ASA-2019-00155 – Telegram: Internationalized domain name (IDN) homograph attacks

Allele Security Alert



ASA-2019-00155, CVE-2019-10044


Internationalized domain name (IDN) homograph attacks


Telegram Messenger LLP


Telegram for iPhone and iPad
Telegram for Android
Telegram for Windows
Telegram for Linux

Affected version(s)

All versions of Telegram for iPhone and iPad
All versions of Telegram for Android
All versions of Telegram for Windows
All versions of Telegram for Linux

Fixed version(s)

Telegram for Windows 1.5.12
The status of other products is unknown

Proof of concept



Telegram (tested on all mobile versions and Linux and Windows for desktop) is vulnerable to an IDN homograph attack when displaying messages containing URLs.

Homograph attack is a security vulnerability that can deceive users into thinking they are visiting a certain website when in fact they are directed to a different, but homograph, domain name. This type of vulnerability can be used to weaponize social engineering, increasing the chances for a successful attack.

Upon receiving a message with a link, Telegram renders it in a clickable format and the font used to display the message makes it impossible to distinguish between the legitimate URL and the malicious URL, for example:

Legitimate URL:
Malicious URL: http://blа – with the ‘a’ as a Cyrillic character, not Latin

On top of that Telegram renders a preview of the web site, making it even more deceptive for a user.

Upon clicking on the malicious link, a user will be taken to instead of the real, despite the fact the link is displayed exactly as the expected web site.

A sample attack scenario against a Telegram user:

  • An activist or person of interest uses Telegram
  • The person receives a URL in a Telegram message disguised as a legitimate, Telegram renders a preview
  • The user clicks on the link
  • The malicious URL serves a one-click browser exploit
  • Target gets infected with mobile or desktop malware

Additionally, the mobile and desktop versions of Tor Browser is vulnerable to the same class of attack. Therefore, users of Telegram with Tor Browser are prone to full-blown phishing attacks.

Technical details



Julio Cesar Fort (Blaze Information Security)


Security advisory: Telegram instant messenger IDN homograph attack



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Last modified: March 25, 2019

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