CVE-2026-109765: Firefox Graphics Text Race Condition Flaw

CVE-2026-109765 Overview

CVE-2026-109765 is a race condition leading to a use-after-free vulnerability in the Graphics: Text component of Mozilla Firefox and Thunderbird. The flaw affects text rendering code paths and can be triggered by crafted web content that causes concurrent access to freed memory. Mozilla fixed the issue in Firefox 158, Firefox ESR 153.5, Thunderbird 158, and Thunderbird 153.5. The vulnerability is categorized under [CWE-362] (Concurrent Execution using Shared Resource with Improper Synchronization).

Critical Impact
Successful exploitation requires user interaction and high attack complexity, but may allow attackers to corrupt memory in the browser process, potentially leading to limited information disclosure or integrity impact.

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 CVE-2026-109765 published to NVD
2026-10-13 - Last updated in NVD database

Technical Details for CVE-2026-109765

Vulnerability Analysis

The vulnerability resides in the Graphics: Text component responsible for text layout and rendering. Multiple threads access a shared text-related object without correct synchronization. One thread can free the object while another retains a reference, producing a use-after-free condition when the dangling reference is later dereferenced.

Exploitation requires rendering of attacker-controlled web content and depends on precise thread timing, which raises attack complexity. The impact scope is limited to confidentiality and integrity within the browser process, with no direct availability impact reported by Mozilla.

Root Cause

The root cause is improper synchronization around a shared object in the text rendering pipeline, classified as [CWE-362]. Lifetime management of the object does not account for concurrent access, allowing deallocation while another thread continues to use the reference.

Attack Vector

An attacker hosts a malicious page or sends an HTML email that triggers specific text rendering operations. The victim must load the content in a vulnerable Firefox or Thunderbird build. When the race window is hit, the browser accesses freed memory, which an attacker may shape to influence program state.

No verified public proof-of-concept code is available. See the Mozilla Bug Report #2073396 and Mozilla Security Advisory MFSA-2026-109 for vendor details.

Detection Methods for CVE-2026-109765

Indicators of Compromise

Unexpected Firefox or Thunderbird process crashes with signatures referencing the Graphics: Text component or gfx modules.
Browser crash reports showing use-after-free faults during page navigation to untrusted sites or HTML email rendering.
Outbound connections to attacker infrastructure immediately following a renderer crash event.

Detection Strategies

Inventory endpoint software to identify Firefox and Thunderbird builds older than the fixed releases (Firefox 158, ESR 153.5, Thunderbird 158, Thunderbird 153.5).
Collect browser crash telemetry and alert on repeated renderer or content-process crashes tied to text rendering.
Correlate web proxy logs with crash events to identify domains that may be probing the vulnerability.

Monitoring Recommendations

Monitor EDR telemetry for anomalous child-process creation or memory region allocations originating from firefox.exe or thunderbird.exe.
Track email gateway logs for HTML messages containing unusual font or text styling payloads delivered to users running vulnerable Thunderbird builds.
Review patch compliance dashboards weekly to confirm Firefox and Thunderbird updates are applied enterprise-wide.

How to Mitigate CVE-2026-109765

Immediate Actions Required

Upgrade Firefox to version 158 or later and Firefox ESR to 153.5 or later across all managed endpoints.
Upgrade Thunderbird to version 158 or later, or ESR 153.5 or later, on all workstations and mail clients.
Prioritize patching for high-risk users such as executives and developers who browse untrusted content.

Patch Information

Mozilla fixed CVE-2026-109765 in Firefox 158, Firefox ESR 153.5, Thunderbird 158, and Thunderbird 153.5. Refer to MFSA-2026-106, MFSA-2026-108, MFSA-2026-109, and MFSA-2026-110 for release-specific guidance.

Workarounds

Disable automatic HTML rendering in Thunderbird and view messages as plain text where feasible.
Restrict browsing to trusted domains using web filtering until patches are deployed.
Enforce enterprise policies that enable Firefox auto-update to prevent regression to vulnerable builds.

bash

# Configuration example
# Enforce Firefox auto-update via policies.json on Linux/macOS/Windows
# Location (Windows): C:\Program Files\Mozilla Firefox\distribution\policies.json
{
  "policies": {
    "DisableAppUpdate": false,
    "AppAutoUpdate": true
  }
}