[mina-sshd] branch master updated: [SSHD-1136] Use configuration property to decide whether to allow fallback to DH group exchange using SHA-1 if no suitable primes found for SHA-256

[lgoldstein]

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lgoldstein pushed a commit to branch master
in repository https://gitbox.apache.org/repos/asf/mina-sshd.git


The following commit(s) were added to refs/heads/master by this push:
     new 595c791  [SSHD-1136] Use configuration property to decide whether to 
allow fallback to DH group exchange using SHA-1 if no suitable primes found for 
SHA-256
595c791 is described below

commit 595c7911e9aa5407dbd3f7c5dc837beba7c43834
Author: Lyor Goldstein <lgoldst...@apache.org>
AuthorDate: Thu Apr 1 18:27:46 2021 +0300

    [SSHD-1136] Use configuration property to decide whether to allow fallback 
to DH group exchange using SHA-1 if no suitable primes found for SHA-256
---
 CHANGES.md                                                     |  1 +
 .../main/java/org/apache/sshd/core/CoreModuleProperties.java   |  6 ++++++
 .../src/main/java/org/apache/sshd/server/kex/DHGEXServer.java  | 10 +++++++++-
 3 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/CHANGES.md b/CHANGES.md
index 8cf7dc8..4681c75 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -54,5 +54,6 @@
 * [SSHD-1133](https://issues.apache.org/jira/browse/SSHD-1133) Added 
capability to specify a custom charset for parsing incoming commands to the 
`ScpShell`
 * [SSHD-1133](https://issues.apache.org/jira/browse/SSHD-1133) Added 
capability to specify a custom charset for returning environment variables 
related data from the `ScpShell`
 * [SSHD-1133](https://issues.apache.org/jira/browse/SSHD-1133) Added 
capability to specify a custom charset for handling the SCP protocol textual 
commands and responses
+* [SSHD-1136](https://issues.apache.org/jira/browse/SSHD-1136) Use 
configuration property to decide whether to allow fallback to DH group exchange 
using SHA-1 if no suitable primes found for SHA-256
 * [SSHD-1137](https://issues.apache.org/jira/browse/SSHD-1137) Added 
capability to override LinkOption(s) when accessing a file/folder via SFTP
 * [SSHD-1147](https://issues.apache.org/jira/browse/SSHD-1147) 
SftpInputStreamAsync: get file size before SSH_FXP_OPEN
\ No newline at end of file
diff --git 
a/sshd-core/src/main/java/org/apache/sshd/core/CoreModuleProperties.java 
b/sshd-core/src/main/java/org/apache/sshd/core/CoreModuleProperties.java
index 062166d..9a21d72 100644
--- a/sshd-core/src/main/java/org/apache/sshd/core/CoreModuleProperties.java
+++ b/sshd-core/src/main/java/org/apache/sshd/core/CoreModuleProperties.java
@@ -131,6 +131,12 @@ public final class CoreModuleProperties {
             = Property.bool("send-immediate-kex-init", true);
 
     /**
+     * Whether allowed to fall back to DH group with SHA-1 KEX if exhausted 
all available primes for SHA-256
+     */
+    public static final Property<Boolean> ALLOW_DHG1_KEX_FALLBACK
+            = Property.bool("allow-dhg1-kex-fallback", false);
+
+    /**
      * Key used to set the heartbeat interval in milliseconds (0 to disable = 
default)
      */
     public static final Property<Duration> HEARTBEAT_INTERVAL
diff --git 
a/sshd-core/src/main/java/org/apache/sshd/server/kex/DHGEXServer.java 
b/sshd-core/src/main/java/org/apache/sshd/server/kex/DHGEXServer.java
index 34c2e9d..bda216b 100644
--- a/sshd-core/src/main/java/org/apache/sshd/server/kex/DHGEXServer.java
+++ b/sshd-core/src/main/java/org/apache/sshd/server/kex/DHGEXServer.java
@@ -274,7 +274,15 @@ public class DHGEXServer extends 
AbstractDHServerKeyExchange {
         List<Moduli.DhGroup> groups = loadModuliGroups(session);
         List<Moduli.DhGroup> selected = selectModuliGroups(session, min, prf, 
max, groups);
         if (GenericUtils.isEmpty(selected)) {
-            log.warn("chooseDH({})[{}][prf={}, min={}, max={}] No suitable 
primes found, defaulting to DHG1",
+            if 
(!CoreModuleProperties.ALLOW_DHG1_KEX_FALLBACK.getRequired(session)) {
+                log.error("chooseDH({})[{}][prf={}, min={}, max={}] No 
suitable primes found - failing",
+                        this, session, prf, min, max);
+                throw new SshException(
+                        SshConstants.SSH2_DISCONNECT_KEY_EXCHANGE_FAILED,
+                        "No suitable primes found for DH group exchange");
+            }
+
+            log.warn("chooseDH({})[{}][prf={}, min={}, max={}] No suitable 
primes found - defaulting to DHG1",
                     this, session, prf, min, max);
             return getDH(new BigInteger(DHGroupData.getP1()), new 
BigInteger(DHGroupData.getG()));
         }