CVE-2019-17191 in Signal Messenger
Summary
by MITRE
The Signal Private Messenger application before 4.47.7 for Android allows a caller to force a call to be answered, without callee user interaction, via a connect message. The existence of the call is noticeable to the callee; however, the audio channel may be open before the callee can block eavesdropping.
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Analysis
by VulDB Data Team • 01/03/2024
The vulnerability identified as CVE-2019-17191 represents a significant security flaw in the Signal Private Messenger application affecting Android devices prior to version 4.47.7. This issue falls under the category of unauthorized call initiation and eavesdropping capabilities that directly compromise user privacy and communication security. The flaw specifically enables an attacker to manipulate the call connection process in a manner that bypasses normal user consent mechanisms, creating a dangerous scenario where personal communications can be intercepted without the recipient's knowledge or approval.
The technical implementation of this vulnerability exploits the application's handling of incoming call connections through what is termed a "connect message" mechanism. When a malicious caller sends this specific message, it triggers an automatic call answering process that occurs without requiring any interaction from the intended recipient. This behavior violates fundamental security principles of user consent and communication integrity that are essential for secure messaging applications. The vulnerability is particularly concerning because while the callee is notified of the call's existence, the audio channel may already be established before the user has an opportunity to respond or block the connection, effectively creating a window for unauthorized eavesdropping.
From an operational security perspective, this vulnerability creates a serious risk for Signal users who rely on the application for private communications. The flaw essentially undermines the core security promise of end-to-end encryption and user control over their communication channels. Attackers could potentially exploit this vulnerability to conduct surveillance operations, gather sensitive information, or engage in unauthorized monitoring of conversations. The timing aspect of the vulnerability is particularly dangerous since the audio channel opens before the user can take defensive actions, making it difficult for recipients to protect their privacy once a call is initiated through this malicious mechanism.
The security implications extend beyond simple privacy concerns to encompass broader threats to user trust and application integrity. This vulnerability demonstrates a critical failure in the application's access control and connection management protocols, allowing unauthorized parties to manipulate the normal call flow process. According to CWE classification, this vulnerability relates to improper control of a resource through a mechanism that allows unauthorized access to communication channels, specifically CWE-284 for improper access control and CWE-310 for cryptographic issues. The flaw also aligns with ATT&CK techniques related to privilege escalation and persistence through manipulation of communication protocols, as attackers can establish unauthorized access to audio channels without proper authentication or user consent.
Mitigation strategies for this vulnerability require immediate application updates to version 4.47.7 or later, which address the flawed connect message handling and restore proper user consent mechanisms for call connections. Users should also implement additional security measures such as regularly updating their applications, monitoring for unusual call activity, and being vigilant about potential social engineering attempts that might exploit this vulnerability. Organizations using Signal for business communications should consider implementing additional monitoring systems to detect unusual call patterns and establish protocols for responding to potential exploitation attempts. The fix implemented by Signal developers likely involved strengthening the connection validation process and ensuring that audio channels cannot be opened without explicit user confirmation, thereby restoring the intended security model of the application.