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Workaround for versions of
CRT and SecureCRT prior to 4.1.9
The remote execution vulnerability only affects users
who have CRT or SecureCRT set to be their default Telnet client.
The appropriate registry key can be modified to no longer
point to CRT or SecureCRT.
The following link resolves to: telnet://localhost. If clicking
on the link does not bring up CRT or SecureCRT, then you are
not vulnerable since CRT or SecureCRT is not set to be your
default Telnet client.
What is my
default Telnet client?
If clicking on the above link launches CRT or SecureCRT,
the situation can be resolved as follows:
WARNING: If you use the registry
editor incorrectly, you may cause serious problems that may
require you to reinstall your operating system. Before making
changes to the registry, you should back up any valued data
on your computer.
- Open the Windows registry editor (Start / Run, regedit)
and browse to the following registry key:
HKEY_CLASSES_ROOT\telnet\shell\open\command
- Modify the "(Default)" value, changing it to:
rundll32.exe url.dll,TelnetProtocolHandler
%l
Note: in the above example, it's "%[el]", rather
than "%[one]"
- Close the registry editor.
At this point, clicking on the following link should bring
up the built-in Windows Telnet client rather than CRT or SecureCRT:
What is my
default Telnet client now?
Once the default Telnet client has been reset, launching
CRT or SecureCRT may present the following prompt when the
application is launched:
"SecureCRT is not currently your default Telnet application.
Would you like to make SecureCRT your default Telnet application?"
To prevent SecureCRT from presenting this prompt again, clear
the "Always perform this check when starting SecureCRT"
option and choose "No".
The following VBScript code can be used to make this change
if this needs to be performed for a large group of users (as
a logon script, for example):
Set WshShell = CreateObject("WScript.Shell")
Key = "HKCR\telnet\shell\open\command\"
Value = "rundll32.exe url.dll,TelnetProtocolHandler
%l"
WshShell.RegWrite Key, Value, "REG_SZ"
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