CVE-2023-25729: Mozilla Firefox Auth Bypass Vulnerability

CVE-2023-25729 Overview

CVE-2023-25729 is an authorization bypass vulnerability affecting Mozilla Firefox, Firefox ESR, and Thunderbird. The flaw exists in how permission prompts for opening external schemes are handled. Permission prompts were only shown for ContentPrincipals, which allowed browser extensions to open external schemes without user interaction by leveraging ExpandedPrincipals. This could lead to further malicious actions such as downloading files or interacting with software already installed on the system.

Critical Impact

Malicious extensions can bypass permission prompts to open external schemes without user consent, potentially leading to unauthorized file downloads or interaction with installed software.

Affected Products

• Mozilla Firefox versions prior to 110
• Mozilla Firefox ESR versions prior to 102.8
• Mozilla Thunderbird versions prior to 102.8

Discovery Timeline

• 2023-06-02 - CVE-2023-25729 published to NVD
• 2025-01-10 - Last updated in NVD database

Technical Details for CVE-2023-25729

Vulnerability Analysis

This vulnerability is classified under CWE-863 (Incorrect Authorization). The core issue lies in the inconsistent handling of permission checks between different types of principals in Mozilla's security model. When a web page or content attempts to open an external scheme (such as mailto:, tel:, or custom protocol handlers), the browser correctly displays a permission prompt to the user via ContentPrincipals. However, the same security check was not applied when requests originated from ExpandedPrincipals, which are used by browser extensions.

This differential treatment creates a security gap where extensions can silently trigger external scheme handlers without the user's knowledge or consent. The impact is significant because external schemes can invoke other applications on the user's system, potentially leading to code execution, data exfiltration, or social engineering attacks.

Root Cause

The root cause is an incomplete authorization check in Mozilla's permission prompt system. The code path that determines whether to display a permission dialog only validated ContentPrincipals, failing to account for ExpandedPrincipals used by browser extensions. This oversight allowed extensions to bypass the intended user confirmation step when opening external URLs or protocol handlers.

Attack Vector

The vulnerability requires user interaction to install a malicious extension, but once installed, the extension can exploit this flaw without further user consent. The attack scenario involves:

1. An attacker distributes a malicious browser extension through social engineering or by compromising a legitimate extension
2. Once installed, the extension uses ExpandedPrincipals to open external schemes
3. The browser fails to show the permission prompt, silently allowing the action
4. The external scheme handler is invoked, potentially launching applications or downloading files

The attack can be conducted remotely through the network, as extensions can be distributed via web downloads. While the vulnerability requires user action to install the extension, subsequent exploitation happens silently.

Detection Methods for CVE-2023-25729

Indicators of Compromise

• Unusual extension activity opening external protocol handlers without corresponding user interaction
• Unexpected application launches triggered by browser extensions
• Unauthorized file downloads initiated through external scheme handlers
• Browser extension logs showing external scheme requests without permission prompt events

Detection Strategies

• Monitor browser extension behavior for unauthorized external scheme access patterns
• Review installed browser extensions for suspicious or recently modified extensions
• Implement endpoint detection rules for unusual protocol handler invocations from browser processes
• Audit browser logs for external scheme requests that lack corresponding permission prompt events

Monitoring Recommendations

• Enable browser telemetry and logging to capture extension activity related to external schemes
• Deploy endpoint detection solutions that monitor for suspicious browser-spawned processes
• Implement network monitoring to detect unusual outbound connections following external scheme invocations
• Review browser extension permissions and audit for extensions with broad access rights

How to Mitigate CVE-2023-25729

Immediate Actions Required

• Update Mozilla Firefox to version 110 or later immediately
• Update Mozilla Firefox ESR to version 102.8 or later
• Update Mozilla Thunderbird to version 102.8 or later
• Review and remove any untrusted browser extensions
• Implement enterprise browser policies to restrict extension installation

Patch Information

Mozilla has addressed this vulnerability in the following releases:

• Firefox 110 - Full fix implemented for permission prompt handling
• Firefox ESR 102.8 - Security backport for ESR channel
• Thunderbird 102.8 - Security fix for mail client

For detailed patch information, refer to the official Mozilla Security Advisories:

• Mozilla Security Advisory MFSA-2023-05
• Mozilla Security Advisory MFSA-2023-06
• Mozilla Security Advisory MFSA-2023-07

Technical details are available in Mozilla Bug Report #1792138.

Workarounds

• Disable or remove all non-essential browser extensions until patching is complete
• Implement strict extension policies via enterprise management tools to whitelist only approved extensions
• Use browser profiles with limited extension permissions for high-risk browsing activities
• Configure external protocol handlers to require confirmation at the OS level where possible

# Firefox enterprise policy to restrict extensions (policies.json)
# Place in Firefox installation directory under /distribution/policies.json
{
  "policies": {
    "ExtensionSettings": {
      "*": {
        "installation_mode": "blocked"
      }
    },
    "DisableDeveloperTools": true
  }
}