Original text:
challenge failed. Different challenges allow the server to obtain proof of different <del> aspects of control over an identifier.  In some challenges, like HTTP, TLS SNI, and DNS, the client directly proves its ability to do certain things related to the identifier.  The choice of which challenges to offer to a client under which circumstances is a matter of server policy. </del>  The identifier validation challenges described in this section all relate to validation of domain names.  If ACME is extended in the 8.4. <del> The TLS with Server Name Indication (TLS SNI) validation method proves control over a domain name by requiring the client to configure a TLS server referenced by the DNS A and AAAA resource records for the domain name to respond to specific connection attempts utilizing the Server Name Indication extension RFC6066.  The server verifies the client's challenge by accessing the TLS server and verifying a particular certificate is presented. The string "tls-sni-02" 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, including padding characters ("="). A client responds to this challenge by constructing a self-signed certificate which the client MUST provision at the domain name concerned in order to pass the challenge. The certificate may be constructed arbitrarily, except that each certificate MUST have exactly two subjectAlternativeNames, SAN A and SAN B.  Both MUST be dNSNames RFC5280. SAN A MUST be constructed as follows: compute the SHA-256 digest FIPS180-4 of the challenge token and encode it in lowercase hexadecimal form.  The dNSName is "x.y.token.acme.invalid", where x is the first half of the hexadecimal representation and y is the second half. SAN B MUST be constructed as follows: compute the SHA-256 digest of the key authorization and encode it in lowercase hexadecimal form. The dNSName is "x.y.ka.acme.invalid" where x is the first half of the hexadecimal representation and y is the second half. The client MUST ensure that the certificate is served to TLS connections specifying a Server Name Indication (SNI) value of SAN A. The response to the TLS-SNI challenge simply acknowledges that the client is ready to fulfill this challenge. The key authorization for this challenge.  This value MUST match the token from the challenge and the client's account key. On receiving a response, the server MUST verify that the key authorization in the response matches the "token" value in the challenge and the client's account key.  If they do not match, then the server MUST return an HTTP error in response to the POST request in which the client sent the challenge. Given a challenge/response pair, the ACME server verifies the client's control of the domain by verifying that the TLS server was configured appropriately, using these steps: Compute SAN A and SAN B in the same way as the client. Open a TLS connection to the domain name being validated, presenting SAN A in the SNI field.  This connection MUST be sent to TCP port 443 on the TLS server.  In the ClientHello initiating the TLS handshake, the server MUST include a server_name extension (i.e., SNI) containing SAN A.  The server SHOULD ensure that it does not reveal SAN B in any way when making the TLS connection, such that the presentation of SAN B in the returned certificate proves association with the client. Verify that the certificate contains a subjectAltName extension containing dNSName entries of SAN A and SAN B and no other entries.  The comparison MUST be insensitive to case and ordering of names. If all of the above verifications succeed, then the validation is successful.  Otherwise, the validation fails. 8.5. </del> When the identifier being validated is a domain name, the client can prove control of that domain by provisioning a TXT resource record containing a designated value for a specific validation domain name. HTTP: Provision files under .well-known on a web server for the domain <del> TLS SNI: Configure a TLS server for the domain </del> DNS: Provision DNS resource records for the domain There are several ways that these assumptions can be violated, both by misconfiguration and by attacks.  For example, on a web server that allows non-administrative users to write to .well-known, any user can claim to own the web server's hostname by responding to an <del> HTTP challenge, and likewise for TLS configuration and TLS SNI. Similarly, if a server that can be used for ACME validation is compromised by a malicious actor, then that malicious actor can use that access to obtain certificates via ACME. </del>  The use of hosting providers is a particular risk for ACME validation.  If the owner of the domain has outsourced operation of The DNS is a common point of vulnerability for all of these challenges.  An entity that can provision false DNS records for a domain can attack the DNS challenge directly and can provision false <del> A/AAAA records to direct the ACME server to send its TLS SNI or HTTP validation query to a remote server of the attacker's choosing. There are a few different mitigations that ACME servers can apply: </del>  Always querying the DNS using a DNSSEC-validating resolver (enhancing security for zones that are DNSSEC-enabled) hosting providers noted above are a particular case. It is RECOMMENDED that the server perform DNS queries and make HTTP <del> and TLS connections from various network perspectives, in order to make MitM attacks harder. </del>  10.3. 11.2. <del> In many cases, TLS-based services are deployed on hosted platforms that use the Server Name Indication (SNI) TLS extension to distinguish between different hosted services or "virtual hosts". When a client initiates a TLS connection with an SNI value indicating a provisioned host, the hosting platform routes the connection to that host. When a connection comes in with an unknown SNI value, one might expect the hosting platform to terminate the TLS connection. However, some hosting platforms will choose a virtual host to be the "default", and route connections with unknown SNI values to that host. In such cases, the owner of the default virtual host can complete a TLS-based challenge (e.g., "tls-sni-02") for any domain with an A record that points to the hosting platform.  