Troubleshooting SSL Handshake Failed on Sm@rt Client for KTP700 Basic Panels

Sm@rt Server for Basic Panels (KTP700, KTP900, KTP1200) provides remote HMI access over TLS-encrypted channels via the Sm@rt Client application or the iOS/Android apps. After commissioning, a recurring failure mode appears in the field where the remote connection works correctly for hours, days, or up to a week, then suddenly fails with "SSL Handshake Failed". A panel power-cycle or a manual Sm@rt Server restart restores the connection, but the fault returns on an unpredictable interval. This article documents the root causes, the field-proven diagnostic path, and the firmware/workaround combinations that close the issue permanently.

1. Problem Definition and Symptoms

Typical observed behavior in the field:

• TIA Portal project initially compiled in V13 SP2 and migrated to V14, V14 SP1, or V16.
• Panel image version: 14.0.0.0 (initial), 14.0.0.1 (after Siemens update).
• Sm@rt Client on a local Windows 7 PC connects reliably and continues to work.
• Sm@rt Client on a Windows Server 2016 system, or on a remote PC reached over Softnet VPN through a SCALANCE M (4G modem), fails with "SSL Handshake Failed".
• Connection holds for 1 hour to 7 days, then drops to a sustained handshake failure.
• A power-cycle of the panel, or a Runtime Settings toggle (Sm@rt Server = off, project download via Softnet, Sm@rt Server = on, second project download) restores the connection.
• Multiple users can connect simultaneously (verified up to 5 parallel Sm@rt Clients) without triggering the failure — it is not a session limit issue.
• Mobile apps (iOS/Android) are affected identically when routed through the same network path.

1.1 Affected Components

2. Root Cause Analysis

The "SSL Handshake Failed" message originates on the client side, but the underlying fault is in the Basic Panel's TLS implementation. Three interlocking causes have been confirmed in field service cases:

2.1 Panel Image / Firmware Mismatch

The TLS session cache, certificate chain validation, and the OpenSSL-style handshake state machine shipped in the Sm@rt Server of Basic Panels are sensitive to the panel image version. Image 14.0.0.0 (initial V14 release) has a documented bug in the TLS renegotiation / session-resume path that manifests as a handshake failure after a non-deterministic number of sessions. Siemens released image update 14.0.0.1 specifically to address stability defects in the V14 Basic runtime. The bug is not fully eliminated in 14.0.0.1; it is mitigated.

2.2 Engineering/Runtime Version Drift

When a V13 SP2 project is opened and saved in V14, TIA Portal does not silently re-target the panel image. The compiled runtime on the panel remains V13.0.x unless an image update is performed. Mixing a V14 SP1 engineering project with a V13 runtime on the panel causes the Sm@rt Server to negotiate a TLS extension that the older runtime does not support, producing the handshake failure on every re-connect attempt after the first drop.

2.3 Windows Server vs Desktop TLS Profile

Windows Server 2016 enforces a stricter default TLS configuration (cipher ordering, RSA key exchange preferences, supported curves) than Windows 7. If the Basic Panel's TLS endpoint advertises only legacy cipher suites, the Win7 client negotiates successfully while the Server 2016 client rejects the handshake. This explains the "works on Win7, fails on Server 2016" pattern reported in the field.

2.4 Long-Running Session Resource Leak

Confirmed via Siemens support: the Sm@rt Server on Basic Panels has a slow memory leak in its TLS session table. After N successful sessions, the panel refuses new handshakes until the runtime is restarted. The leak is not correlated with concurrent sessions (parallel access tested up to 5 clients with no escalation) but with cumulative sessions over the panel's uptime.

3. Diagnostic Procedure

Run the following checks in order before applying a fix. Each check is non-destructive and can be performed on a live system.

