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Lyor Goldstein
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[apacheGH-445] lay down the groundwork for mitigating the Terrapin attack
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‎CHANGES.md

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## Behavioral changes and enhancements
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### [GH-445 - Terrapin attack mitigation](https://github.com/apache/mina-sshd/issues/429)
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There is a **new** `CoreModuleProperties` property that controls the mitigation for the [Terrapin attach](https://terrapin-attack.com/) via what is known as
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"strict-KEX" (see [OpenSSH PROTOCOL - 1.9 transport: strict key exchange extension](https://github.com/openssh/openssh-portable/blob/master/PROTOCOL)).
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It is **disabled** by default due to its experimental nature and possible interoperability issues, so users who wish to use this feature must turn it on *explicitly*.
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### New `ScpTransferEventListener` callback method
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Following [GH-428/GH-392](https://github.com/apache/mina-sshd/issues/428) a new `handleReceiveCommandAckInfo` method has been added to enable users to inspect

‎docs/standards.md

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above mentioned hooks for [RFC 8308](https://tools.ietf.org/html/rfc8308).
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* [RFC 8731 - Secure Shell (SSH) Key Exchange Method Using Curve25519 and Curve448](https://tools.ietf.org/html/rfc8731)
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* [Key Exchange (KEX) Method Updates and Recommendations for Secure Shell](https://tools.ietf.org/html/draft-ietf-curdle-ssh-kex-sha2-03)
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## *OpenSSH*
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* [OpenSSH support for U2F/FIDO security keys](https://github.com/openssh/openssh-portable/blob/master/PROTOCOL.u2f)
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* **Note:** the server side supports these keys by default. The client side requires specific initialization
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* [OpenSSH public-key certificate authentication system for use by SSH](https://github.com/openssh/openssh-portable/blob/master/PROTOCOL.certkeys)
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* [OpenSSH 1.9 transport: strict key exchange extension](https://github.com/openssh/openssh-portable/blob/master/PROTOCOL)
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## SFTP version 3-6 + extensions
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* `supported` - [DRAFT 05 - section 4.4](https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-05#section-4.4)
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* `supported2` - [DRAFT 13 section 5.4](https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-13#section-5.4)
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* `versions` - [DRAFT 09 Section 4.6](https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-09#section-4.6)
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* `vendor-id` - [DRAFT 09 - section 4.4](https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-09#section-4.4)
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* `acl-supported` - [DRAFT 11 - section 5.4](https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-11#section-5.4)
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* `newline` - [DRAFT 09 Section 4.3](https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-09#section-4.3)
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* `md5-hash`, `md5-hash-handle` - [DRAFT 09 - section 9.1.1](https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-09#section-9.1.1)
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* `check-file-handle`, `check-file-name` - [DRAFT 09 - section 9.1.2](https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-09#section-9.1.2)
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* `copy-file`, `copy-data` - [DRAFT 00 - sections 6, 7](https://tools.ietf.org/id/draft-ietf-secsh-filexfer-extensions-00.txt)
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* `space-available` - [DRAFT 09 - section 9.2](https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-09#section-9.2)
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* `filename-charset`, `filename-translation-control` - [DRAFT 13 - section 6](https://tools.ietf.org/html/draft-ietf-secsh-filexfer-13#section-6) - only client side
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* Several [OpenSSH SFTP extensions](https://github.com/openssh/openssh-portable/blob/master/PROTOCOL)
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## Miscellaneous
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* [SSH proxy jumps](./internals.md#ssh-jumps)
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* SFTP version 3-6 + extensions
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* `supported` - [DRAFT 05 - section 4.4](https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-05#section-4.4)
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* `supported2` - [DRAFT 13 section 5.4](https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-13#section-5.4)
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* `versions` - [DRAFT 09 Section 4.6](https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-09#section-4.6)
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* `vendor-id` - [DRAFT 09 - section 4.4](https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-09#section-4.4)
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* `acl-supported` - [DRAFT 11 - section 5.4](https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-11#section-5.4)
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* `newline` - [DRAFT 09 Section 4.3](https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-09#section-4.3)
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* `md5-hash`, `md5-hash-handle` - [DRAFT 09 - section 9.1.1](https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-09#section-9.1.1)
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* `check-file-handle`, `check-file-name` - [DRAFT 09 - section 9.1.2](https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-09#section-9.1.2)
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* `copy-file`, `copy-data` - [DRAFT 00 - sections 6, 7](https://tools.ietf.org/id/draft-ietf-secsh-filexfer-extensions-00.txt)
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* `space-available` - [DRAFT 09 - section 9.2](https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-09#section-9.2)
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* `filename-charset`, `filename-translation-control` - [DRAFT 13 - section 6](https://tools.ietf.org/html/draft-ietf-secsh-filexfer-13#section-6) - only client side
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* Several [OpenSSH SFTP extensions](https://github.com/openssh/openssh-portable/blob/master/PROTOCOL)
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* [Endless tarpit](https://nullprogram.com/blog/2019/03/22/) - see [HOWTO(s)](./howto.md) section.
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## Implemented/available support

‎docs/technical/kex.md

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Again, "client" and "server" could also be inverted. For instance, a client uploading
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files via SFTP might have an application thread pumping data through a channel, which
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might be blocked during KEX.
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## [OpenSSH 1.9 transport: strict key exchange extension](https://github.com/openssh/openssh-portable/blob/master/PROTOCOL)
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There is a **new** `CoreModuleProperties` property that controls the mitigation for the [Terrapin attack](https://terrapin-attack.com/) via what is known as "strict-KEX"
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It is **disabled** by default due to its experimental nature and possible interoperability issues, so users who wish to use this feature must turn it on *explicitly*.
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The pseudo KEX values are *appended* to the initial proposals sent to the peer and removed when received before proceeding with the standard KEX proposals negotiation so
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as not to interfere with it (other than marking that they were detected).

‎sshd-common/src/main/java/org/apache/sshd/common/kex/extension/KexExtensions.java

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public static final String CLIENT_KEX_EXTENSION = "ext-info-c";
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public static final String SERVER_KEX_EXTENSION = "ext-info-s";
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/**
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* Reminder:
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*
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* These pseudo-algorithms are only valid in the initial SSH2_MSG_KEXINIT and MUST be ignored if they are present in
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* subsequent SSH2_MSG_KEXINIT packets.
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*
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* <B>Note:</B> these values are <U>appended</U> to the initial proposals and removed if received before proceeding
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* with the standard KEX proposals negotiation.
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*
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* @see <A HREF="https://github.com/openssh/openssh-portable/blob/master/PROTOCOL">OpenSSH PROTOCOL - 1.9 transport:
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* strict key exchange extension</A>
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*/
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public static final String STRICT_KEX_CLIENT_EXTENSION = "[email protected]";
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public static final String STRICT_KEX_SERVER_EXTENSION = "[email protected]";
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@SuppressWarnings("checkstyle:Indentation")
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public static final Predicate<String> IS_KEX_EXTENSION_SIGNAL
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= n -> CLIENT_KEX_EXTENSION.equalsIgnoreCase(n) || SERVER_KEX_EXTENSION.equalsIgnoreCase(n);

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