CVE-2025-1019: Mozilla Firefox Spoofing Vulnerability

CVE-2025-1019 Overview

CVE-2025-1019 is a user interface spoofing vulnerability affecting Mozilla Firefox and Mozilla Thunderbird. The flaw allows the z-order of browser windows to be manipulated to hide the fullscreen notification. An attacker can leverage this behavior to conceal the visual indicator that alerts users when a page enters fullscreen mode. This creates conditions for spoofing attacks, where a malicious page can impersonate a trusted application or operating system interface without the user noticing the transition. The vulnerability is classified under [CWE-1021] Improper Restriction of Rendered UI Layers or Frames. Mozilla addressed the issue in Firefox 135 and Thunderbird 135.

Critical Impact
Attackers can hide the fullscreen notification in Firefox and Thunderbird, enabling UI spoofing attacks that impersonate trusted interfaces to harvest credentials or deceive users.

Affected Products

Mozilla Firefox versions prior to 135
Mozilla Thunderbird versions prior to 135
Systems where users interact with untrusted web content in the affected browsers

Discovery Timeline

2025-02-04 - CVE-2025-1019 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-1019

Vulnerability Analysis

The vulnerability resides in how Firefox and Thunderbird manage the stacking order of browser windows during fullscreen transitions. When a web page requests fullscreen mode, the browser displays a notification informing the user of the change. This notification is intended to prevent malicious sites from mimicking operating system dialogs or trusted application interfaces.

An attacker can manipulate the z-order so that another window renders above the fullscreen notification. The notification remains present in the rendering tree but is visually obscured. Users therefore receive no warning that the page has entered fullscreen mode. The impact vector is limited to integrity, with no direct effect on confidentiality or availability. Exploitation requires user interaction, such as visiting a crafted page.

Root Cause

The root cause is improper restriction of rendered UI layers, tracked as [CWE-1021]. The browser fails to guarantee that the fullscreen notification remains the topmost layer during the transition. Content controlled by the page can occupy a higher z-order position, defeating the security purpose of the notification.

Attack Vector

Exploitation occurs over the network through a crafted web page. An attacker hosts content that programmatically triggers fullscreen mode and simultaneously renders overlays that obscure the notification. Once fullscreen is active without visible warning, the page can render fake login prompts, fake operating system dialogs, or counterfeit banking interfaces. No verified public proof-of-concept or exploit is documented for this CVE. Technical details are available in Mozilla Bug Report #1940162.

Detection Methods for CVE-2025-1019

Indicators of Compromise

Firefox or Thunderbird installations reporting version numbers earlier than 135 in endpoint inventory data
Browser telemetry showing repeated fullscreen API invocations from a single domain within a short window
User reports of unexpected credential prompts or system dialogs appearing inside a browser session

Detection Strategies

Monitor endpoint software inventory for Mozilla Firefox and Thunderbird builds below version 135
Correlate DNS and proxy logs against threat intelligence feeds identifying domains used in phishing kits that abuse fullscreen APIs
Review browser console logs and content security policy reports for pages invoking Element.requestFullscreen() alongside overlay elements

Monitoring Recommendations

Deploy endpoint visibility to track browser versions and flag installations that fall behind the fixed release
Ingest browser and proxy telemetry into a centralized data lake for hunting patterns of fullscreen abuse combined with credential input events
Alert on outbound traffic to newly registered domains that serve pages requesting fullscreen mode immediately on load

How to Mitigate CVE-2025-1019

Immediate Actions Required

Update Mozilla Firefox to version 135 or later on all managed endpoints
Update Mozilla Thunderbird to version 135 or later on all managed endpoints
Enforce automatic browser updates through enterprise policy to prevent regression
Communicate to users that fullscreen prompts inside the browser should be treated with caution when requesting credentials

Patch Information

Mozilla released fixes in Firefox 135 and Thunderbird 135. Details are published in Mozilla Security Advisory MFSA-2025-07 and Mozilla Security Advisory MFSA-2025-11. Administrators should validate that deployed builds match or exceed these versions across Windows, macOS, and Linux endpoints.

Workarounds

Restrict the use of the fullscreen API through browser enterprise policies where supported
Disable fullscreen requests site-wide using the full-screen-api.enabled preference in about:config as a temporary control
Train users to press the Escape key and verify the browser chrome before entering credentials on unfamiliar pages

bash

# Configuration example: disable fullscreen API in Firefox via policies.json
# Place this file in the Firefox distribution directory
{
  "policies": {
    "Preferences": {
      "full-screen-api.enabled": {
        "Value": false,
        "Status": "locked"
      }
    },
    "DisableAppUpdate": false
  }
}