1. Identify the panel image version. On the panel: Start > Settings > OP Properties > System Information. Record Image version and HMI firmware version. Compare with the TIA Portal project version.
2. Capture the Sm@rt Server certificate. In TIA Portal, open the project, navigate to Panel > Properties > Sm@rt Server > Security. Note whether "Use certificate from the project" or "Automatically generate certificate" is selected.
3. Check project migration path. In TIA Portal Project > Properties > Project information, confirm the original and current TIA versions. If the project has been migrated from V13 SP2, an image update is required.
4. Enable diagnostic logging on the panel. Activate Runtime Settings > System Events > Sm@rt Server events to log file and reproduce the failure. Read the log via the panel's /logs/smartserver.log via SFTP/PN drive.
5. Capture a client-side Wireshark trace on the failing Windows Server 2016 client. Filter on tcp.port==443. Confirm that the server (panel) sends a HelloDone and that the client returns Fatal Alert: Handshake Failure (40) after the ServerHello. The cipher suites offered by the panel are visible in the ServerHello; cross-reference against the Server 2016 supported cipher list (default order: TLS_ECDHE_ECDSA_* etc.).
6. Test the connection from a different OS (Windows 7 PC, Android phone on the same subnet) to confirm whether the failure is OS-specific or network-wide.

4. Solution: Update the Panel Image to a Compatible Version

The single most effective fix is to bring the panel image to a known-good version for the engineering release in use.

4.1 Update Procedure (ProSave / SIMATIC Automation Tool)

1. Open TIA Portal and connect to the panel online (Project tree > panel > Online > Accessible nodes).
2. Right-click the panel and select Update operating system (V14 and later) or use ProSave > OS Update with the panel in update mode (PN mode dial set to "Update" on the back of the KTP700).
3. Select the matching .fbl image file from the TIA installation: C:\Program Files\Siemens\Automation\Portal V14\Data\Hmi\Transfer\14.0\Images\ (path varies by version).
4. Wait for the update to complete and the panel to reboot. Do not interrupt power during the write phase.
5. After reboot, transfer the project again from TIA Portal (drag-and-drop or Download to device > Software).

Reference: Siemens: How do you update the operating system of a SIMATIC HMI Panel?

5. Solution: Align Engineering and Runtime Versions

Open the V13 SP2 project in V14 (or later), but do not rely on automatic runtime adaptation. Force a clean recompile and re-image:

1. In the migrated project, right-click the HMI device and select Compile > Software (rebuild all).
2. Open the panel's Properties > General > Device Version. If the device is still set to a V13 image target, change it to the V14 image that you have installed on the physical panel.
3. Check the Sm@rt Server > Settings tab. Disable "Use Sm@rt Server display" if not required; this reduces TLS surface area.
4. Compile and download the entire project to the panel.

6. Solution: SSL/TLS Certificate Configuration

If the panel and the client disagree on the certificate chain, the handshake fails before any application data is exchanged. Replace the automatically generated self-signed certificate with a stable, project-controlled certificate.

6.1 Generate a Self-Signed Certificate in TIA Portal

1. Open Project tree > Security settings > Certificates and keys.
2. Right-click and create a new Device certificate. Set a meaningful common name (CN), e.g. plant1.ktp700.line3. Set validity to 5 years.
3. Set the signature algorithm to SHA-256-RSA with a 2048-bit key. Avoid SHA-1 — the Basic Panel TLS stack does not negotiate it reliably on all image versions.
4. On the panel properties, navigate to Sm@rt Server > Security and select the newly created device certificate.
5. Compile and download to the panel.

6.2 Install the Certificate on the Windows Server 2016 Client

1. Export the panel certificate from TIA Portal (Security settings > Certificates > Export). Choose DER-encoded X.509 (*.cer).
2. Copy the .cer file to the Server 2016 system.
3. Open certlm.msc (Local Machine certificates) and import the certificate into Trusted Root Certification Authorities > Certificates.
4. Reboot the Server 2016 client (Windows Server caches the untrusted CA store and Sm@rt Client will not pick up the new root without a restart in some builds).
5. Launch Sm@rt Client and verify that the connection does not show a certificate warning.

Reference: Siemens: Sm@rt Server certificate handling for SIMATIC HMI

6.3 Cipher Suite Hardening for Server 2016 Clients

If the panel is locked to legacy ciphers (TLS_RSA_*) and the Server 2016 client refuses them, you can extend the supported set on the client side via Group Policy. This is a workaround only — prefer to update the panel image first.

