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Blocks IP addresses generating invalid Terminal Services logons
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ts_block.vbs - Blocks brute force Terminal Services login attempts Copyright 2011 Wellbury LLC - See LICENSE for license information Release 20110901 - Adapted from sshd_block release 20100120 Release 20120530 - Added MSI to distribution For support, please contact Evan Anderson at Wellbury LLC EAnderson@wellbury.com, (866) 569-9799, ext 801 <http://serverfault.com/users/7200/evan-anderson> If you like this program please drop me an email. If you have any ideas for enhancements or find a bug, I'd love to hear about that too. Overview ======== ts_block is a VBScript program that acts as a WMI event sink to receive events logged by Windows in response to invalid Terminal Services logons. It parses these log entries and acts upon them as follows: - If the IP address attempts to logon with a username flagged as "block immediately" the IP address is blocked immediately. - If the IP address attempts to logon with more frequently than is allowed in a given time period the IP address is blocked. The "block immediately" usernames and thresholds associated with repeated logon attempts are configurable in the "Configuration" section of the script. Default settings are as follows: Block Immediately Usernames - administrator, root, guest Logon attempts allowed - 5 in 120 seconds (2 minutes) Duration of block - 300 seconds (5 minutes) The configuration variables for these values are reasonably self-explanatory. Additional variables to enable/disable debugging and event log usage are also present and self-explanatory. Review the section "Registry Configuration Parameters" for information about configuring via the registry (which is useful for management via Group Policy). Four times per second IP addresses that have remained blocked for their assigned block duration are unblocked. ts_block does not run under Windows 2000 Server (because I have no machines handy to test it on and because the Terminal Services logon type, 10, is not logged on Windows 2000 Server) and under Windows XP (because the remote IP address is not included in the event log message). Pre-Requisite Computer Configuration ==================================== For Windows Vista, 2008, 7, and 2008 R2 the "Advanced Firewall" is used to create inbound firewall rules blocking traffic from the blocked host. On these operating systems no special configuration of the registry or network adapters is necessary. Because Windows Server 2003 lacks sufficient features in its built-in firewall functionality a black-hole host route is used. Unfortunately, the "trick" commonly used on Linux to black-hole traffic with a route to "lo" (127.0.0.1) doesn't work on Windows. The "route" command will fail if the destination specified isn't local to one of machine's interfaces, as well. As such, there are two options for selecting the destination address used for the black-hole route. You may specify the black-hole destination IP address as REG_SZ value as described below in the "Registry Configuration Parameters" section. This address must be local to (in the same IP subnet as) one of the server computer's interfaces. It is recommended that you select an address that is unused in your network. This is my preferred method of installation because no device drivers need to be installed. Alternatively you may install a network interface with a static IP address assigned and no default gateway specified be present on the server computer. A physical hardware device is not necessary as the Microsoft Loopback Adapter serves the purposes of this application. Details about installing the Microsoft Loopback Adapter is available from: http://support.microsoft.com/kb/842561 After you have installed the Microsoft Loopback Adapter (or chosen an unused physical hardware NIC) specify a static IP address and no default gateway in the TCP/IP version 4 properties for the adapter. The IP address and subnet mask assigned to this adapter should not match any network in use in your enterprise and should be in the RFC 1918 space. The ts_block script will locate the adapter with no default gateway specified and use it as the destination for the black-hole route. Registry Configuration Parameters ================================= The following configuration paramters are available under the registry path: HKLM\Software\Policies\Wellbury LLC\ts_block Parameter: BlockAttempts Type: REG_DWORD Explanation: The number of sequential failed logon attempts (with accounts that are not considered "block immediately" accounts) that will trigger a block. Parameter: BlockDuration Type: REG_DWORD Explanation: The duration, in seconds, of a block (either because of reaching the BlockAttempts threshhold or because of a "block immediately"). Parameter: BlockTimeout Type: REG_DWORD Explanation: The duration, in seconds, that must elapse between failed logon attempts to reset the count of failed logon attempts for a given IP address. Parameter: BlackholeIP Type: REG_SZ Explanation: The IP address