CVE-2019-15514 in Telegram App
Summary
by MITRE
The Privacy > Phone Number feature in the Telegram app 5.10 for Android and iOS provides an incorrect indication that the access level is Nobody, because attackers can find these numbers via the Group Info feature, e.g., by adding a significant fraction of a region's assigned phone numbers.
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Analysis
by VulDB Data Team • 12/01/2023
The vulnerability described in CVE-2019-15514 represents a critical privacy flaw within the Telegram messaging application affecting versions 5.10 on both Android and iOS platforms. This issue specifically targets the application's handling of phone number privacy settings, where users expect their contact information to be restricted according to the configured access levels. The fundamental problem lies in the application's failure to properly enforce privacy controls for phone number visibility, creating an unintended information disclosure channel that undermines user expectations of confidentiality.
The technical implementation flaw stems from how Telegram processes and displays phone number information within group contexts. When users configure their phone number privacy settings to "Nobody," the application should prevent any unauthorized access to this information. However, the vulnerability allows attackers to bypass these restrictions through the Group Info feature, which enables the enumeration of phone numbers from group members. This occurs because the application's access control mechanisms do not adequately distinguish between direct user queries and group-based information retrieval, creating a pathway for adversaries to collect significant portions of regional phone number databases through systematic group membership exploration.
The operational impact of this vulnerability extends beyond simple information disclosure to represent a substantial privacy risk for Telegram users. Attackers can systematically build comprehensive phone number databases by joining groups and extracting contact information from members, effectively circumventing the intended privacy controls. This capability enables malicious actors to perform reconnaissance activities, conduct targeted phishing campaigns, or engage in social engineering attacks using the collected phone numbers. The vulnerability particularly affects users in regions where phone number assignment follows predictable patterns, making the enumeration process more efficient and effective.
From a cybersecurity perspective, this vulnerability aligns with CWE-284, which addresses improper access control mechanisms, and represents a specific implementation failure in access control enforcement. The flaw demonstrates how seemingly minor configuration options can create significant security gaps when not properly validated across all application interfaces. The attack pattern employed here corresponds to techniques described in the ATT&CK framework under Initial Access and Credential Access domains, where adversaries exploit application weaknesses to gather user information without direct authentication.
The mitigation strategy for this vulnerability requires immediate implementation of proper access control validation within the Group Info feature and related group membership functions. Developers must ensure that privacy settings are consistently enforced regardless of the information retrieval method used by the application. This includes implementing robust access control checks that verify user permissions before exposing phone number information, particularly in group contexts where multiple users may have varying privacy preferences. Additionally, rate limiting and monitoring mechanisms should be implemented to detect and prevent systematic enumeration activities that could indicate malicious intent. The fix should also consider implementing more granular privacy controls that allow users to specify different access levels for different types of information within group contexts, ensuring that the application's privacy model aligns with user expectations and security best practices.