Blocking RDWeb brute force attempts

F.vanSteen — Thu, 15 Aug 2024 16:26:50 GMT

Hi,

we have a customer that's under a slow brute force attempt by a persistent party. Blocked their IP several times, but they switched and even started using multiple addresses at the same time now.

It's doing around 3 to 4 attempts per second per IP on the remote desktop gateway to brute force the passwords. As they seem to possess several valid user accounts these users accounts get locked out which is highly annoying for the users.

Have now added the certificate to the PA and created a decrypt policy. Unfortunately at this stage I'm not sure how they were attempting this.

Basically there are 2 ways I've identified so far. The webserver is presenting a webpage with a form on the /RDWeb website. This form unfortunately doesn't seem to adhere to any HTTP standards for authentication. Failing authentication on it doesn't result in any 401 errors from the webserver, it doesn't seem to follow any HTTP authentication protocol either.

So far generic brute force settings aren't picking up on it.

The other is the remote desktop gateway protocol itself. Made a loop with xfreerdp that calls something like this:

xfreerdp /gateway:g:my.rdgateway.tld,u:myuser,d:DOMAIN,p:verybad /v:my.backendserver.local /u:myuser@domain.tld

And that works fine.

Have a vulnerability protection profile attached with exceptions for:

40006 HTTP: User Authentication Brute Force Attempt
40021 MS-RDP Brute Force Attempt

40030 HTTP NTLM Authentication Brute Force Attack

40031 HTTP Unauthorized Brute Force Attack

With the settings modified to 5 per 10 minutes and action block-ip for 1 hour.

That works fine for the xfreerdp way thus, which does RPC over HTTP calls, but it doesn't do anything at all for the /RDWeb form via browser/POST unfortunately and no clue how to get that blocked as it happily returns 200 with a message in the page that authentication failed.

Anyone have any ideas on how to block that?

Re: Blocking RDWeb brute force attempts

F.vanSteen — Thu, 15 Aug 2024 16:33:48 GMT

This is with the xfreerdp method, the one mstsc will also be using. But no fix for /RDWeb so far.

topic Blocking RDWeb brute force attempts in General Topics

Blocking RDWeb brute force attempts

Re: Blocking RDWeb brute force attempts