| ACME Working Group | Y. Sheffer |
| Internet-Draft | Intuit |
| Intended status: Standards Track | D. Lopez |
| Expires: April 26, 2020 | O. Gonzalez de Dios |
| A. Pastor Perales | |
| Telefonica I+D | |
| T. Fossati | |
| ARM | |
| October 24, 2019 |
Support for Short-Term, Automatically-Renewed (STAR) Certificates in Automated Certificate Management Environment (ACME)
draft-ietf-acme-star-11
Public-key certificates need to be revoked when they are compromised, that is, when the associated private key is exposed to an unauthorized entity. However the revocation process is often unreliable. An alternative to revocation is issuing a sequence of certificates, each with a short validity period, and terminating this sequence upon compromise. This memo proposes an ACME extension to enable the issuance of short-term and automatically renewed (STAR) X.509 certificates.
[RFC Editor: please remove before publication]
While the draft is being developed, the editor’s version can be found at https://github.com/yaronf/I-D/tree/master/STAR.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 26, 2020.
Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved.
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The ACME protocol [RFC8555] automates the process of issuing a certificate to a named entity (an Identifier Owner or IdO). Typically, but not always, the identifier is a domain name.
If the IdO wishes to obtain a string of short-term certificates originating from the same private key (see [Topalovic] about why using short-lived certificates might be preferable to explicit revocation), she must go through the whole ACME protocol each time a new short-term certificate is needed – e.g., every 2-3 days. If done this way, the process would involve frequent interactions between the registration function of the ACME Certification Authority (CA) and the identity provider infrastructure (e.g.: DNS, web servers), therefore making the issuance of short-term certificates exceedingly dependent on the reliability of both.
This document presents an extension of the ACME protocol that optimizes this process by making short-term certificates first class objects in the ACME ecosystem. Once the Order for a string of short-term certificates is accepted, the CA is responsible for publishing the next certificate at an agreed upon URL before the previous one expires. The IdO can terminate the automatic renewal before the negotiated deadline, if needed – e.g., on key compromise.
For a more generic treatment of STAR certificates, readers are referred to [I-D.nir-saag-star].
The proposed mechanism can be used as a building block of an efficient name-delegation protocol, for example one that exists between a CDN or a cloud provider and its customers [I-D.ietf-acme-star-delegation]. At any time, the service customer (i.e., the IdO) can terminate the delegation by simply instructing the CA to stop the automatic renewal and letting the currently active certificate expire shortly thereafter.
Note that in the name delegation use case the delegated entity needs to access the auto-renewed certificate without being in possession of the ACME account key that was used for initiating the STAR issuance. This leads to the optional use of unauthenticated GET in this protocol (Section 3.4).
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.
The following subsections describe the three main phases of the protocol:
The IdO, in its role as an ACME client, requests the CA to issue a STAR certificate, i.e., one that:
Other than that, the ACME protocol flows as usual between IdO and CA. In particular, IdO is responsible for satisfying the requested ACME challenges until the CA is willing to issue the requested certificate. Per normal ACME processing, the IdO is given back an Order resource associated with the STAR certificate to be used in subsequent interaction with the CA (e.g., if the certificate needs to be terminated.)
The bootstrap phase ends when the ACME CA updates the Order resource to include the URL for the issued STAR certificate.
The CA issues the initial certificate after the authorization completes successfully. It then automatically re-issues the certificate using the same CSR (and therefore the same identifier and public key) before the previous one expires, and publishes it to the URL that was returned to the IdO at the end of the bootstrap phase. The certificate user, which could be either the IdO itself or a delegated third party, as described in [I-D.ietf-acme-star-delegation], obtains the certificate (Section 3.3) and uses it.
The refresh process (Figure 1) goes on until either:
Certificate ACME/STAR User Server | Retrieve cert | [...] |---------------------->| | | +------. / | | | / | | Automatic renewal : | | | \ | |<-----' \ | Retrieve cert | | |---------------------->| short validity period | | | | +------. / | | | / | | Automatic renewal : | | | \ | |<-----' \ | Retrieve cert | | |---------------------->| short validity period | | | | +------. / | | | / | | Automatic renewal : | | | \ | |<-----' \ | | | | [...] | [...]
Figure 1: Auto renewal
The IdO may request early termination of the STAR certificate by sending a cancellation request to the Order resource, as described in Section 3.1.2. After the CA receives and verifies the request, it shall:
Note that it is not necessary to explicitly revoke the short-term certificate.