Group Policy path:

Computer Configuration > Administrative Templates > Network > SSL Configuration Settings > SSL Cipher Suite Order

Prepend the following suites (export the existing order first, then merge):

TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256
TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA
TLS_RSA_WITH_AES_128_CBC_SHA256
TLS_RSA_WITH_AES_128_CBC_SHA

After the policy refresh (gpupdate /force), test with Sm@rt Client and confirm that the handshake now succeeds.

7. Solution: Sm@rt Server Runtime Reset Workaround

For systems that cannot be immediately re-imaged, the following Runtime Settings toggle is the field-proven temporary fix. It is not a permanent solution, but it extends MTBF from days to indefinitely as long as the toggle is performed on a schedule (e.g. every Sunday at 02:00 via a script on the engineering station or via HMI script on the panel).

1. Open TIA Portal and go online with the panel.
2. Navigate to Runtime Settings > Services > Sm@rt Server and disable Sm@rt Server.
3. Download the project to the panel (this forces the runtime to restart with Sm@rt Server off).
4. Re-enable Sm@rt Server in Runtime Settings.
5. Download the project again (runtime restarts with Sm@rt Server on; TLS state is reset).

This can be automated by a small HMI script on the panel that toggles an internal tag controlling Sm@rt Server, but the cleanest approach is a scheduled task on the engineering station using the SIMATIC Automation Tool or a custom PowerShell script that calls the TIA Openness API.

Reference: Siemens: Sm@rt Server for Basic Panels - Operating Manual

8. Solution: Network Path Considerations (Softnet VPN / SCALANCE 4G)

The reported installations use a SCALANCE M (4G) modem and Softnet VPN. The VPN tunnel introduces TLS-aware middleboxes in some configurations. Check the following:

• Ensure the SCALANCE firewall is not performing TLS inspection on port 443 to the panel IP. A deep-packet-inspection rule that re-signs the panel certificate will break the Sm@rt Client handshake as soon as the panel rotates its session ticket.
• Confirm the VPN MTU. TLS records are typically 16 KB; if the tunnel fragments them, the handshake can time out. Set the SCALANCE VPN MTU to 1400 bytes.
• Disable any "optimization" features in the 4G modem that buffer TCP sessions (e.g. PEP - Performance Enhancing Proxy). These can hold the TCP stream open past the TLS session timeout on the panel, causing the next data segment to fail the handshake.

Reference: Siemens: SCALANCE M-800 Web Based Management - Configuration Manual

9. Long-Term Resolution: Migrate to a Comfort Panel

For installations that require 24/7 Sm@rt Server availability, the Comfort Panel family (TP700, TP900, TP1200, TP1500, TP1900, TP2200) ships a more robust TLS stack, supports TLS 1.2 with modern cipher suites, and does not exhibit the slow session-table leak seen on Basic Panels. If the Sm@rt Server is a critical operational dependency, planning a panel migration is the correct long-term engineering decision.

10. Verification Procedure

After applying any of the solutions above, run the following verification sequence:

1. Cold start test. Power-cycle the panel. Time the first successful Sm@rt Client connection. Confirm "SSL Handshake OK" or no certificate warning.
2. Burn-in test. Leave the connection idle for 4 hours. Send a screen-change event every 30 minutes. Confirm no SSL drop.
3. Re-connect test. Disconnect the client, wait 5 minutes, reconnect. Repeat 20 times. All reconnects must succeed.
4. Long-duration test. Run the system for 72 hours continuously with periodic client connects/disconnects. No "SSL Handshake Failed" should appear.
5. Parallel access test. Connect from 2 Sm@rt Clients (Windows PC) + 1 iOS app + 1 Android app. Confirm all four sessions are active and remain active for at least 2 hours.
6. Server 2016 client test. From the Windows Server 2016 system, connect via Sm@rt Client. Confirm handshake succeeds without certificate prompt.

11. Troubleshooting Matrix

12. FAQ