Vulnerability Note VU#735416

UltraVNC repeater does not restrict IP addresses or ports by default

Original Release date: 08 Aug 2016 | Last revised: 08 Aug 2016


UltraVNC repeater versions prior to ultravnc_repeater_1300 do not restrict usage by IP address by default and cannot restrict by ports, which may be leveraged to induce connections to arbitrary hosts using any port.


CWE-16: Configuration - CVE-2016-5673

UltraVNC repeater acts as a proxy to route remote desktop VNC connections. IP addresses are not restricted in default configurations, and ports cannot be selectively restricted. Consequently, in a default installation, a repeater can be caused to initiate connections to arbitrary hosts using any port. To initiate a connection to a common web service, for instance, an attacker may request a connection to <IP>::<80><padding>, where padding consists of null bytes and the request length is 250 bytes.


A remote, unauthenticated attacker may induce a default-configured repeater to initiate connections to arbitrary hosts and services.


Update repeater configuration

New installations of UltraVNC repeater now default to restricting access to all IP addresses and support more granular port restrictions. Existing installations should consider updating to ultravnc_repeater_1300, review the vendor's advisory, and make modifications as appropriate:

"WARNING:  In MODE I the repeater works like a proxy.  If you don't limit the destination and or ports your repeater can be used to connect to all ip adresses and all ports that can be reached from the repeater.

You need to restrict the ip addreses and ports to prevent unwanted access."

Vendor Information (Learn More)

VendorStatusDate NotifiedDate Updated
UltraVNCAffected13 May 201601 Aug 2016
If you are a vendor and your product is affected, let us know.

CVSS Metrics (Learn More)

Group Score Vector
Base 5.0 AV:N/AC:L/Au:N/C:N/I:P/A:N
Temporal 3.9 E:POC/RL:OF/RC:C
Environmental 1.0 CDP:ND/TD:L/CR:ND/IR:ND/AR:ND



Thanks to Yonathan Klijnsma and Dan Tentler for reporting this vulnerability.

This document was written by Joel Land.

Other Information

  • CVE IDs: CVE-2016-5673
  • Date Public: 06 Aug 2016
  • Date First Published: 08 Aug 2016
  • Date Last Updated: 08 Aug 2016
  • Document Revision: 21


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