Certificate ACME/STAR User IdO Server | | | | | Cancel Order | | +---------------------->| | | +-------. | | | | | | | End auto renewal | | | Remove cert link | | | etc. | | | | | | Done |<------' | |<----------------------+ | | | | | | Retrieve cert | +---------------------------------------------->| | Error: autoRenewalCanceled | |<----------------------------------------------+ | |
Figure 2: Termination
This section describes the protocol details, namely the extensions to the ACME protocol required to issue STAR certificates.
This protocol extends the ACME protocol, to allow for automatically renewed Orders.
The Order resource is extended with a new “auto-renewal” object that MUST be present for STAR certificates. The “auto-renewal” object has the following structure:
These attributes are included in a POST message when creating the Order, as part of the “payload” encoded object. They are returned when the Order has been created, and the ACME server MAY adjust them at will, according to its local policy (see also Section 3.2).
The optional notBefore and notAfter fields defined in Section 7.1.3 of [RFC8555] MUST NOT be present in a STAR Order. If they are included, the server MUST return an error with status code 400 “Bad Request” and type “malformedRequest”.
Section 7.1.6 of [RFC8555] defines the following values for the Order resource’s status: “pending”, “ready”, “processing”, “valid”, and “invalid”. In the case of auto-renewal Orders, the status MUST be “valid” as long as STAR certificates are being issued. We add a new status value: “canceled”, see Section 3.1.2.
A STAR certificate is by definition a dynamic resource, i.e., it refers to an entity that varies over time. Instead of overloading the semantics of the “certificate” attribute, this document defines a new attribute “star-certificate” to be used instead of “certificate”.
An important property of the auto-renewal Order is that it can be canceled by the IdO, with no need for certificate revocation. To cancel the Order, the ACME client sends a POST to the Order URL as shown in Figure 3.
POST /acme/order/ogfr8EcolOT HTTP/1.1
Host: example.org
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"kid": "https://example.com/acme/acct/gw06UNhKfOve",
"nonce": "Alc00Ap6Rt7GMkEl3L1JX5",
"url": "https://example.com/acme/order/ogfr8EcolOT"
}),
"payload": base64url({
"status": "canceled"
}),
"signature": "g454e3hdBlkT4AEw...nKePnUyZTjGtXZ6H"
}
Figure 3: Canceling an Auto-renewal Order
After a successful cancellation, the server MUST NOT issue any additional certificates for this Order.
When the Order is canceled, the server:
Issuing a cancellation for an Order that is not in “valid” state is not allowed. A client MUST NOT send such a request, and a server MUST return an error response with status code 400 (Bad Request) and type “urn:ietf:params:acme:error:autoRenewalCancellationInvalid”.
The state machine described in Section 7.1.6 of [RFC8555] is extended as illustrated in Figure 4 (State Transitions for Order Objects).
pending --------------+
| |
| All authz |
| "valid" |
V |
ready ---------------+
| |
| Receive |
| finalize |
| request |
V |
processing ------------+
| |
| First |
| certificate | Error or
| issued | Authorization failure
V V
valid invalid
|
| STAR
| Certificate
| canceled
V
canceled
Figure 4
Explicit certificate revocation using the revokeCert interface (Section 7.6 of [RFC8555]) is not supported for STAR certificates. A server receiving a revocation request for a STAR certificate MUST return an error response with status code 403 (Forbidden) and type “urn:ietf:params:acme:error:autoRenewalRevocationNotSupported”.
In order to support the discovery of STAR capabilities, the “meta” field inside the directory object defined in Section 9.7.6 of [RFC8555] is extended with a new “auto-renewal” object. The “auto-renewal” object MUST be present if the server supports STAR. Its structure is as follows:
An example directory object advertising STAR support with one day min-lifetime and one year max-duration, and supporting certificate fetching with an HTTP GET is shown in Figure 5.
{
"new-nonce": "https://example.com/acme/new-nonce",
"new-account": "https://example.com/acme/new-account",
"new-order": "https://example.com/acme/new-order",
"new-authz": "https://example.com/acme/new-authz",
"revoke-cert": "https://example.com/acme/revoke-cert",
"key-change": "https://example.com/acme/key-change",
"meta": {
"terms-of-service": "https://example.com/acme/terms/2017-5-30",
"website": "https://www.example.com/",
"caa-identities": ["example.com"],
"auto-renewal": {
"min-lifetime": 86400,
"max-duration": 31536000,
"allow-certificate-get": true
}
}
}
Figure 5: Directory object with STAR support
The certificate is fetched from the star-certificate endpoint with POST-as-GET as per [RFC8555] Section 7.4.2, unless client and server have successfully negotiated the “unauthenticated GET” option described in Section 3.4. In such case, the client can simply issue a GET to the star-certificate resource without authenticating itself to the server as illustrated in Figure 6.