used for the black-hole route (for Windows Server 2003). If not specified the default algorithm of selecting the IP address of a network interface with no default gateway specified will be used. This setting is not used in Windows Server 2008 and later versions of Windows. A Group Policy Administrative Template (ADM) file is included with this distribution that is capable of setting these values. Deploying a GPO near the top of the domain with the BlockAttempts, BlockDuration, and BlockTimeout values specified and Site or OU-level GPOs with the BlackholeIP value specified (as this will vary based on the subnets where the server computers are located, and is only necessary for Windows Server 2003 machines) is recommended. Script Testing ============== It is recommended that you copy the ts_block.vbs script to your desired location, modify the configuration parameters if you are unsatisfied with the defaults, and execute the script either. It is recommended that you execute the script using the CSCRIPT.EXE utility, but it is possible to execute the script using WSCRIPT.EXE via double-clicking on the script file in Windows Explorer. Be aware that, should debugging be enabled, execution is only effectively possible through CSCRIPT.EXE because message logging to pop-up dialogs will "stall" the script until the dialogs are dismissed. Test the functionality of the script by performing both invalid logons using both a "block immediately" account and attempting repeated logons with a valid or invalid account that is not in the "block immediately" list. Blocking and unblocking events will be logged in the Application event log. (It is recommended that you perform your tests via a protocol that won't cause you to lose communication with the server computer during testing or from an IP address that isn't going to be blocked.) Windows Service Installation ============================ A binary copy of the public domain "Non-Sucking Service Manager" (nssm, available from http://iain.cx/src/nssm/) is included with ts_block to facilitate installation as a Windows service. The Microsoft SRVANY tool may also be used to run ts_block as a Windows Service. If you choose to use nssm, copy the nssm.exe file to the location of your choice (in "%ProgramFiles%\ts_block", for example). After you are satisfied with the performance of the script in testing and have copied nssm.exe to the desired location, install the script as a Windows service using the following command-line (from the directory where nssm was installed): nssm install ts_block %SystemRoot%\System32\cscript.exe "\"%ProgramFiles%\ts_block\ts_block.vbs\"" The command is depicted as two lines above but should be entered on a single line. It is necessary to enter the "\" characters as depicted such that the resulting registry entry is surrounded by double quotes. This command will create a service set to start automatically. (If your ts_block.vbs is stored in a path w/o spaces then you don't need to go through those gyrations.) After installing the service start it and verify that it functions properly. External Dependencies ===================== The following external programs are required to be in the PATH for the user context under which ssdh_block is executing: ROUTE.EXE - For black-hole routing blocked IP addresses under Windows XP NETSH.EXE - For creating Advanced Firewall rules on Windows Vista and later versions of Windows EVENTCREATE.EXE - For writing to the event log (only if event logging is enabled) Performance and Security ======================== A simple and fairly unscientific stress test function is included in the script (and disabled by default). Testing with the parameters listed in the script (but with the BLOCK_DURATION decreased from the default to 60 seconds) on a Windows Server 2003 SP2 x86 Stadard Edition machine resulted in peak memory usage of 6,780KB. As the blocked queue drained at the end of the test, the memory usage decreased slightly. On the face of it, it would appear that the script can handle at least thousands of unique IP addresses being blocked at a rate of one IP address every 10 to 50ms with no major issues. Parameters passed to calls to external programs for creating Windows Event Log entries or altering IP routes are sanitized through a regular expression match (allowing only the characters 0-9, a-z, A-Z, and period, underscore, space, right-leaning slash, colon, and minus). Windows Installer Package (MSI) =============================== A Windows Installer package (MSI) version of ts_block is included with this distribution (along with the WiX source file used to create the MSI). The MSI is self-contained (all necessary files are compressed and embedded within it) and can be used for automated deployment of ts_block as a service under NSSM. It has been tested using Group Policy Software Installation Policy under Windows Server 2003, Windows Server 2008 R2, and Windows 7. Uninstallation via Group Policy has also been tested. Future Roadmap ============== Exciting enhancement possibilities include: - Loading values for "block immediately" usernames from the registry.