Troubleshooting Tip: Special characters displayed incorrectly when using the RDP clipboard toolbox in SSL VPN web mode

Description

This article describes an issue where special characters and language-specific symbols, are replaced by diamond-shaped symbols or garbled text when the RDP clipboard toolbox is utilized in SSL VPN web mode. This behavior is typically observed when text snippets are transferred from a remote Windows server to a local workstation via the "Request Remote Clipboard" function.

Scope

FortiGate.

Solution

In SSL VPN web mode, a specialized RDP toolbar (accessed via the F8 key) is provided to synchronize clipboards between the remote server and the local workstation. While standard alphanumeric characters are transferred without issue, corruption is frequently observed when extended character sets are utilized.

When the 'Request Remote Clipboard' function is invoked, the text retrieved from the remote session may be displayed incorrectly. The following symptoms are typically encountered:

• Character Replacement: Specialized characters are replaced by diamond-shaped symbols or question marks.

• Encoding Mismatch: The text preview within the SSL VPN clipboard window is shown as garbled or incoherent strings.

This behavior is found to be independent of the FortiOS firmware version, as it is observed across various releases (v7.2, v7.4, and v7.6). The root cause is identified as an encoding incompatibility between the application sourcing the text on the remote Windows server and the SSL VPN web proxy logic.

It is observed that legacy applications, such as the standard Windows Notepad, utilize encoding methods that are not always correctly interpreted by the web-based RDP transformation engine. Conversely, applications that support a broader range of character sets are found to function without error.

To ensure that special characters are preserved during the clipboard transfer process, the following measures are recommended:

• Utilization of Modern Text Editors: The use of Notepad++ or other advanced editors is advised on the remote server. These applications utilize encoding standards that are compatible with the SSL VPN portal.

• Alternative Access Methods: If the clipboard functionality is critical and the use of modern applications is not possible, the use of SSL VPN Tunnel Mode (via FortiClient) or IPsec VPN remote access is recommended. In Tunnel Mode, the native RDP client is used, and character encoding is handled directly by the RDP protocol rather than the web proxy.