GET /acme/cert/g7m3ZQeTEqa HTTP/1.1 Host: example.org Accept: application/pem-certificate-chain HTTP/1.1 200 OK Content-Type: application/pem-certificate-chain Link: <https://example.com/acme/some-directory>;rel="index" Cert-Not-Before: Thu, 3 Oct 2019 00:00:00 GMT Cert-Not-After: Thu, 10 Oct 2019 00:00:00 GMT -----BEGIN CERTIFICATE----- [End-entity certificate contents] -----END CERTIFICATE----- -----BEGIN CERTIFICATE----- [Issuer certificate contents] -----END CERTIFICATE----- -----BEGIN CERTIFICATE----- [Other certificate contents] -----END CERTIFICATE-----
Figure 6: Fetching a STAR certificate with unauthenticated GET
The Server SHOULD include the “Cert-Not-Before” and “Cert-Not-After” HTTP header fields in the response. When they exist, they MUST be equal to the respective fields inside the end-entity certificate. Their format is “HTTP-date” as defined in Section 7.1.1.2 of [RFC7231]. Their purpose is to enable client implementations that do not parse the certificate.
Following are further clarifications regarding usage of these header fields, as per [RFC7231] Sec. 8.3.1. All apply to both headers.
To improve robustness, the next certificate MUST be made available by the ACME CA at the URL pointed by “star-certificate” at the latest halfway through the lifetime of the currently active certificate. It is worth noting that this has an implication in case of cancellation: in fact, from the time the next certificate is made available, the cancellation is not completely effective until the “next” certificate also expires. To avoid the client accidentally entering a broken state, the notBefore of the “next” certificate MUST be set so that the certificate is already valid when it is published at the “star-certificate” URL. Note that the server might need to increase the auto-renewal lifetime-adjust value to satisfy the latter requirement. For a detailed description of the renewal scheduling logic, see Section 3.5. For further rationale on the need for adjusting the certificate validity, see Section 4.1.
The server MUST NOT issue any certificates for this Order with notAfter after the auto-renewal end-date.
For expired Orders, the server MUST respond with 403 (Forbidden) to any requests to the star-certificate endpoint. The response SHOULD provide additional information using a problem document [RFC7807] with type “urn:ietf:params:acme:error:autoRenewalExpired”. Note that the Order resource’s state remains “valid”, as per the base protocol.
In order to enable the name delegation workflow defined in [I-D.ietf-acme-star-delegation] as well as to increase the reliability of the STAR ecosystem (see Section 4.3 for details), this document defines a mechanism that allows a server to advertise support for accessing star-certificate resources via unauthenticated GET (in addition to POST-as-GET), and a client to enable this service with per-Order granularity.
Specifically, a server states its availability to grant unauthenticated access to a client’s Order star-certificate by setting the allow-certificate-get attribute to true in the auto-renewal object of the meta field inside the Directory object:
A client states its desire to access the issued star-certificate via unauthenticated GET by adding an allow-certificate-get attribute to the auto-renewal object of the payload of its newOrder request and setting it to true.
If the server accepts the request, it MUST reflect the attribute setting in the resulting Order object.
Note that even when the use of unauthenticated GET has been agreed, the server MUST also allow POST-as-GET requests to the star-certificate resource.
We define “nominal renewal date” as the point in time when a new short-term certificate for a given STAR Order is due. Its cadence is a multiple of the Order’s auto-renewal lifetime that starts with the issuance of the first short-term certificate and is upper-bounded by the Order’s auto-renewal end-date (Figure 7).
T - STAR Order's auto-renewal lifetime
end - STAR Order's auto-renewal end-date
nrd[i] - nominal renewal date of the i-th STAR certificate
.- T -. .- T -. .- T -. .__.
