CVE-2026-100803: Firefox WebExtensions Auth Bypass Flaw

CVE-2026-100803 Overview

CVE-2026-100803 is a same-origin policy (SOP) bypass in the Mozilla WebExtensions component. The flaw affects Mozilla Firefox, Firefox Extended Support Release (ESR), and Thunderbird. Mozilla fixed the issue in Firefox 157, Firefox ESR 115.42, Firefox ESR 140.17, Firefox ESR 153.4, Thunderbird 157, Thunderbird 153.4, and Thunderbird 140.17. The vulnerability allows extension-mediated cross-origin access that the browser's SOP enforcement should have blocked.

Critical Impact
A same-origin policy bypass in the WebExtensions layer can expose data from origins the extension or attacker should not be able to read, undermining a foundational browser security boundary.

Affected Products

Mozilla Firefox (versions prior to 157)
Mozilla Firefox ESR (versions prior to 115.42, 140.17, and 153.4)
Mozilla Thunderbird (versions prior to 140.17, 153.4, and 157)

Discovery Timeline

2026-09-29 - CVE-2026-100803 published to the National Vulnerability Database (NVD)
2026-09-30 - Last updated in NVD database

Technical Details for CVE-2026-100803

Vulnerability Analysis

The defect resides in the WebExtensions component that mediates access between extension contexts and web content. The same-origin policy is a browser security control that prevents documents or scripts loaded from one origin from reading data belonging to another origin. A bypass in this layer allows content or code paths to reach data across origin boundaries in ways the browser should reject.

Mozilla's advisories MFSA-2026-97 through MFSA-2026-103 track the fixes across the Firefox, Firefox ESR, and Thunderbird channels. Because Thunderbird embeds the Gecko platform and shares the WebExtensions implementation, the mail client inherits the same exposure and requires equivalent patches.

Root Cause

Mozilla's public advisory describes the issue as a same-origin policy bypass in the WebExtensions component. Detailed root-cause internals are not disclosed in the public NVD entry. See Mozilla Bug Report #2057988 for the tracking bug and Mozilla Security Advisory MFSA-2026-100 for advisory context.

Attack Vector

Exploitation requires processing extension-mediated or web-facing content that triggers the flawed origin check. A user visiting a malicious page, or interacting with a crafted message in Thunderbird, could allow the bypass to read or manipulate data across origin boundaries. No public exploit or proof-of-concept has been referenced in the advisory data at the time of publication.

No verified code example is available. Refer to the Mozilla Foundation Security Advisories linked above for further technical detail as it becomes public.

Detection Methods for CVE-2026-100803

Indicators of Compromise

No file-based indicators of compromise have been published for CVE-2026-100803.
Unexpected cross-origin data access or script activity originating from installed browser extensions may indicate exploitation attempts.
Presence of Firefox, Firefox ESR, or Thunderbird versions older than the fixed builds on managed endpoints.

Detection Strategies

Inventory installed Mozilla applications and compare versions against the fixed releases listed in the Mozilla advisories.
Monitor browser and extension telemetry for anomalous cross-origin HTTP requests initiated by extension processes.
Correlate outbound traffic from Firefox and Thunderbird processes with newly installed or updated WebExtensions.

Monitoring Recommendations

Track software inventory changes via endpoint management to identify unpatched Firefox, Firefox ESR, and Thunderbird instances.
Alert on new extension installations from untrusted sources on corporate devices.
Ingest browser and process telemetry into a centralized data lake to hunt for cross-origin access anomalies following extension activity.

How to Mitigate CVE-2026-100803

Immediate Actions Required

Update Firefox to 157 or later on all endpoints.
Update Firefox ESR to 115.42, 140.17, or 153.4 depending on the ESR channel in use.
Update Thunderbird to 140.17, 153.4, or 157 depending on the release channel.
Prioritize systems where users install third-party browser extensions or process untrusted web content.

Patch Information

Mozilla released fixes across the Firefox and Thunderbird product lines. Refer to the vendor advisories for build-specific details: MFSA-2026-97, MFSA-2026-98, MFSA-2026-99, MFSA-2026-100, MFSA-2026-101, MFSA-2026-102, and MFSA-2026-103.

Workarounds

Restrict installation of WebExtensions through enterprise policy until systems are patched.
Disable or remove non-essential browser extensions on managed endpoints.
Enforce automatic updates for Firefox and Thunderbird through group policy or configuration management to shorten exposure windows.

bash

# Enforce Firefox auto-update via enterprise policy (policies.json)
{
  "policies": {
    "DisableAppUpdate": false,
    "AppAutoUpdate": true,
    "ExtensionSettings": {
      "*": {
        "installation_mode": "blocked"
      }
    }
  }
}