Advisory: 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 ============================= RedTeam Pentesting is looking for penetration testers to join our team in Aachen, Germany. If you are interested please visit: https://www.redteam-pentesting.de/jobs/ -- RedTeam Pentesting GmbH Tel.: +49 241 510081-0 Dennewartstr. 25-27 Fax : +49 241 510081-99 52068 Aachen https://www.redteam-pentesting.de Germany Registergericht: Aachen HRB 14004 Geschäftsführer: Patrick Hof, Jens Liebchen
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