/ \ / \ / \ / end
-----------o---------o---------o---------o----X-------> t
nrd[0] nrd[1] nrd[2] nrd[3]
Figure 7: Nominal Renewal Date
The rules to determine the notBefore and notAfter values of the i-th STAR certificate are as follows:
notAfter = min(nrd[i] + T, end)
notBefore = nrd[i] - max(adjust_client, adjust_server)
Where “adjust_client” is the min between the auto-renewal lifetime-adjust value (“la”), optionally supplied by the client, and the auto-renewal lifetime of each short-term certificate (“T”); “adjust_server” is the amount of padding added by the ACME server to make sure that all certificates being published are valid at the time of publication. The server padding is a fraction f of T (i.e., f * T with .5 <= f < 1, see Section 3.3):
adjust_client = min(T, la)
adjust_server = f * T
Note that the ACME server MUST NOT set the notBefore of the first STAR certificate to a date prior to the auto-renewal start-date.
Given a server that intends to publish the next STAR certificate halfway through the lifetime of the previous one, and a STAR Order with the following attributes:
"auto-renewal": {
"start-date": "2019-01-10T00:00:00Z",
"end-date": "2019-01-20T00:00:00Z",
"lifetime": 345600, // 4 days
"lifetime-adjust": 259200 // 3 days
}
The amount of time that needs to be subtracted from each nominal renewal date is 3 days – i.e., max(min(345600, 259200), 345600 * .5).
The notBefore and notAfter of each short-term certificate are:
| notBefore | notAfter |
|---|---|
| 2019-01-10T00:00:00Z | 2019-01-14T00:00:00Z |
| 2019-01-11T00:00:00Z | 2019-01-18T00:00:00Z |
| 2019-01-15T00:00:00Z | 2019-01-20T00:00:00Z |
The value of the notBefore is also the time at which the client should expect the new certificate to be available from the star-certificate endpoint.
“Short Term” is a relative concept, therefore trying to define a cut-off point that works in all cases would be a useless exercise. In practice, the expected lifetime of a STAR certificate will be counted in minutes, hours or days, depending on different factors: the underlying requirements for revocation, how much clock synchronization is expected among relying parties and the issuing CA, etc.
Nevertheless, this section attempts to provide reasonable suggestions for the Web use case, informed by current operational and research experience.
Acer et al. [Acer] find that one of the main causes of “HTTPS error” warnings in browsers is misconfigured client clocks. In particular, they observe that roughly 95% of the “severe” clock skews – the 6.7% of clock-related breakage reports which account for clients that are more than 24 hours behind – happen to be within 6-7 days.
In order to avoid these spurious warnings about a not (yet) valid server certificate, site owners could use the auto-renewal lifetime-adjust attribute to control the effective lifetime of their Web facing certificates. The exact number depends on the percentage of the “clock-skewed” population that the site owner expects to protect – 5 days cover 97.3%, 7 days cover 99.6% – as well as the nominal auto-renewal lifetime of the STAR Order. Note that exact choice is also likely to depend on the kinds of client that is prevalent for a given site or app – for example, Android and Mac OS clients are known to behave better than Windows clients. These considerations are clearly out of scope of the present document.
In terms of security, STAR certificates and certificates with OCSP must-staple [RFC7633] can be considered roughly equivalent if the STAR certificate’s and the OCSP response’s lifetimes are the same. Given OCSP responses can be cached on average for 4 days [Stark], it is RECOMMENDED that a STAR certificate that is used on the Web has an “effective” lifetime (excluding any adjustment to account for clock skews) no longer than 4 days.
Even in the highly unlikely case STAR becomes the only certificate issuance model, discussion with the IETF TRANS Working Group and Certificate Transparency (CT) logs implementers suggests that existing CT Log Server implementations are capable of sustaining the resulting 100-fold increase in ingestion rate. Additionally, such a future, higher load could be managed with a variety of techniques (e.g., sharding by modulo of certificate hash, using “smart” load-balancing CT proxies, etc.). With regards to the increase in the log size, current CT log growth is already being managed with schemes like Chrome’s Log Policy [OBrien] which allow Operators to define their log life-cycle; and allowing the CAs, User Agents, Monitors, and any other interested entities to build-in support for that life-cycle ahead of time.
When using authenticated POST-as-GET, the HTTPS endpoint from where the STAR certificate is fetched can’t be easily replicated by an on-path HTTP cache. Reducing the caching properties of the protocol makes STAR clients increasingly dependent on the ACME server availability. This might be problematic given the relatively high rate of client-server interactions in a STAR ecosystem and especially when multiple endpoints (e.g., a high number of CDN edge nodes) end up requesting the same certificate. Clients and servers should consider using the mechanism described in Section 3.4 to mitigate the risk.
When using unauthenticated GET to fetch the STAR certificate, the server SHALL use the appropriate cache directives to set the freshness lifetime of the response (Section 5.2 of [RFC7234]) such that on-path caches will consider it stale before or at the time its effective lifetime is due to expire.
