CVE-2026-92055: Firefox DevTools Privilege Escalation Flaw

CVE-2026-92055 Overview

CVE-2026-92055 is a privilege escalation vulnerability in the DevTools component of Mozilla Firefox and Thunderbird. Mozilla fixed the flaw in Firefox 156, Firefox ESR 153.3, Thunderbird 156, and Thunderbird 153.3. The vulnerability is categorized under [CWE-269] Improper Privilege Management. Exploitation requires user interaction over a network vector, and successful attacks can compromise confidentiality, integrity, and availability of the affected browser or mail client. Mozilla published security advisories MFSA-2026-90, MFSA-2026-93, MFSA-2026-94, and MFSA-2026-96 addressing the issue across the affected product lines.

Critical Impact
An attacker who convinces a user to interact with malicious content can elevate privileges through the DevTools component and gain code execution within a higher-privileged browser context.

Affected Products

Mozilla Firefox versions prior to 156
Mozilla Firefox ESR versions prior to 153.3
Mozilla Thunderbird versions prior to 156 and 153.3

Discovery Timeline

2026-09-15 - CVE-2026-92055 published to NVD
2026-09-16 - Last updated in NVD database

Technical Details for CVE-2026-92055

Vulnerability Analysis

The flaw resides in the Firefox DevTools component, which is the built-in developer tooling exposed to web content and extensions. Improper privilege management allows content operating at a lower trust level to influence DevTools operations that execute with elevated browser privileges. When a user interacts with attacker-controlled content, the DevTools boundary can be crossed, giving the attacker capabilities normally reserved for chrome-privileged code. Because Thunderbird embeds the same Gecko rendering and DevTools stack, the mail client is affected under identical conditions when remote content is processed.

Root Cause

The root cause is improper privilege management ([CWE-269]) inside DevTools code paths. Trust boundaries between untrusted web content and privileged DevTools APIs were not consistently enforced, permitting privilege boundaries to be crossed during specific interactions. Mozilla addressed the issue in the fixed versions but has not published low-level exploitation details in the public advisory.

Attack Vector

The attack vector is network-based and requires user interaction, consistent with a browser-delivered exploit chain. A remote attacker hosts crafted content that a victim visits or opens in a supported client. Interaction with the malicious page or message triggers the DevTools code path that elevates privileges. No prior authentication is required, and the scope remains unchanged, but the resulting privileges apply to the affected browser process.

No public proof-of-concept or verified exploit code is available. Refer to the Mozilla Bug Report #2063652 and Mozilla Security Advisory MFSA-2026-90 for the vendor's technical description.

Detection Methods for CVE-2026-92055

Indicators of Compromise

Unexpected launches of firefox.exe or thunderbird.exe child processes spawning shells, scripting hosts, or LOLBins after web browsing activity.
Creation or modification of files under Firefox or Thunderbird profile directories that correlate with DevTools activity on end-user systems where developers are not present.
Outbound network connections from browser processes to previously unseen domains immediately following user interaction with a link or attachment.

Detection Strategies

Inventory Firefox, Firefox ESR, and Thunderbird versions across the fleet and flag any host running versions below Firefox 156, ESR 153.3, or Thunderbird 156 / 153.3.
Alert on browser processes creating executable content or invoking cmd.exe, powershell.exe, wscript.exe, or bash shortly after page loads.
Correlate DevTools-related telemetry (extension changes, remote-debugging protocol connections on localhost:6000+ ranges) with unusual user-agent activity.

Monitoring Recommendations

Ingest endpoint process telemetry and DNS logs into a centralized data lake for cross-source correlation on browser-originated activity.
Monitor for enablement of devtools.chrome.enabled or devtools.debugger.remote-enabled preferences on systems that do not require them.
Track patch compliance for Mozilla products via vulnerability management scans and enforce reporting on any host missing the fixed builds.

How to Mitigate CVE-2026-92055

Immediate Actions Required

Upgrade Firefox to version 156, Firefox ESR to 153.3, Thunderbird to 156, and Thunderbird ESR to 153.3 as soon as validation completes.
Prioritize patching for developer workstations and any users with elevated local privileges, since exploitation grants attacker code execution in the browser context.
Restart affected applications after patching to ensure the vulnerable DevTools code paths are unloaded.

Patch Information

Mozilla released fixes in Firefox 156, Firefox ESR 153.3, Thunderbird 156, and Thunderbird 153.3. Full details are available in Mozilla Security Advisory MFSA-2026-90, MFSA-2026-93, MFSA-2026-94, and MFSA-2026-96.

Workarounds

Disable browser DevTools for non-developer users through enterprise policy where operationally feasible.
Restrict users from installing untrusted extensions and enforce Enterprise Policies to lock down DevTools preferences.
Apply network-layer controls that block known malicious domains and enforce web content filtering until patches are deployed.

bash

# Configuration example: Firefox Enterprise Policy to disable DevTools
# policies.json placed alongside the Firefox binary or in /etc/firefox/policies/
{
  "policies": {
    "DisableDeveloperTools": true,
    "BlockAboutConfig": true,
    "ExtensionSettings": {
      "*": {
        "installation_mode": "blocked"
      }
    }
  }
}