CVE-2025-9183: Mozilla Firefox Address Bar Spoofing Flaw

CVE-2025-9183 Overview

CVE-2025-9183 is an address bar spoofing vulnerability affecting Mozilla Firefox and Firefox Extended Support Release (ESR). The flaw resides in the browser's Address Bar component and allows a remote attacker to display misleading URL information to the user. Mozilla addressed the issue in Firefox 142 and Firefox ESR 140.2. The vulnerability maps to CWE-451, User Interface (UI) Misrepresentation of Critical Information.

Exploitation requires user interaction, such as visiting a crafted web page. Successful exploitation supports phishing and credential-theft campaigns by making malicious pages appear to originate from trusted domains.

Critical Impact
An attacker can manipulate the Firefox address bar to spoof a trusted origin, enabling convincing phishing pages that bypass URL-based user verification.

Affected Products

Mozilla Firefox versions prior to 142
Mozilla Firefox ESR versions prior to 140.2
Desktop builds tracked under CPE cpe:2.3:a:mozilla:firefox:*

Discovery Timeline

2025-08-19 - CVE-2025-9183 published to the National Vulnerability Database (NVD)
2026-06-17 - Last updated in the NVD database

Technical Details for CVE-2025-9183

Vulnerability Analysis

The vulnerability is a UI misrepresentation flaw in the Firefox Address Bar (URL bar) component. The address bar is the primary trust signal that users rely on to verify the origin of a page. When the component fails to correctly render or reconcile the effective document origin with the displayed URL, an attacker-controlled page can display an arbitrary or misleading location string.

The defect falls under CWE-451, which covers cases where security-relevant information is presented to the user in a misleading way. Because the address bar is treated as authoritative, spoofing content there undermines phishing defenses that depend on URL inspection. Details on the underlying bug are tracked in Mozilla Bug 1976102.

Root Cause

The root cause is improper handling of URL presentation logic within the Address Bar component. The browser renders content or navigation state in a way that does not accurately reflect the true active document origin. Mozilla resolved the condition in the Firefox 142 and ESR 140.2 code lines as documented in MFSA-2025-64 and MFSA-2025-67.

Attack Vector

An attacker hosts a crafted page and lures a victim into visiting it, typically through a phishing link, malvertising, or a compromised third-party site. The page triggers navigation or scripting sequences that cause the address bar to display an attacker-chosen URL while the victim interacts with attacker-controlled content. The victim then submits credentials, tokens, or payment data believing the page belongs to the spoofed domain.

No code execution or memory corruption is involved. The vulnerability is exploited entirely through legitimate browser UI behavior. See Mozilla Bug 1976102 for technical bug tracking references.

Detection Methods for CVE-2025-9183

Indicators of Compromise

Outbound traffic from Firefox clients running versions prior to 142 or ESR versions prior to 140.2 to newly registered or low-reputation domains.
User reports of credential submission on pages that later appear unfamiliar in browser history.
Web proxy logs showing full-page navigations that do not match the URL later reported by users.

Detection Strategies

Inventory Firefox versions across the fleet and flag any endpoint running a build older than Firefox 142 or ESR 140.2.
Correlate phishing-report telemetry with the Firefox user agent to identify users targeted by spoofing campaigns.
Use secure web gateway URL categorization to block phishing infrastructure before the address bar is ever rendered.

Monitoring Recommendations

Monitor browser version telemetry through endpoint management tooling and alert on unpatched Firefox installations.
Track authentication events for anomalous logins that follow visits to unclassified domains from Firefox clients.
Feed browser and proxy logs into a centralized data lake to enable retroactive hunting once new phishing indicators emerge.

How to Mitigate CVE-2025-9183

Immediate Actions Required

Update Firefox to version 142 or later, and Firefox ESR to 140.2 or later, on all managed endpoints.
Push the update through enterprise deployment tooling rather than relying on user-initiated upgrades.
Reinforce phishing awareness training, emphasizing that URL bar contents alone are not a sufficient trust signal.

Patch Information

Mozilla shipped the fix in Firefox 142 and Firefox ESR 140.2. Refer to MFSA-2025-64 and MFSA-2025-67 for full advisory details, affected version ranges, and cross-referenced bug reports.

Workarounds

Enforce URL filtering and phishing protection at the network edge to reduce exposure to spoofing pages.
Require multi-factor authentication (MFA) so that credentials harvested through a spoofed page cannot be reused directly.
Restrict Firefox usage to enterprise-managed profiles with Safe Browsing and enhanced tracking protection enabled until patches are deployed.

bash

# Verify installed Firefox version on Linux/macOS endpoints
firefox --version

# Windows: query the installed version via registry
reg query "HKLM\SOFTWARE\Mozilla\Mozilla Firefox" /v CurrentVersion