SSH connection hangs with Unknown SSH message types 26, 120, 136 on macOS (ProtocolsSharp 2026.1.29.1)
SSH connection hangs with Unknown SSH message types 26, 120, 136 on macOS (ProtocolsSharp 2026.1.29.1)
**Summary**
SSH connections hang silently after authentication when connecting to an OpenSSH 9.2p1 server (Debian 12 / Univention Corporate Server 5.2). The terminal opens but no output appears and the session is unresponsive. The following messages are logged:
```
Devolutions.ProtocolsSharp: 2026.1.29.1 OS: macOS
Unknown SSH message type: 120
Unknown SSH message type: 136
Unknown SSH message type: 26
```
**Environment**
- RDM macOS, ProtocolsSharp 2026.1.29.1
- macOS (Apple Silicon)
- Server: OpenSSH_9.2p1, OpenSSL 3.0.18 (Debian 12 / UCS 5.2)
**Steps to Reproduce**
1. Create an SSH Shell entry targeting an OpenSSH 9.2p1 server on Debian 12
2. Connect using key-based authentication
3. Observe the above log messages — the session hangs with no prompt or output
**Expected Behavior**
Unknown message types should be handled gracefully (ignored or responded to with SSH_MSG_UNIMPLEMENTED per RFC 4253) and the connection should proceed normally.
**Actual Behavior**
The connection hangs indefinitely after authentication. No shell prompt is displayed.
**Authentication Method**
The issue occurs regardless of how the SSH key is provided:
- Linking a vault SSH key entry directly to the session
- Using the RDM SSH key agent
Both authentication methods produce the exact same hang. Ideally I would like to use the SSH key agent workflow, but it is currently unusable due to this bug.
**Additional Notes**
- The same server, same SSH key, and same credentials work perfectly when connecting via iTerm2 (native macOS OpenSSH client)
- The server configuration is entirely standard — no unusual extensions or custom sshd_config options
- Message types 26 (KEX/algorithm negotiation range), 120 (channel-related range), and 136 (client protocols range) appear to be unrecognized by ProtocolsSharp
- Since the server is OpenSSH 9.2p1 and does NOT support newer extensions like ObscureKeystrokeTiming (added in 9.5+), these message types are not being sent by the server — this suggests the issue may be on the client side within ProtocolsSharp itself
**Workaround**
Setting the SSH entry display mode to External (iTerm2) bypasses ProtocolsSharp entirely and works correctly.
All Comments (3)
Hello,
Thanks for the detailed report. Would it be possible for you to enable logging with verbose level 2 and send the logs it produces after trying to connect ? It would help us pinpoint the error better.
You can set the settings here:
Best regards,
Maxim Buisson
Screenshot 2026-05-04 at 11.23.06 AM.png
Hello,
thank you for your fast reply, here are the requestes logs:
[04.05.2026 18:27:10] Devolutions.ProtocolsSharp: 2026.2.12.1 OS: macOS [04.05.2026 18:27:10] Terminal font: JetBrains Mono [JetBrains Mono, fixed=True] [04.05.2026 18:27:10] Starting SSH, verbose level: 2 [04.05.2026 18:27:10] Setting up connection [04.05.2026 18:27:10] Connecting to: 10.0.4.10:22 (IPV_ANY) [04.05.2026 18:27:10] SSH version banner: SSH-2.0-OpenSSH_9.2p1 Debian-2+deb12u7 [04.05.2026 18:27:10] Sending kex init [04.05.2026 18:27:10] Received kex init [04.05.2026 18:27:10] Selected algorithms: sntrup761x25519-sha512(strict), ssh-ed25519, chacha20-poly1305@openssh.com, chacha20-poly1305@openssh.com, implicit by cipher, implicit by cipher, zlib@openssh.com, zlib@openssh.com [04.05.2026 18:27:10] Sending sntrup x25519 kex init [04.05.2026 18:27:10] Received sntrup x25519 kex reply [04.05.2026 18:27:10] Successfully authentified server [04.05.2026 18:27:10] Sending new keys message [04.05.2026 18:27:10] Received new keys message [04.05.2026 18:27:10] Sending userauth service request [04.05.2026 18:27:10] Received extension info message [04.05.2026 18:27:10] Server accepts public key types: ssh-ed25519,sk-ssh-ed25519@openssh.com,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,sk-ecdsa-sha2-nistp256@openssh.com,webauthn-sk-ecdsa-sha2-nistp256@openssh.com,ssh-dss,ssh-rsa,rsa-sha2-256,rsa-sha2-512 [04.05.2026 18:27:10] Received service accepted message [04.05.2026 18:27:10] Using OpenSSH agent [04.05.2026 18:27:10] Using egent socket: /var/run/com.apple.launchd.xgrhSm2UZa/Listeners [04.05.2026 18:27:10] Agent has 3 keys [04.05.2026 18:27:10] Sending userauth init request [04.05.2026 18:27:10] Received userauth failure: publickey,gssapi-keyex,gssapi-with-mic,keyboard-interactive [04.05.2026 18:27:10] Starting public key authentication [04.05.2026 18:27:10] Validating public key: ssh-ed25519 [04.05.2026 18:27:10] Key is not accepted by server [04.05.2026 18:27:10] Validating public key: ssh-ed25519 [04.05.2026 18:27:10] Key is not accepted by server [04.05.2026 18:27:10] Validating public key: rsa-sha2-512 [04.05.2026 18:27:10] Received userauth key ok [04.05.2026 18:27:10] Sending userauth public key signature: rsa-sha2-512 [04.05.2026 18:27:10] User authenticated successfuly using agent [04.05.2026 18:27:10] Sending session channel open request: 0|- [04.05.2026 18:27:10] Unknown SSH message type: 120 [04.05.2026 18:27:10] Unknown SSH message type: 136 [04.05.2026 18:27:10] Unknown SSH message type: 26
Thanks for the logs. Our protocol expert was not able to reproduce the issue, but will attempt a blind fix based on your logs and description. We will let you know when we have updates.
Best regards,
Maxim Buisson