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Directory Traversal in Cisco Expressway Gateway

RedTeam Pentesting discovered a directory traversal vulnerability in
Cisco Expressway which enables access to administrative web interfaces.


Details
=======

Product: Cisco Expressway Gateway
Affected Versions: 11.5.1, possibly others
Fixed Versions: See Cisco Bug ID CSCvo47769 [1]
Vulnerability Type: Directory Traversal
Security Risk: medium
Vendor URL: https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20190501-expressway-traversal
Vendor Status: fixed version released
Vendor ID: Cisco Bug ID CSCvo47769
Advisory URL: https://www.redteam-pentesting.de/advisories/rt-sa-2019-002
Advisory Status: published
CVE: CVE-2019-1854
CVE URL: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2019-1854


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

"Cisco Expressway offers users outside your firewall simple, highly
secure access to all collaboration workloads, including video, voice,
content, IM, and presence. Collaborate with people who are on
third-party systems and endpoints or in other companies. Help
teleworkers and Cisco Jabber mobile users work more effectively on their
device of choice."
(from the Cisco Expressway Series website [2]) 


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

Cisco Expressway Gateway is a kind of reverse proxy which implements
authentication mechanisms and forwards authorised requests to services
based on information encoded within the requested URL. It supports two
different URI layouts. The first layout only includes the base64-encoded
domain name of the gateway itself:

------------------------------------------------------------------------
https://example.com:8443/$(echo -n example.com|base64)/test123

https://example.com:8443/ZXhhbXBsZS5jb20=/test123
------------------------------------------------------------------------

The second layout additionally specifies the protocol, hostname and port
number of a target system which is to be contacted through the gateway:

------------------------------------------------------------------------
https://example.com:8443/$(echo -n example.com/https/example.int/8443|base64)/test123

https://example.com:8443/ZXhhbXBsZS5jb20vaHR0cHMvZXhhbXBsZS5pbnQvODQ0Mw==/test123
------------------------------------------------------------------------

RedTeam Pentesting analysed a Cisco Unified Communication Manager (CUCM)
instance which was accessible via a Cisco Expressway Gateway. In this
configuration, a directory traversal vulnerability was identified. It
leverages the methodology described in "Breaking Parser Logic" [3] by
Orange Tsai. The CUCM service, which is implemented using the Tomcat [4]
application server, interprets URLs different to the upstream
reverse proxy. By accessing a specially crafted URL, attackers can
access the CUCM manager application, even though it is not exposed by
the Cisco Expressway Gateway.


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

First, an attacker must authenticate to the Cisco Expressway Gateway.
A login can be performed by accessing the following URL using
HTTP-Basic-Authentication:

------------------------------------------------------------------------
https://example.com:8443/ZXhhbXBsZS5jb20=/get_edge_config
------------------------------------------------------------------------

Afterwards, the resources on the CUCM may be accessed through the Cisco
Expressway Gateway. By inserting repeated occurrences of "/..;/" into
the URL, the directory traversal vulnerability can be exploited. Using
the following URL, a list of applications installed on the CUCM system
can be retrieved:

------------------------------------------------------------------------
https://example.com:8443/ZXhhbXBsZS5jb20vaHR0cHMvZXhhbXBsZS5pbnQvODQ0Mw==/cucm-uds/user/example_user/..;/..;/..;/
------------------------------------------------------------------------

Similarly, the CUCM manager application can be accessed as follows:

------------------------------------------------------------------------
https://example.com:8443/ZXhhbXBsZS5jb20vaHR0cHMvZXhhbXBsZS5pbnQvODQ0Mw==/cucm-uds/user/example_user/..;/..;/..;/ccmadmin
------------------------------------------------------------------------


Workaround
==========

Prevent access to the Cisco Expressway Gateway by untrusted parties.


Fix
===

See Cisco Bug ID CSCvo47769 [1] for affected software releases and
available patches.


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

The vulnerability can be used to access administrative interfaces which
are usually not reachable. Attackers could potentially read or modify
sensitive information via these interfaces. However, it is necessary to
have an authorised user account to access the Cisco Expressway Gateway.
Therefore, the vulnerability poses a medium risk.


Timeline
========

2019-02-01 Vulnerability identified
2019-02-20 Customer approved disclosure to vendor
2019-02-21 Vendor notified
2019-02-21 Receipt of advisory acknowledged by vendor
2019-04-16 Vendor announces public disclosure for May 1st to RedTeam Pentesting
2019-05-01 Vendor publishes advisory
2019-05-16 Customer approves release of this advisory
2019-05-17 Advisory released


References
==========
[1] https://bst.cloudapps.cisco.com/bugsearch/bug/CSCvo47769
[2] https://www.cisco.com/c/en/us/products/unified-communications/expressway-series/index.html
[3] https://media.defcon.org/DEF%20CON%2026/DEF%20CON%2026%20presentations/Orange%20Tsai%20-%20Updated/DEFCON-26-Orange-Tsai-Breaking-Parser-Logic-Take-Your-Path-Normalization-Off-and-Pop-0days-Out-Updated.pdf
[4] https://tomcat.apache.org


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

RedTeam Pentesting offers individual penetration tests performed by a
team of specialised IT-security experts. Hereby, security weaknesses in
company networks or products are uncovered and can be fixed immediately.

As there are only few experts in this field, RedTeam Pentesting wants to
share its knowledge and enhance the public knowledge with research in
security-related areas. The results are made available as public
security advisories.

More information about RedTeam Pentesting can be found at:
https://www.redteam-pentesting.de/


Working at RedTeam Pentesting
=============================

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