Telegram Desktop before 1.5.12 on Windows, and the Telegram applications for Android, iOS, and Linux, is vulnerable to an IDN homograph attack when displaying messages containing URLs. This occurs because the application produces a clickable link even if (for example) Latin and Cyrillic characters exist in the same domain name, and the available font has an identical representation of characters from different alphabets.

An exploitable information disclosure vulnerability exists in the "Secret Chats" functionality of the Telegram Android messaging application version 4.9.0. The "Secret Chats" functionality allows a user to delete all traces of a chat, either by using a time trigger or by direct request. There is a bug in this functionality that leaves behind photos taken and shared on the secret chats, even after the chats are deleted. These photos will be stored in the device and accessible to all applications installed on the Android device.

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. NOTE: a third party has reported that potentially unwanted behavior is caused by misconfiguration of the "Secret chats > Preview links" setting

An issue was discovered in the org.telegram.messenger application 4.8.11 for Android. The FingerprintManager class for Biometric validation allows authentication bypass through the callback method from onAuthenticationFailed to onAuthenticationSucceeded with null, because the fingerprint API in conjunction with the Android keyGenerator class is not implemented. In other words, an attacker could authenticate with an arbitrary fingerprint. NOTE: the vendor indicates that this is not an attack of interest within the context of their threat model, which excludes Android devices on which rooting has occurred

An issue was discovered in the org.telegram.messenger application 4.8.11 for Android. The Passcode feature allows authentication bypass via runtime manipulation that forces a certain method's return value to true. In other words, an attacker could authenticate with an arbitrary passcode. NOTE: the vendor indicates that this is not an attack of interest within the context of their threat model, which excludes Android devices on which rooting has occurred

Telegram Desktop (aka tdesktop) 1.3.14, and Telegram 3.3.0.0 WP8.1 on Windows, leaks end-user public and private IP addresses during a call because of an unsafe default behavior in which P2P connections are accepted from clients outside of the My Contacts list.

Telegram Desktop (aka tdesktop) 1.3.16 alpha, when "Use proxy" is enabled, sends credentials and application data in cleartext over the SOCKS5 protocol.

Telegram Desktop (aka tdesktop) 1.3.14 might allow attackers to cause a denial of service (assertion failure and application exit) via an "Edit color palette" search that triggers an "index out of range" condition. NOTE: this issue is disputed by multiple third parties because the described attack scenario does not cross a privilege boundary

The saveFile method in MediaController.java in the Telegram Messenger application before 2017-12-08 for Android allows directory traversal via a pathname obtained in a file-transfer request from a remote peer, as demonstrated by writing to tgnet.dat or tgnet.dat.bak.

Telegram Desktop 0.10.19 uses 0755 permissions for $HOME/.TelegramDesktop, which allows local users to obtain sensitive authentication information via standard filesystem operations.