CVE-2018-20436 in Telegram
Summary
by MITRE
The "secret chat" feature in Telegram 4.9.1 for Android has a "side channel" in which Telegram servers send GET requests for URLs typed while composing a chat message, before that chat message is sent. There are also GET requests to other URLs on the same web server. This also affects one or more other Telegram products, such as Telegram Web-version 0.7.0. In addition, it can be interpreted as an SSRF issue.
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Analysis
by VulDB Data Team • 08/05/2024
The vulnerability identified as CVE-2018-20436 represents a significant privacy and security flaw within Telegram's secret chat functionality across multiple platform versions including Android 4.9.1 and Web version 0.7.0. This issue manifests through unintended information leakage mechanisms that occur during the composition of secret chat messages, fundamentally undermining the confidentiality assurances that users expect from encrypted communication channels.
The technical implementation of this vulnerability stems from the improper handling of URL detection and processing within Telegram's client-side code. When users type messages containing URLs in secret chats, the application performs background GET requests to these addresses before the message is actually transmitted. This behavior creates a side channel attack vector where metadata about user activities and browsing patterns can be inferred by adversaries monitoring the Telegram servers. The flaw operates at the application layer and demonstrates poor separation between user input processing and network communication protocols.
The operational impact of this vulnerability extends beyond simple information leakage to encompass potential reconnaissance activities by threat actors. The side channel exposure allows attackers to discover which websites users are visiting through URL typing, potentially enabling targeted phishing campaigns, tracking of user behavior, or correlation of activities across different platforms. This weakness particularly affects the security model of secret chats, which are designed to provide end-to-end encryption and protection against metadata analysis. The vulnerability also represents a server-side request forgery (SSRF) issue under CWE-918, where the server inadvertently makes requests to arbitrary URLs based on user input, potentially exposing internal network resources.
The implications for users include compromised privacy and potential exposure of sensitive browsing patterns that should remain confidential within the secret chat environment. Security practitioners should note that this vulnerability aligns with ATT&CK technique T1071.004 for application layer protocol: DNS, as it involves the misuse of DNS resolution and URL handling mechanisms. The flaw demonstrates how seemingly benign features can introduce critical security weaknesses when proper input validation and network isolation principles are not applied. Organizations using Telegram for sensitive communications should consider the risk of this information leakage when evaluating the platform's security posture, particularly given that the vulnerability affects multiple product lines and persists across different client implementations.