This could result in mis-issuance in cases where there are multiple hosts with different owners resident on the hosting platform. A CA that accepts TLS-based proof of domain control should attempt to check whether a domain is hosted on a domain with a default virtual host before allowing an authorization request for this host to use a TLS-based challenge.  Typically, systems with default virtual hosts do not allow the holder of the default virtual host to control what certificates are presented on a request-by-request basis.  Rather, the default virtual host can configure which certificate is presented in TLS on a fairly static basis, so that the certificate presented should be stable over small intervals. A CA can detect such a bounded default vhost by initiating TLS connections to the host with random SNI values within the namespace used for the TLS-based challenge (the "acme.invalid" namespace for "tls-sni-02").  If it receives the same certificate on two different connections, then it is very likely that the server is in a default virtual host configuration.  Conversely, if the TLS server returns an unrecognized_name alert, then this is an indication that the server is not in a default virtual host configuration. </del>  11.3. <del> The http-01, tls-sni-02 and dns-01 validation methods mandate the usage of a random token value to uniquely identify the challenge. The value of the token is required to contain at least 128 bits of entropy for the following security properties.  First, the ACME client should not be able to influence the ACME server's choice of token as this may allow an attacker to reuse a domain owner's previous challenge responses for a new validation request.  Secondly, the entropy requirement prevents ACME clients from implementing a "naive" validation server that automatically replies to challenges without participating in the creation of the initial authorization request. 11.4. </del> ACME provides certificate chains in the widely-used format known colloquially as PEM (though it may diverge from the actual Privacy Enhanced Mail specifications RFC1421, as noted in RFC7468).  Some

Reviewer comments:
Recent developments[[0]] [[1]] [[2]] have identified real-world server/hosting configurations that violate the assumptions of TLS-SNI-01 and its currently specified replacement, TLS-SNI-02. In light of these issues and the feasibility of addressing them across the entire Internet it seems prudent that the ACME specification remove the affected challenge type pending the development of a better alternative (TLS-SNI-03?). This will allow the draft last-call to proceed while the details of TLS-SNI-03 are worked out. The "Default Virtual Hosts" sub-section of the "Operational Considerations" section is removed since it spoke exclusively to a concern with TLS-SNI-02 validation. When the dust has settled on TLS-SNI-03 we should certainly include a section that describes why TLS-SNI-01/02 were removed that could replace this information. [0]: URL [1]: URL [2]: URL
NAME I don't think we need a place-holder in-doc for this
r+ with the one comment addressedShould there be any mention of that makes sure we use in a future RFC in case if gets re-added?

Revised text:
challenge failed. Different challenges allow the server to obtain proof of different  <ins> aspects of control over an identifier.  In some challenges, like HTTP and DNS, the client directly proves its ability to do certain things related to the identifier.  The choice of which challenges to offer to a client under which circumstances is a matter of server policy. </ins> The identifier validation challenges described in this section all relate to validation of domain names.  If ACME is extended in the 8.4.  When the identifier being validated is a domain name, the client can prove control of that domain by provisioning a TXT resource record containing a designated value for a specific validation domain name. HTTP: Provision files under .well-known on a web server for the domain  DNS: Provision DNS resource records for the domain There are several ways that these assumptions can be violated, both by misconfiguration and by attacks.  For example, on a web server that allows non-administrative users to write to .well-known, any user can claim to own the web server's hostname by responding to an  <ins> HTTP challenge.  Similarly, if a server that can be used for ACME validation is compromised by a malicious actor, then that malicious actor can use that access to obtain certificates via ACME. </ins> The use of hosting providers is a particular risk for ACME validation.  If the owner of the domain has outsourced operation of The DNS is a common point of vulnerability for all of these challenges.  An entity that can provision false DNS records for a domain can attack the DNS challenge directly and can provision false  <ins> A/AAAA records to direct the ACME server to send its HTTP validation query to a remote server of the attacker's choosing.  There are a few different mitigations that ACME servers can apply: </ins> Always querying the DNS using a DNSSEC-validating resolver (enhancing security for zones that are DNSSEC-enabled) hosting providers noted above are a particular case. It is RECOMMENDED that the server perform DNS queries and make HTTP  <ins> connections from various network perspectives, in order to make MitM attacks harder. </ins> 10.3. 11.2.  <ins> The http-01, and dns-01 validation methods mandate the usage of a random token value to uniquely identify the challenge.  The value of the token is required to contain at least 128 bits of entropy for the following security properties.  First, the ACME client should not be able to influence the ACME server's choice of token as this may allow an attacker to reuse a domain owner's previous challenge responses for a new validation request.  Secondly, the entropy requirement prevents ACME clients from implementing a "naive" validation server that automatically replies to challenges without participating in the creation of the initial authorization request. </ins> 11.3.  ACME provides certificate chains in the widely-used format known colloquially as PEM (though it may diverge from the actual Privacy Enhanced Mail specifications RFC1421, as noted in RFC7468).  Some