Security advisory: Telegram instant messenger IDN homograph attack

advisory, telegram, security research, homographs
Advisory information

Title: Telegram instant messenger IDN homograph attacks
Advisory reference: BLAZE-02-2019 (CVE-2019-10044)
Product: Telegram
Disclosure mode: Coordinated disclosure

Product description

Telegram is a messaging app with a focus on speed and security, it’s super-fast, simple and free. You can use Telegram on all your devices at the same time — your messages sync seamlessly across any number of your phones, tablets or computers.

With Telegram, you can send messages, photos, videos and files of any type (doc, zip, mp3, etc), as well as create groups for up to 200,000 people or channels for broadcasting to unlimited audiences. You can write to your phone contacts and find people by their usernames. As a result, Telegram is like SMS and email combined — and can take care of all your personal or business messaging needs. In addition to this, we support end-to-end encrypted voice calls.
(from Telegram FAQ:

Vulnerability details

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 http://blа 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.

Fix and recommendations

Blaze Information Security recommends Telegram to "defang" URLs with mixed scripts (e.g., Latin and Cyrillic in the same domain name) or URLs using scripts not installed or enabled in the device should be displayed in the message as un-clickable.

Alternatively, Telegram could implement the same strategy used by WhatsApp to mark the link as insecure, with a clear warning message to the user, when an IDN homograph domain name is encountered in the message.

The vulnerability seems to have been addressed by Telegram in its Windows version 1.5.12. The status of the other platforms is unknown, as the vendor has not responded to multiple contact attempts.


This vulnerability was discovered and researched by Julio Cesar Fort from Blaze Information Security (

Disclosure timeline

31/01/2019: Initial contact asking for the vendor's PGP key
01/02/2019: Vendor responded with PGP key, asking for details of the vulnerability to be sent via encrypted e-mail
01/02/2019: Vulnerability details sent via encrypted e-mail
Unknown: A fix was released at least for Telegram Desktop for Windows, version 1.5.12. The vendor has never contacted Blaze to inform about the fix.
25/03/2019: Advisory released


IDN homograph attack:
The Homograph Attack:

About Blaze Information Security

Blaze Information Security is a privately held, independent information security firm born from years of combined experience. With presence in South America and Europe, Blaze has a team of senior analysts with past experience in leading information security consulting companies around the world and a proven track record of published security research.

E-Mail: [email protected]
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PGP key fingerprint: DB53 D9D9 F0E1 E513 4F52 8219 C33B C7FA C5D0 E926