single protocol name
"acme-tls/1" and a SNI extension containing only the domain name
being validated during the TLS handshake.
4. The ACME server verifies that during the TLS handshake the
application-layer protocol "acme-tls/1" was successfully
negotiated (and that the ALPN extension contained only the value
"acme-tls/1") and that the certificate returned contains:
* a subjectAltName extension containing the dNSName being
validated and no other entries
* a critical acmeIdentifier extension containing the expected
SHA-256 digest computed in step 1
The comparison of dNSNames MUST be case insensitive [RFC4343]. Note
that as ACME doesn't support Unicode identifiers all dNSNames MUST be
encoded using [RFC3492] rules.
If all of the above steps succeed then the validation is successful,
otherwise it fails.
4. acme-tls/1 Protocol Definition
The "acme-tls/1" protocol MUST only be used for validating ACME tls-
alpn-01 challenges. The protocol consists of a TLS handshake in
which the required validation information is transmitted. Once the
handshake is completed the client MUST NOT exchange any further data
with the server and MUST immediately close the connection.
5. Security Considerations
The design of this challenge relies on some assumptions centered
around how a server behaves during validation.
The first assumption is that when a server is being used to serve
content for multiple DNS names from a single IP address that it
properly segregates control of those names to the users that own
them. This means that if User A registers Host A and User B
registers Host B the server should not allow a TLS request using a
SNI value for Host A to be served by User B or Host B to be served by
User A. If the server allows User B to serve this request it allows
them to illegitimately validate control of Host A to the ACME server.
The second assumption is that a server will not violate [RFC7301] by
blindly agreeing to use the "acme-tls/1" protocol without actually
understanding it.
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To further mitigate the risk of users claiming domain names used by
other users on the same infrastructure hosting providers, CDNs, and
other service providers SHOULD NOT allow users to provide their own
certificates for the TLS ALPN validation process. If providers wish
to implement TLS ALPN validation they SHOULD only generate
certificates used for validation themselves and not expose this
functionality to users.
The extensions to the ACME protocol described in this document build
upon the Security Considerations and threat model defined in
[RFC8555] Section 10.1.
6. IANA Considerations
[[RFC Editor: please replace XXXX below by the RFC number.]]
6.1. SMI Security for PKIX Certificate Extension OID
Within the SMI-numbers registry, the "SMI Security for PKIX
Certificate Extension (1.3.6.1.5.5.7.1)" table is to be updated to
add the following entry:
+---------+----------------------+------------+
| Decimal | Description | References |
+---------+----------------------+------------+
| 31 | id-pe-acmeIdentifier | RFC XXXX |
+---------+----------------------+------------+
6.2. ALPN Protocol ID
Within the Transport Layer Security (TLS) Extensions registry, the
"Application-Layer Protocol Negotiation (ALPN) Protocol IDs" table is
to be updated to add the following entry:
+------------+------------------------------------------+-----------+
| Protocol | Identification Sequence | Reference |
+------------+------------------------------------------+-----------+
| ACME-TLS/1 | 0x61 0x63 0x6d 0x65 0x2d 0x74 0x6c 0x73 | RFC XXXX |
| | 0x2f 0x31 ("acme-tls/1") | |
+------------+------------------------------------------+-----------+
6.3. ACME Validation Method
The "ACME Validation Methods" registry is to be updated to include
the following entry:
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+-------------+-----------------+-----------+
| Label | Identifier Type | Reference |
+-------------+-----------------+-----------+
| tls-alpn-01 | dns | RFC XXXX |
+-------------+-----------------+-----------+
7. Appendix: Design Rationale
The TLS ALPN challenge exists to replace the TLS SNI challenge
defined in the early ACME drafts. This challenge was convenient for
service providers who were either operating large TLS layer load
balancing systems at which they wanted to perform validation or
running servers fronting large numbers of DNS names from a single
host as it allowed validation purely within the TLS layer.
A security issue was discovered in the TLS SNI challenge by Frans
Rosen which allowed users of various service providers to
illegitimately validate control of the DNS names of other users of
the provider. When the TLS SNI challenge was designed it was assumed
that a user would only be able to respond to TLS traffic via SNI for
domain names they controlled (i.e. if User A registered Host A and
User B registered Host B with a service provider that User A wouldn't
be able to respond to SNI traffic for Host B). This turns out not to
be a security property provided by a number of large service
providers. Because of this users were able to respond to SNI traffic
for the SNI names used by the TLS SNI challenge validation process.
This meant that if User A and User B had registered Host A and Host B
respectively User A would be able to claim the SNI name for Host B
and when the validation connection was made that User A would be able
to answer, proving 'control' of Host B. As the SNI name used was a
subdomain of the domain name being validated, rather than the domain
name itself, it was likely to not already be registered with the
service provider for traffic routing, making it much easier for a
hijack to occur.
8. Acknowledgements
The author would like to thank all those whom have provided design
insights and editorial review of this document, including Richard
Barnes, Ryan Hurst, Adam Langley, Ryan Sleevi, Jacob Hoffman-Andrews,
Daniel McCarney, Marcin Walas, Martin Thomson and especially Frans
Rosen who discovered the vulnerability in the TLS SNI method which
necessitated the writing of this specification.
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9. Normative References
[FIPS180-4]
Department of Commerce, National., "NIST FIPS 180-4,
Secure Hash Standard", March 2012,
<http://csrc.nist.gov/publications/fips/fips180-4/
fips-180-4.pdf>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
for Internationalized Domain Names in Applications
(IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003,
<https://www.rfc-editor.org/info/rfc3492>.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/info/rfc4086>.
[RFC4343] Eastlake 3rd, D., "Domain Name System (DNS) Case
Insensitivity Clarification", RFC 4343,
DOI 10.17487/RFC4343, January 2006,
<https://www.rfc-editor.org/info/rfc4343>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
<https://www.rfc-editor.org/info/rfc6066>.
[RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
July 2014, <https://www.rfc-editor.org/info/rfc7301>.
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[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8555] Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
Kasten, "Automatic Certificate Management Environment
(ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,
<https://www.rfc-editor.org/info/rfc8555>.
[X.680] International Telecommunication Union, ., "Information
technology -- Abstract Syntax Notation One (ASN.1):
Specification of basic notation", 2015,
<https://www.itu.int/ITU-T/studygroups/com17/languages/
X.680-0207.pdf>.
[X.690] International Telecommunication Union, ., "Information
Technology -- ASN.1 encoding rules: Specification of Basic
Encoding Rules (BER), Canonical Encoding Rules (CER) and
Distinguished Encoding Rules (DER)", 2015,
<https://www.itu.int/ITU-T/studygroups/com17/languages/
X.690-0207.pdf>.
Author's Address
Roland Bracewell Shoemaker
Internet Security Research Group
Email: roland@letsencrypt.org
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