Re: Connection Reset with specific HTML white-spac...

diogoreis · ‎2022-03-22

There is a specific substring in HTML files that causes a connection reset when serving the request, and it seems to be caused by the Check Point VPN.

I've tested in three different clients' Check Point VPNs, in several browsers (Chrome, Firefox, Edge, Internet Explorer). The file is being hosted on a IIS server. Browsing on the server, the page shows correctly, only when outside the server, when the request must pass through the VPN, does the issue occur. I've tried on different VPNs without any issues.

The minimum HTML file content that causes the problem (the newline is necessary), and yes I know this is not a valid HTML file, this was the least amount of HTML that manages to cause the connection reset.:

<p>{white-space: pre-line;}</p>
X

Here is a valid HTML file that also causes the problem:

<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<style type="text/css">
body { white-space: pre-line; }
</style>
</head>
<body>
<h1>HTML test</h1>
</body>
</html>

Has anyone reported a similar issue, or can anyone reproduce the problem please?

Thank you very for your help

PhoneBoy · ‎2022-03-22

What precise version/JHF level are we talking about?
Also, what drops do you see on the gateway side in the logs?
I suspect an IPS or Core protection is being triggered.

diogoreis · ‎2022-03-25

First of all, thank you for your reply. We were able to use a workaround for the problem.

The person responsible for the changes in the Check Point configuration described to us the issue and workaround. Any errors or mistranslations are my fault, so I apologise in advance.

> What precise version/JHF level are we talking about?
Checkpoint 6000 em R80.40 JHF-Take156.

> Also, what drops do you see on the gateway side in the logs?
There were no drops on the gateway. When the IPS Software Blade was specified, they were able to confirm that Check Point was blocking the access to that specific URL.

> I suspect an IPS or Core protection is being triggered.
The IPS Software Blade was preventing access to the URL. Source Atack Name: Web Client Enforcement Violation / Atack Information: Microsoft Internet Explorer Memory Corruption (MS16-104: CVE-2016-3247)
The workaround used was: an exception to the action of that Violation (MS16-104: CVE-2016-3247) in the internal networks, because of VPN use, when accessing the URL by http/https.

I believe the issue is described here:
https://www.exploit-db.com/exploits/40797

The mitigation used seems more broad than necessary, given that the following HTML is blocked:

<p>{white-space: pre-line;}</p>
X

Thank you again for your help

Are you a member of CheckMates?

Connection Reset with specific HTML white-space: pre-line