CVE-2020-5753 in Signal
Summary
by MITRE
Signal Private Messenger Android v4.59.0 and up and iOS v3.8.1.5 and up allows a remote non-contact to ring a victim's Signal phone and disclose currently used DNS server due to ICE Candidate handling before call is answered or declined.
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Analysis
by VulDB Data Team • 05/21/2020
The vulnerability identified as CVE-2020-5753 affects Signal Private Messenger applications across both Android and iOS platforms, specifically versions 4.59.0 and higher for Android and 3.8.1.5 and higher for iOS. This security flaw resides in the application's implementation of Interactive Connectivity Establishment (ICE) candidate handling during voice call initiation processes. The vulnerability enables remote attackers who are not in the victim's contact list to exploit the signaling mechanism to trigger a ringing notification on the victim's device while simultaneously disclosing the victim's currently configured DNS server information.
The technical flaw manifests in the improper handling of ICE candidates before a call is either answered or declined by the victim. During the initial call setup phase, Signal's implementation fails to adequately validate or secure the ICE candidate exchange process, allowing unauthorized remote actors to inject malicious ICE candidates that trigger the ringing behavior. This vulnerability operates at the network signaling level where the application's WebRTC implementation does not sufficiently authenticate or filter incoming ICE candidate data, creating an attack surface that can be exploited without requiring any prior relationship or contact information from the victim.
The operational impact of this vulnerability extends beyond simple notification abuse, as it represents a significant privacy disclosure risk. When an attacker successfully triggers the ringing behavior, they can simultaneously extract the victim's DNS server configuration, which serves as a critical piece of information for potential further attacks. This DNS information can reveal network infrastructure details, including the specific DNS servers being used by the victim's device, which may be leveraged for more sophisticated reconnaissance activities. The vulnerability essentially provides an attacker with a method to establish contact with a victim without their knowledge while simultaneously gathering network configuration data that could be used for subsequent attacks.
This vulnerability maps to CWE-200 (Information Exposure) and CWE-345 (Insufficient Verification of Data Authenticity) within the CWE taxonomy, as it exposes sensitive information through improper verification of ICE candidate data and provides unauthorized access to network configuration details. From an ATT&CK framework perspective, this vulnerability aligns with T1071.004 (Application Layer Protocol: DNS) and T1566 (Phishing) techniques, as it enables both information gathering and social engineering attacks through unauthorized contact establishment. The vulnerability also demonstrates characteristics of T1592 (Gather Victim Network Information) and T1590 (Gather Victim Identity Information) as it provides attackers with network infrastructure details and potential identity correlation opportunities.
Mitigation strategies for this vulnerability should focus on implementing proper ICE candidate validation mechanisms within the application's signaling implementation. The Signal development team should enforce strict authentication and verification of all ICE candidates before processing them, ensuring that only legitimate candidates from authorized sources are accepted. Additionally, the application should implement rate limiting and connection validation measures to prevent abuse of the ringing functionality. Network-level mitigations could include implementing DNS query filtering and monitoring for unusual DNS server disclosure patterns. Users should be advised to keep their Signal applications updated to the latest versions where this vulnerability has been addressed, and organizations should consider implementing network monitoring solutions to detect potential exploitation attempts. The fix should include comprehensive input validation for all ICE candidate data and proper state management during call setup processes to prevent unauthorized triggering of ringing notifications.