An exploitable unsafe default configuration vulnerability exists in the TURN server functionality of coTURN prior to 126.96.36.199. By default, the TURN server allows relaying external traffic to the loopback interface of its own host. This can provide access to other private services running on that host, which can lead to further attacks. An attacker can set up a relay with a loopback address as the peer on an affected TURN server to trigger this vulnerability.
7.7 - CVSS:3.0/AV:N/AC:L/PR:L/UI:N/S:C/C:N/I:H/A:N
CWE-250: Execution with Unnecessary Privileges
coTURN is an open-source implementation of TURN and STUN servers that can be used as a general-purpose networking traffic TURN server. TURN servers are usually deployed in so-called “DMZ” zones — any server reachable by the internet — to provide firewall traversal solutions. Attackers who are able to take over such servers may be able to bypass firewalls and conduct further attacks.
According to Shodawn, thousands of coTURN servers are directly reachable on the internet.
The default options of affected coTURN servers allow TURN clients to set up peers being a loopback address. This setup forwards traffic from an external interface to a loopback interface of the server, and provides access to other services running on the loopback interface that would otherwise be private.
Run the coTURN server with the following option to disable loopback forwarding:
--no-loopback-peers Disallow peers on the loopback addresses (127.x.x.x and ::1)
2017-09-04 - Vendor Disclosure
2019-01-28 - Vendor Patched
2019-01-29 - Public Release
Discovered by Nicolas Edet of Cisco.