[apacheGH-445] lay down the groundwork for mitigating the Terrapin at…

…tack
lgoldstein · Dec 21, 2023 · 1943506 · 1943506
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diff --git a/CHANGES.md b/CHANGES.md
@@ -36,6 +36,12 @@
 
 ## Behavioral changes and enhancements
 
+### [GH-445 - Terrapin attack mitigation](https://github.com/apache/mina-sshd/issues/429)
+
+There is a **new** `CoreModuleProperties` property that controls the mitigation for the [Terrapin attach](https://terrapin-attack.com/) via what is known as
+"strict-KEX" (see [OpenSSH PROTOCOL - 1.9 transport: strict key exchange extension](https://github.com/openssh/openssh-portable/blob/master/PROTOCOL)).
+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*.
+
 ### New `ScpTransferEventListener` callback method
 
 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

diff --git a/docs/standards.md b/docs/standards.md
@@ -29,23 +29,31 @@
     above mentioned hooks for [RFC 8308](https://tools.ietf.org/html/rfc8308).
 * [RFC 8731 - Secure Shell (SSH) Key Exchange Method Using Curve25519 and Curve448](https://tools.ietf.org/html/rfc8731)
 * [Key Exchange (KEX) Method Updates and Recommendations for Secure Shell](https://tools.ietf.org/html/draft-ietf-curdle-ssh-kex-sha2-03)
+
+## *OpenSSH*
 * [OpenSSH support for U2F/FIDO security keys](https://github.com/openssh/openssh-portable/blob/master/PROTOCOL.u2f)
     * **Note:** the server side supports these keys by default. The client side requires specific initialization
 * [OpenSSH public-key certificate authentication system for use by SSH](https://github.com/openssh/openssh-portable/blob/master/PROTOCOL.certkeys)
+* [OpenSSH 1.9 transport: strict key exchange extension](https://github.com/openssh/openssh-portable/blob/master/PROTOCOL)
+
+## SFTP version 3-6 + extensions
+
+* `supported` - [DRAFT 05 - section 4.4](https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-05#section-4.4)
+* `supported2` - [DRAFT 13 section 5.4](https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-13#section-5.4)
+* `versions` - [DRAFT 09 Section 4.6](https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-09#section-4.6)
+* `vendor-id` - [DRAFT 09 - section 4.4](https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-09#section-4.4)
+* `acl-supported` - [DRAFT 11 - section 5.4](https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-11#section-5.4)
+* `newline` - [DRAFT 09 Section 4.3](https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-09#section-4.3)
+* `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)
+* `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)
+* `copy-file`, `copy-data` - [DRAFT 00 - sections 6, 7](https://tools.ietf.org/id/draft-ietf-secsh-filexfer-extensions-00.txt)
+* `space-available` - [DRAFT 09 - section 9.2](https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-09#section-9.2)
+* `filename-charset`, `filename-translation-control` - [DRAFT 13 - section 6](https://tools.ietf.org/html/draft-ietf-secsh-filexfer-13#section-6) - only client side
+* Several [OpenSSH SFTP extensions](https://github.com/openssh/openssh-portable/blob/master/PROTOCOL)
+
+## Miscellaneous
+
 * [SSH proxy jumps](./internals.md#ssh-jumps)
-* SFTP version 3-6 + extensions
-    * `supported` - [DRAFT 05 - section 4.4](https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-05#section-4.4)
-    * `supported2` - [DRAFT 13 section 5.4](https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-13#section-5.4)
-    * `versions` - [DRAFT 09 Section 4.6](https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-09#section-4.6)
-    * `vendor-id` - [DRAFT 09 - section 4.4](https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-09#section-4.4)
-    * `acl-supported` - [DRAFT 11 - section 5.4](https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-11#section-5.4)
-    * `newline` - [DRAFT 09 Section 4.3](https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-09#section-4.3)
-    * `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)
-    * `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)
-    * `copy-file`, `copy-data` - [DRAFT 00 - sections 6, 7](https://tools.ietf.org/id/draft-ietf-secsh-filexfer-extensions-00.txt)
-    * `space-available` - [DRAFT 09 - section 9.2](https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-09#section-9.2)
-    * `filename-charset`, `filename-translation-control` - [DRAFT 13 - section 6](https://tools.ietf.org/html/draft-ietf-secsh-filexfer-13#section-6) - only client side
-    * Several [OpenSSH SFTP extensions](https://github.com/openssh/openssh-portable/blob/master/PROTOCOL)
 * [Endless tarpit](https://nullprogram.com/blog/2019/03/22/) - see [HOWTO(s)](./howto.md) section.
 
 ## Implemented/available support

diff --git a/docs/technical/kex.md b/docs/technical/kex.md
@@ -129,3 +129,11 @@ thread is not overrun by producers and actually can finish.
 Again, "client" and "server" could also be inverted. For instance, a client uploading
 files via SFTP might have an application thread pumping data through a channel, which
 might be blocked during KEX.
+
+## [OpenSSH 1.9 transport: strict key exchange extension](https://github.com/openssh/openssh-portable/blob/master/PROTOCOL)
+
+
+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"
+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*.
+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
+as not to interfere with it (other than marking that they were detected).
diff --git a/sshd-common/src/main/java/org/apache/sshd/common/kex/extension/KexExtensions.java b/sshd-common/src/main/java/org/apache/sshd/common/kex/extension/KexExtensions.java
@@ -59,6 +59,21 @@ public final class KexExtensions {
     public static final String CLIENT_KEX_EXTENSION = "ext-info-c";
     public static final String SERVER_KEX_EXTENSION = "ext-info-s";
 
+    /**
+     * Reminder:
+     *
+     * These pseudo-algorithms are only valid in the initial SSH2_MSG_KEXINIT and MUST be ignored if they are present in
+     * subsequent SSH2_MSG_KEXINIT packets.
+     *
+     * <B>Note:</B> these values are <U>appended</U> to the initial proposals and removed if received before proceeding
+     * with the standard KEX proposals negotiation.
+     *
+     * @see <A HREF="https://github.com/openssh/openssh-portable/blob/master/PROTOCOL">OpenSSH PROTOCOL - 1.9 transport:
+     *      strict key exchange extension</A>
+     */
+    public static final String STRICT_KEX_CLIENT_EXTENSION = "[email protected]";
+    public static final String STRICT_KEX_SERVER_EXTENSION = "[email protected]";
+
     @SuppressWarnings("checkstyle:Indentation")
     public static final Predicate<String> IS_KEX_EXTENSION_SIGNAL
             = n -> CLIENT_KEX_EXTENSION.equalsIgnoreCase(n) || SERVER_KEX_EXTENSION.equalsIgnoreCase(n);