CVE-2020-11881

MikroTik SMB Remote Denial of Service (DoS)

This report describes CVE-2020-11881, an unauthenticated remote DoS for MikroTik's SMB service running on RouterOs. The vulnerability allows an attacker to crash the running SMB service and was responsible disclosed to security<@>mikrotik.com on 06.04.2020.

The Server Message Block (SMB) protocol was introduced by Microsoft and reimplemented by multiple vendors in order to maintain file exchange compatibility to Windows systems and services. The protocol in general is used for file exchange between Windows systems.

Affected Versions(tested)

• 6.41.3 (long term release)

• 6.45.8 (long term release)

• 6.45.9 (long term release)

• 6.46.4 (stable release)

• 6.47.2 (stable)

• 6.47.3 (stable)

• 7.0beta5 (beta)

• 7.1beta2 and below

Probably more versions released before 12.09.2020 are affected. With the exception of version 6.47.0, which seems to be fixed for one release cycle as response to the initial responsible disclosure process.

The SMB Vulnerability

The SMB server will crash after receiving a negotiation-request packet followed by a modified setup-request packet. In order to trigger the vulnerability, the setup-request packet needs to have the MessageID field set to 1 and the SessionID field set to 0. The service crashes since it apparently can't handle the SessionID field.

Reproducing the Bug

In order to reliably reproduce the bug, please rebuild the used test environment provided in this repo.

1. Download and start the RouterOs environment.

   Please execute the following script on a fresh installed version of Ubuntu 18.04, since the script will install all required software. The script ends by providing the IP of the RouterOs VM.

   sudo ./setupMikroTikEnvironment.sh

2. Enable SMB on RouterOs

   Run the following commands to activate the SMB service.

   telnet <RouterOs IP>
   (user: admin, <no password>)
   ip smb set enabled=yes
   ip smb print

   The service can be checked manually from the host system with the following command and should respond wiht Anonymous login successful as long the service is running.

   smbclient -N -L \\\\<RouterOs IP>

3. The denial of service attack

   The python3 script cve-2020-11881.py validates the availability of the SMB service before and after the exploitation process. A successful execution will end up with the unavailability of the SMB service. Please execute the script as shown below.

   python3 cve-2020-11881.py --ip <RouterOs Ip>

   Testing the SMB service with the following command will fail.

   smbclient -N -L \\\\<RouterOs IP>

4. Remove VM

   In order to remove the VM just run the script again.

   sudo ./setupMikroTikEnvironment.sh

Fix

The bug was reported on 06.04.2020 and wasn't fixed on 12.09.2020 even after multiple requests.

Discussion about the CVE: https://forum.mikrotik.com/viewtopic.php?f=2&t=166137

Currently only the long-term version channel (v6.46.7) has all the necessary fixes for this CVE. We are working on getting them published in stable and testing channels as well. Sorry for any inconvenience.