CVE-2026-109723: Firefox Integer Overflow Vulnerability

CVE-2026-109723 Overview

CVE-2026-109723 is an integer overflow vulnerability [CWE-190] in the Layout: Text and Fonts component used by Mozilla Firefox and Thunderbird. Mozilla fixed the issue in Firefox 158, Firefox ESR 153.5, Thunderbird 158, and Thunderbird 153.5. Exploitation requires user interaction, such as rendering an attacker-controlled page containing crafted text or font data. Successful exploitation can lead to limited impacts on confidentiality and integrity within the browser process.

Critical Impact
An attacker can trigger an integer overflow in the text and fonts layout path by delivering crafted web content, resulting in limited information exposure and memory state corruption inside the renderer.

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-2026-109723 published to the National Vulnerability Database (NVD)
2026-10-15 - Last updated in NVD database

Technical Details for CVE-2026-109723

Vulnerability Analysis

The flaw resides in the Layout: Text and Fonts component, which handles measurement, shaping, and rendering of text runs and font glyph data. An integer overflow occurs when arithmetic operations on size or offset values exceed the bounds of the underlying integer type. The overflowed value then feeds subsequent allocation or indexing logic, producing an undersized buffer or an out-of-range offset. The practical impact is scoped to the renderer process, which aligns with the user interaction requirement and the limited confidentiality and integrity effects reported by Mozilla.

Root Cause

The root cause is improper validation of arithmetic in text and font layout calculations [CWE-190]. When attacker-supplied inputs drive values such as glyph counts, run lengths, or buffer dimensions, the computed result wraps around. Downstream code treats the wrapped value as a valid size, breaking memory safety invariants.

Attack Vector

Exploitation requires an attacker to serve crafted HTML, CSS, or font resources that the victim loads in a vulnerable Firefox or Thunderbird client. Thunderbird is affected when it renders remote content in email messages. No authentication is required, but the victim must interact by visiting a page or opening a message. See the Mozilla Bug Report #2074171 and the Mozilla Security Advisory MFSA-2026-106 for additional details.

The vulnerability manifests in the text and font layout path. Mozilla has not published proof-of-concept code for this issue, and no verified exploit is publicly available.

Detection Methods for CVE-2026-109723

Indicators of Compromise

Unexpected Firefox or Thunderbird renderer process crashes with signatures referencing text shaping, font loading, or layout code paths.
Repeated crash reports originating from a single email message or web page containing unusual font or text content.
Outbound connections to recently registered domains serving custom web fonts combined with renderer instability.

Detection Strategies

Inventory Firefox, Firefox ESR, and Thunderbird versions across managed endpoints and flag any installation below the fixed releases.
Collect browser crash telemetry and correlate renderer faults with URLs or message IDs visited at the time of failure.
Monitor email gateways for messages that reference remote content, custom @font-face declarations, or embedded font payloads.

Monitoring Recommendations

Forward endpoint process and crash telemetry to a central analytics platform for cross-host correlation.
Alert on child process termination events for firefox.exe, thunderbird.exe, and their Linux and macOS equivalents when accompanied by access violations.
Track user agent strings in web proxy logs to identify hosts still running vulnerable Firefox or Thunderbird builds.

How to Mitigate CVE-2026-109723

Immediate Actions Required

Upgrade Firefox to version 158 or later, and Firefox ESR to 153.5 or later, on all managed endpoints.
Upgrade Thunderbird to version 158 or later, or 153.5 or later for the ESR branch.
Enforce automatic browser and mail client updates through enterprise policy to reduce patch latency.

Patch Information

Mozilla addressed the issue in Firefox 158, Firefox ESR 153.5, Thunderbird 158, and Thunderbird 153.5. Full remediation details are available in Mozilla Security Advisory MFSA-2026-106, MFSA-2026-108, MFSA-2026-109, and MFSA-2026-110.

Workarounds

Disable automatic loading of remote content in Thunderbird to prevent passive rendering of crafted messages.
Restrict download and use of custom web fonts via enterprise content filtering where feasible.
Isolate browsing of untrusted content to dedicated virtual machines or hardened browser profiles until patches are applied.

bash

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

# Verify installed Thunderbird version on Linux endpoints
thunderbird --version