Note to RFC Editor: please remove this section before publication, including the reference to [RFC7942] and [I-D.sheffer-acme-star-request].
This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in [RFC7942]. The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors. This is not intended as, and must not be construed to be, a catalog of available implementations or their features. Readers are advised to note that other implementations may exist.
According to [RFC7942], “this will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature. It is up to the individual working groups to use this information as they see fit”.
The implementation is constructed around 3 elements: STAR Client for the Name Delegation Client (NDC), STAR Proxy for IdO and ACME Server for CA. The communication between them is over an IP network and the HTTPS protocol.
The software of the implementation is available at: https://github.com/mami-project/lurk
The following subsections offer a basic description, detailed information is available in https://github.com/mami-project/lurk/blob/master/proxySTAR_v2/README.md
This is a fork of the Let’s Encrypt Boulder project that implements an ACME compliant CA. It includes modifications to extend the ACME protocol as it is specified in this draft, to support recurrent Orders and cancelling Orders.
The implementation understands the new “recurrent” attributes as part of the Certificate issuance in the POST request for a new resource. An additional process “renewalManager.go” has been included in parallel that reads the details of each recurrent request, automatically produces a “cron” Linux based task that issues the recurrent certificates, until the lifetime ends or the Order is canceled. This process is also in charge of maintaining a fixed URI to enable the NDC to download certificates, unlike Boulder’s regular process of producing a unique URI per certificate.
The STAR Proxy has a double role as ACME client and STAR Server. The former is a fork of the EFF Certbot project that implements an ACME compliant client with the STAR extension. The latter is a basic HTTP REST API server.
The STAR Proxy understands the basic API request with a server. The current implementation of the API is defined in draft-ietf-acme-star-01. Registration or Order cancellation triggers the modified Certbot client that requests, or cancels, the recurrent generation of certificates using the STAR extension over ACME protocol. The URI with the location of the recurrent certificate is delivered to the STAR client as a response.
This is a prototype.
A STAR Client is not included in this implementation, but done by direct HTTP request with any open HTTP REST API tool. This is expected to be covered as part of the [I-D.sheffer-acme-star-request] implementation.
This implementation completely covers STAR Proxy and ACME Server with STAR extension.
The implementation is compatible with version draft-ietf-acme-star-01. The implementation is based on the Boulder and Certbot code release from 7-Aug-2017.
This implementation inherits the Boulder license (Mozilla Public License 2.0) and Certbot license (Apache License Version 2.0 ).
To prove the concept all the implementation has been done with a self-signed CA, to avoid impact on real domains. To be able to do it we use the FAKE_DNS property of Boulder and static /etc/hosts entries with domains names. Nonetheless this implementation should run with real domains.
Most of the implementation has been made to avoid deep changes inside of Boulder or Certbot, for example, the recurrent certificates issuance by the CA is based on an external process that auto-configures the standard Linux “cron” daemon in the ACME CA server.
The reference setup recommended is one physical host with 3 virtual machines, one for each of the 3 components (client, proxy and server) and the connectivity based on host bridge.
Network security is not enabled (iptables default policies are “accept” and all rules removed) in this implementation to simplify and test the protocol.
See author details below.
[[RFC Editor: please replace XXXX below by the RFC number.]]
This document requests that IANA create the following new registries:
All of these registries are administered under a Specification Required policy [RFC8126].
This document adds the following entries to the ACME Error Type registry:
| Type | Description | Reference |
|---|---|---|
| autoRenewalCanceled | The short-term certificate is no longer available because the auto-renewal Order has been explicitly canceled by the IdO | RFC XXXX |
| autoRenewalExpired | The short-term certificate is no longer available because the auto-renewal Order has expired | RFC XXXX |
| autoRenewalCancellationInvalid | A request to cancel a auto-renewal Order that is not in state “valid” has been received | RFC XXXX |
| autoRenewalRevocationNotSupported | A request to revoke a auto-renewal Order has been received | RFC XXXX |
This document adds the following entries to the ACME Order Object Fields registry:
| Field Name | Field Type | Configurable | Reference |
|---|---|---|---|
| auto-renewal | object | true | RFC XXXX |
| star-certificate | string | false | RFC XXXX |
The “ACME Order Auto Renewal Fields” registry lists field names that are defined for use in the JSON object included in the “auto-renewal” field of an ACME order object.
