CVE-2026-109742: Firefox PDF Viewer Information Disclosure

CVE-2026-109742 Overview

CVE-2026-109742 is an information disclosure vulnerability in the PDF Viewer component of Mozilla Firefox, Firefox ESR, and Thunderbird. The flaw is classified under [CWE-200] (Exposure of Sensitive Information to an Unauthorized Actor). An attacker can leverage a crafted PDF document to expose data that should remain confined to the browser or mail client context. Exploitation requires user interaction, typically opening or previewing a malicious PDF. Mozilla addressed the issue in Firefox 158, Firefox ESR 153.5, Thunderbird 158, and Thunderbird 153.5.

Critical Impact
A remote attacker can trigger disclosure of sensitive data from the PDF Viewer by convincing a user to open a crafted PDF file.

Affected Products

Mozilla Firefox versions prior to 158
Mozilla Firefox ESR versions prior to 153.5
Mozilla Thunderbird versions prior to 158 and prior to 153.5

Discovery Timeline

2026-10-13 - CVE-2026-109742 published to the National Vulnerability Database (NVD)
2026-10-13 - Last updated in NVD database

Technical Details for CVE-2026-109742

Vulnerability Analysis

The vulnerability resides in the PDF Viewer component shipped with Firefox and Thunderbird. Mozilla's PDF Viewer (pdf.js) parses and renders PDF content inside a privileged browser context. When processing a crafted document, the component exposes information that should not be reachable from the embedded PDF content. The disclosed data falls within the confidentiality boundary of the PDF Viewer and surrounding runtime. Mozilla rated the issue moderate and bundled the fix with the advisories listed below.

Full technical details are tracked in the Mozilla Bug Report #2058800 and the vendor notes in Mozilla Security Advisory MFSA-2026-106, MFSA-2026-108, MFSA-2026-109, and MFSA-2026-110.

Root Cause

The root cause is improper isolation inside the PDF Viewer rendering path, allowing a crafted document to read data that should remain inaccessible to untrusted PDF content. This maps to the Information Exposure weakness class under [CWE-200]. The fix hardens the viewer so that malicious PDF structures can no longer trigger the disclosure path.

Attack Vector

Exploitation requires the victim to open or preview a malicious PDF file. In Firefox, this typically means navigating to a page that serves the PDF inline. In Thunderbird, the vector extends to messages that embed or link PDF attachments. No elevated privileges are required on the attacker side, and the attack is deliverable over the network through standard web or email delivery channels.

No verified public exploit code is available for CVE-2026-109742 at the time of writing. See the Mozilla advisories referenced above for vendor-supplied technical context.

Detection Methods for CVE-2026-109742

Indicators of Compromise

Inbound email messages or web content delivering unsolicited PDF attachments to users running outdated Firefox, Firefox ESR, or Thunderbird builds.
Browser or mail client telemetry showing PDF Viewer renders followed by unexpected outbound network connections to attacker-controlled hosts.
Endpoints still reporting Firefox below 158, Firefox ESR below 153.5, or Thunderbird below 158 or 153.5 after the patch window.

Detection Strategies

Inventory installed Mozilla client versions through endpoint management tooling and flag any build below the fixed releases.
Correlate PDF file opens in Firefox or Thunderbird with subsequent anomalous DOM, script, or network behavior.
Use web and email gateway inspection to identify crafted PDFs with unusual object structures or embedded JavaScript targeting the viewer.

Monitoring Recommendations

Monitor process telemetry for firefox.exe and thunderbird.exe child processes that render PDFs, and alert on outbound requests immediately following PDF load events.
Track software update compliance for Firefox, Firefox ESR, and Thunderbird across managed endpoints.
Review email gateway logs for PDF attachments originating from low-reputation senders or newly registered domains.

How to Mitigate CVE-2026-109742

Immediate Actions Required

Upgrade Firefox to version 158 or later on all managed workstations.
Upgrade Firefox ESR deployments to 153.5 or later.
Upgrade Thunderbird to version 158 or 153.5, matching the deployed release channel.
Prioritize patching for high-risk users who routinely handle PDFs from external senders.

Patch Information

Mozilla released fixes in Firefox 158, Firefox ESR 153.5, Thunderbird 158, and Thunderbird 153.5. Refer to Mozilla Security Advisory MFSA-2026-106, MFSA-2026-108, MFSA-2026-109, and MFSA-2026-110 for release-specific guidance.

Workarounds

Configure Firefox to open PDFs in an external, patched viewer by setting pdfjs.disabled to true in about:config until updates are deployed.
In Thunderbird, disable inline PDF preview and require users to save attachments before opening them in a trusted viewer.
Block or quarantine inbound PDF attachments from untrusted senders at the mail gateway during the patch rollout window.

bash

# Example: enforce Firefox auto-update and disable built-in PDF Viewer via policies.json
{
  "policies": {
    "DisableAppUpdate": false,
    "AppAutoUpdate": true,
    "PDFjs": {
      "Enabled": false,
      "EnablePermissions": false
    }
  }
}