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Signal App Information Disclosure

Low

Synopsis

CVE-2020–5753 - Remote DNS Disclosure

Signal Messenger App has a vulnerability which allows a remote non-contact to ring a user's Signal phone and disclose the Signal user's current DNS server. We found Android Signal versions v4.59.0-4.58.10 to be vulnerable and iOS Signal versions v3.8.0.34-v3.8.3.5 vulnerable. 

The impact can result in a remote attacker obtaining coarse information via leaking DNS server IP of a Signal user, which may disclose coarse location as well as changes in internet connections at any given moment. In the event the Signal user is using a DNS that uses EDNS Client Subnet, then part of the Signal user's IP address could also be disclosed which results in finer location and network disclosure. 

Cause

The issue lies in how WebRTC is used to processes ICE Candidates, which takes place before a user decides to answer an incoming Signal call. While a callee's IP address can be hidden from the ICE Candidate exchange using Signal "Always Relay Calls" setting, ICE Candidates support domain names which allows an attacker to force a callee to resolve during the signaling process. If the attacker owns the authoritative nameserver for given domain, then callee's current DNS server is disclosed.

 

Solution

Update Signal Android to version 4.59.11 or Signal iOS to version 3.8.4

Disclosure Timeline

05-05-2020 - Tenable discloses vulnerability to [email protected]
05-06-2020 - Signal security confirms receipt
05-15-2020 - Tenable notices patch was committed to public http://github.com/signalapp repository and notifies public disclosure will occur following week.
05-15-2020 - Signal apologizes for lack of coordination, says dev teams started work on it more quickly than realized. Asks to schedule phone call
05-15-2020 - Tenable schedules phone call
05-18-2020 - Tenable provides disclosure draft explaining impact
05-18-2020 - Signal plans to submit patch to Chromium security team, asks if that's ok.
05-19-2020 - Tenable has no objection to reaching out to Chromium. Updates Signal on EDNS impact scenario

All information within TRA advisories is provided “as is”, without warranty of any kind, including the implied warranties of merchantability and fitness for a particular purpose, and with no guarantee of completeness, accuracy, or timeliness. Individuals and organizations are responsible for assessing the impact of any actual or potential security vulnerability.

Tenable takes product security very seriously. If you believe you have found a vulnerability in one of our products, we ask that you please work with us to quickly resolve it in order to protect customers. Tenable believes in responding quickly to such reports, maintaining communication with researchers, and providing a solution in short order.

For more details on submitting vulnerability information, please see our Vulnerability Reporting Guidelines page.

If you have questions or corrections about this advisory, please email [email protected]

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