Template:
Initial contents: The fields and descriptions defined in Section 3.1.1.
| Field Name | Field Type | Configurable | Reference |
|---|---|---|---|
| start-date | string | true | RFC XXXX |
| end-date | string | true | RFC XXXX |
| lifetime | integer | true | RFC XXXX |
| lifetime-adjust | integer | true | RFC XXXX |
| allow-certificate-get | boolean | true | RFC XXXX |
This document adds the following entry to the ACME Directory Metadata Fields:
| Field Name | Field Type | Reference |
|---|---|---|
| auto-renewal | object | RFC XXXX |
The “ACME Directory Metadata Auto Renewal Fields” registry lists field names that are defined for use in the JSON object included in the “auto-renewal” field of an ACME directory “meta” object.
Template:
Initial contents: The fields and descriptions defined in Section 3.2.
| Field Name | Field Type | Reference |
|---|---|---|
| min-lifetime | integer | RFC XXXX |
| max-duration | integer | RFC XXXX |
| allow-certificate-get | boolean | RFC XXXX |
The “Message Headers” registry should be updated with the following additional values:
| Header Field Name | Protocol | Status | Reference |
|---|---|---|---|
| Cert-Not-Before | http | standard | RFC XXXX, Section 3.3 |
| Cert-Not-After | http | standard | RFC XXXX, Section 3.3 |
STAR certificates eliminate an important security feature of PKI which is the ability to revoke certificates. Revocation allows the administrator to limit the damage done by a rogue node or an adversary who has control of the private key. With STAR certificates, expiration replaces revocation so there is potential for lack of timeliness in the revocation taking effect. To that end, see also the discussion on clock skew in Section 4.1.
It should be noted that revocation also has timeliness issues, because both CRLs and OCSP responses have nextUpdate fields that tell relying parties (RPs) how long they should trust this revocation data. These fields are typically set to hours, days, or even weeks in the future. Any revocation that happens before the time in nextUpdate goes unnoticed by the RP.
One situation where the lack of explicit revocation could create a security risk to the IdO is when the Order is created with start-date some appreciable amount of time in the future. Recall that when authorizations have been fulfilled, the Order moves to the “valid” state and the star-certificate endpoint is populated with the first cert (Figure 4). So, if an attacker manages to get hold of the private key as well as of the first (post-dated) certificate, there is a time window in the future when they will be able to successfully impersonate the IdO. Note that cancellation is pointless in this case. In order to mitigate the described threat, it is RECOMMENDED that IdO place their Orders at a time that is close to the Order’s start-date.
More discussion of the security of STAR certificates is available in [Topalovic].
STAR adds a new attack vector that increases the threat of denial of service attacks, caused by the change to the CA’s behavior. Each STAR request amplifies the resource demands upon the CA, where one Order produces not one, but potentially dozens or hundreds of certificates, depending on the auto-renewal “lifetime” parameter. An attacker can use this property to aggressively reduce the auto-renewal “lifetime” (e.g. 1 sec.) jointly with other ACME attack vectors identified in Sec. 10 of [RFC8555]. Other collateral impact is related to the certificate endpoint resource where the client can retrieve the certificates periodically. If this resource is external to the CA (e.g. a hosted web server), the previous attack will be reflected to that resource.
Mitigation recommendations from ACME still apply, but some of them need to be adjusted. For example, applying rate limiting to the initial request, by the nature of the auto-renewal behavior cannot solve the above problem. The CA server needs complementary mitigation and specifically, it SHOULD enforce a minimum value on auto-renewal “lifetime”. Alternatively, the CA can set an internal certificate generation processes rate limit. Note that this limit has to take account of already-scheduled renewal issuances as well as new incoming requests.
In order to avoid correlation of certificates by account, if unauthenticated GET is negotiated (Section 3.4) the recommendation in Section 10.5 of [RFC8555] regarding the choice of URL structure applies, i.e. servers SHOULD choose URLs of certificate resources in a non-guessable way, for example using capability URLs [W3C.WD-capability-urls-20140218].
This work is partially supported by the European Commission under Horizon 2020 grant agreement no. 688421 Measurement and Architecture for a Middleboxed Internet (MAMI). This support does not imply endorsement.
Thanks to Ben Kaduk, Richard Barnes, Roman Danyliw, Jon Peterson, Eric Rescorla, Ryan Sleevi, Sean Turner, Alexey Melnikov, Adam Roach, Martin Thomson and Mehmet Ersue for helpful comments and discussions that have shaped this document.
[[Note to RFC Editor: please remove before publication.]]
IESG processing:
Richard and Ryan’s review resulted in the following updates: