CVE-2026-0877: Mozilla Firefox Auth Bypass Vulnerability

CVE-2026-0877 Overview

CVE-2026-0877 is a high-severity mitigation bypass vulnerability in the DOM: Security component of Mozilla Firefox and Mozilla Thunderbird. The flaw weakens browser security mitigations that protect the Document Object Model (DOM) layer, allowing attackers to circumvent defensive controls intended to block hostile web content. Exploitation requires user interaction, such as visiting a crafted page or opening a malicious email message. Mozilla fixed the issue in Firefox 147, Firefox ESR 115.32, Firefox ESR 140.7, Thunderbird 147, and Thunderbird 140.7. The vulnerability is classified under CWE-693: Protection Mechanism Failure.

Critical Impact
A network-based attacker can bypass DOM security mitigations to compromise confidentiality and integrity of browser-rendered content with only user interaction.

Affected Products

Mozilla Firefox (versions prior to 147)
Mozilla Firefox ESR (prior to 115.32 and 140.7)
Mozilla Thunderbird (prior to 147 and 140.7)

Discovery Timeline

2026-01-13 - CVE-2026-0877 published to NVD
2026-04-13 - Last updated in NVD database

Technical Details for CVE-2026-0877

Vulnerability Analysis

The vulnerability resides in the DOM: Security component, which enforces protection mechanisms such as Content Security Policy (CSP), same-origin restrictions, and sandboxing for web content. A flaw in this component allows attackers to bypass at least one of these mitigations. Once bypassed, hostile web content can perform actions the browser would normally prevent. The issue affects both the standard Firefox release branch and the Extended Support Release (ESR) branch, and is shared with Thunderbird, which inherits the Gecko rendering engine.

Root Cause

The root cause is a protection mechanism failure [CWE-693] in the DOM security enforcement logic. The component fails to consistently apply security checks under specific conditions, allowing crafted content to evade controls that should otherwise block it. Mozilla has not published low-level technical details for this issue. Refer to Mozilla Bug Report #1999257 for further technical context.

Attack Vector

Exploitation occurs over the network and requires user interaction. An attacker hosts a crafted web page or delivers HTML email content to a Thunderbird user. When the victim loads the content, the DOM security mitigation is bypassed, exposing browser confidentiality and integrity. The vulnerability does not require authentication or elevated privileges and does not affect availability. No public proof-of-concept or in-the-wild exploitation has been reported.

No verified exploit code is publicly available. See Mozilla Security Advisory MFSA-2026-01 for vendor details.

Detection Methods for CVE-2026-0877

Indicators of Compromise

Browser telemetry showing Firefox or Thunderbird versions below the patched releases (Firefox 147, ESR 115.32, ESR 140.7, Thunderbird 147, Thunderbird 140.7) on managed endpoints.
Outbound connections from firefox.exe or thunderbird.exe to unfamiliar domains immediately after user click activity.
Unexpected child processes spawned by browser or mail client processes following user interaction with external content.

Detection Strategies

Inventory installed Firefox and Thunderbird versions across the environment and flag any builds older than the fixed releases.
Monitor web proxy and DNS logs for known phishing infrastructure that delivers crafted HTML content to internal users.
Correlate user click events with subsequent anomalous DOM activity, script execution, or credential prompts.

Monitoring Recommendations

Enable centralized browser version reporting through endpoint management tooling.
Forward browser and email client process telemetry to a SIEM or data lake for retrospective hunting against new IOCs.
Subscribe to Mozilla security advisories and ingest new MFSA bulletins into vulnerability management workflows.

How to Mitigate CVE-2026-0877

Immediate Actions Required

Upgrade Firefox to version 147 or later, Firefox ESR to 115.32 or 140.7, and Thunderbird to 147 or 140.7.
Push updates through enterprise software distribution to ensure no users remain on vulnerable builds.
Restart browser and mail client processes after patching to ensure the updated binaries are loaded.

Patch Information

Mozilla addressed CVE-2026-0877 in Firefox 147, Firefox ESR 115.32, Firefox ESR 140.7, Thunderbird 147, and Thunderbird 140.7. Patch details are documented across MFSA-2026-01, MFSA-2026-02, MFSA-2026-03, MFSA-2026-04, and MFSA-2026-05.

Workarounds

Disable rendering of remote content in Thunderbird until the patched version is deployed.
Restrict browsing to trusted sites through proxy allow-lists where feasible, since exploitation requires user interaction with crafted content.
Apply enterprise browser policies that enforce strict site isolation and disable unnecessary scripting on untrusted origins.

bash

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

# Verify installed Thunderbird version
thunderbird --version

# Example: enforce auto-update via Firefox policies.json
# /etc/firefox/policies/policies.json
# {
#   "policies": {
#     "DisableAppUpdate": false,
#     "AppAutoUpdate": true
#   }
# }