CVE-2026-2345

Executive Summary

CVE-2026-2345 identifies a critical security flaw in the Proctorio Chrome Extension, a widely used online proctoring tool. The vulnerability stems from multiple window.addEventListener('message', ...) handlers that fail to validate the event.origin attribute. An internal messaging bridge processes messages solely based on the presence of a fromWebsite property, allowing any arbitrary website to send crafted messages to the extension. This can lead to unauthorized data access or manipulation of the extension’s internal state, potentially compromising exam integrity and user privacy.

The CVSS score of 3.6 reflects a low severity due to the requirement of user interaction (e.g., visiting a malicious site while the extension is active). However, the lack of a known patch and the sensitive nature of proctoring data elevate the risk for educational institutions and test-takers.

Supply Chain & Package Security

This vulnerability affects a Chrome extension distributed via the Chrome Web Store. The extension is not an NPM, PyPI, Go, or Ruby package, but it is part of the browser extension supply chain. Users should verify the extension’s integrity by checking its ID and ensuring it is obtained from the official store. No open-source patch data is available from OSV or GitHub.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:N/A:N
• Base Score: 3.6 (Low)
• Attack Vector: Network (AV:N) – The attack is launched remotely.
• Attack Complexity: Low (AC:L) – No special conditions required.
• Privileges Required: None (PR:N) – No authentication needed.
• User Interaction: Required (UI:R) – Victim must visit a malicious page.
• Scope: Unchanged (S:U) – The vulnerable component and impacted component are the same.
• Confidentiality Impact: Low (C:L) – Limited data exposure possible.
• Integrity Impact: None (I:N) – No data modification.
• Availability Impact: None (A:N) – No denial of service.

Threat Level: Low, but with potential for escalation if combined with other vulnerabilities or if sensitive proctoring data (e.g., screen captures, audio) is leaked.

Deep Dive & Mechanics

The Proctorio Chrome Extension uses window.addEventListener('message', ...) to communicate with web pages. This is a standard mechanism for cross-origin communication, but it requires strict origin validation. In this case, the extension’s message handlers check only for the presence of a fromWebsite property in the message data, ignoring the event.origin attribute. This means any website (e.g., attacker.com) can send a message to the extension if the user is browsing that site.

The internal messaging bridge processes these messages, potentially allowing an attacker to:

• Trigger extension actions (e.g., start/stop recording).
• Read sensitive data from the extension’s storage.
• Manipulate the proctoring session.

The vulnerability is rooted in CWE-346 (Origin Validation Error), where the software does not properly verify the source of a communication channel.

Vulnerable vs. Patched Code

Vulnerable Code (Conceptual):

window.addEventListener('message', function(event) {
    if (event.data && event.data.fromWebsite) {
        // Process message without checking event.origin
        processMessage(event.data);
    }
});

Patched Code (Conceptual):

window.addEventListener('message', function(event) {
    // Validate origin against allowed domains
    if (event.origin !== 'https://proctorio.com' && event.origin !== 'chrome-extension://' + chrome.runtime.id) {
        return;
    }
    if (event.data && event.data.fromWebsite) {
        processMessage(event.data);
    }
});

The patched version adds a strict check on event.origin to ensure messages only come from trusted sources (e.g., the extension’s own pages or the official Proctorio domain).

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• Victim has Proctorio Chrome Extension installed and active.
• Victim visits a malicious website controlled by the attacker.

Exploit Steps:

1. Host a malicious HTML page on an attacker-controlled server.
2. Include JavaScript that sends a crafted postMessage to the extension:

   // Exploit payload
   window.postMessage({
       fromWebsite: true,
       action: 'getExamData',
       // Additional parameters
   }, '*');
   

3. The extension processes the message due to missing origin validation.
4. Attacker receives sensitive data via a callback or by reading extension storage.

Note: No public exploit code is available, but the above demonstrates the attack vector.

Blue Team (Detection & Defense)

Detection:

• Monitor for unusual postMessage events from unknown origins in browser console logs.
• Use browser extension security tools to audit message listeners.

Defense:

• Apply the conceptual patch by validating event.origin in all message handlers.
• Restrict the extension’s permissions to only necessary domains.
• Educate users to avoid visiting untrusted sites while the extension is active.

Indicators of Compromise (IOCs)

• Network: Unusual postMessage traffic to chrome-extension:// origins.
• File: Malicious HTML/JS files that contain window.postMessage with fromWebsite property.
• Behavior: Extension performing actions (e.g., recording) without user initiation.
• Registry/Config: No specific registry IOCs; focus on browser extension behavior.

Detection Engineering

YARA Rule

rule Detect_Proctorio_Origin_Bypass {
    meta:
        description = "Detects JavaScript code exploiting CVE-2026-2345 by sending postMessage with fromWebsite property"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2026-2345"
    strings:
        $msg = "postMessage" nocase
        $prop = "fromWebsite" nocase
    condition:
        all of them
}

Sigma Rule

title: Proctorio Extension Origin Validation Bypass
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects potential exploitation of CVE-2026-2345 via postMessage with fromWebsite property
author: Valters IT
logsource:
    category: browser_extension
    product: chrome
detection:
    selection:
        EventID: 1001  # Example: Chrome extension message event
        MessageData|contains: 'fromWebsite'
    condition: selection
falsepositives:
    - Legitimate Proctorio communication (if patched)
level: medium

Suricata Rule

alert http any any -> any any (msg:"CVE-2026-2345 Proctorio Origin Bypass Attempt"; flow:to_server; content:"postMessage"; http_client_body; content:"fromWebsite"; distance:0; within:100; classtype:attempted-recon; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable the extension until a patch is released by Proctorio.
2. Apply a Content Security Policy (CSP) that restricts message origins.
3. Monitor browser logs for suspicious postMessage events.

Long-term Fix:

• Proctorio must release an update that validates event.origin in all message handlers.
• Users should enable automatic updates for Chrome extensions.

Workarounds:

• Use a different proctoring solution that is not vulnerable.
• Run the extension in a sandboxed browser profile with limited permissions.

Valters IT
Securing Digital Ecosystems