CVE-2026-109768: Firefox Site Reports DOS Vulnerability

CVE-2026-109768 Overview

CVE-2026-109768 is a denial-of-service vulnerability in the Site Reports component of Mozilla Firefox and Thunderbird. The flaw affects versions prior to Firefox 158 and Thunderbird 158. An attacker can trigger resource exhaustion when a user interacts with crafted content, causing the application to become unresponsive. The weakness is classified under [CWE-400] (Uncontrolled Resource Consumption). Mozilla addressed the issue in advisories MFSA-2026-106 and MFSA-2026-109.

Critical Impact
Successful exploitation causes high availability loss in the Firefox or Thunderbird client, requiring user interaction with attacker-controlled content delivered over the network.

Affected Products

Mozilla Firefox versions prior to 158
Mozilla Thunderbird versions prior to 158
Site Reports component shared across both applications

Discovery Timeline

2026-10-13 - CVE-2026-109768 published to the National Vulnerability Database
2026-10-13 - Last updated in NVD database

Technical Details for CVE-2026-109768

Vulnerability Analysis

The vulnerability resides in the Site Reports component used by Firefox and Thunderbird. Processing a crafted site report input leads to uncontrolled resource consumption, degrading application availability. The flaw does not expose confidentiality or integrity, but it can render the client unusable until the process is terminated or restarted. Mozilla shipped the correction in Firefox 158 and Thunderbird 158. Full technical analysis is tracked in Mozilla Bug Report #2011368.

Root Cause

The root cause is uncontrolled resource consumption ([CWE-400]) within the Site Reports code path. The component fails to bound the resources allocated when handling report data, allowing attacker-controlled input to exhaust memory or CPU. Mozilla has not released additional implementation detail beyond the advisories.

Attack Vector

Exploitation requires network delivery and user interaction. A victim visiting a crafted web page or opening attacker-prepared content triggers the Site Reports code path. No privileges or authentication are required on the target system. The resulting condition is availability loss within the browser or mail client. See Mozilla Security Advisory MFSA-2026-109 for vendor guidance.

No verified proof-of-concept code is public. The vulnerability mechanism is described in prose per Mozilla's advisory; see the referenced bug and advisories for technical detail.

Detection Methods for CVE-2026-109768

Indicators of Compromise

Firefox or Thunderbird processes consuming abnormally high CPU or memory for extended periods
Application hangs or crashes correlating with navigation to untrusted sites or opening suspicious email content
Repeated restarts of firefox.exe or thunderbird.exe by the same user session

Detection Strategies

Inventory endpoint software versions and flag Firefox or Thunderbird installations below version 158
Correlate browser crash telemetry with recent URL navigation history to identify crafted content sources
Monitor web proxy logs for repeated requests to pages that precede client crashes

Monitoring Recommendations

Alert on sustained memory growth in browser or mail client processes beyond baseline thresholds
Track crash reporter submissions from Firefox and Thunderbird fleets for spikes tied to Site Reports
Review endpoint EDR telemetry for abnormal process termination events on user workstations

How to Mitigate CVE-2026-109768

Immediate Actions Required

Upgrade Firefox to version 158 or later on all managed endpoints
Upgrade Thunderbird to version 158 or later on all managed endpoints
Enforce automatic Mozilla update channels through enterprise policy where feasible
Communicate the risk to users and advise caution when opening links or email content from untrusted senders

Patch Information

Mozilla released fixes in Firefox 158 and Thunderbird 158. Refer to Mozilla Security Advisory MFSA-2026-106 and Mozilla Security Advisory MFSA-2026-109 for the complete list of addressed issues and version metadata.

Workarounds

No vendor-supplied workaround is documented; patching is the recommended path
Restrict browsing to trusted sites until patched versions are deployed
Disable automatic loading of remote content in Thunderbird to limit exposure to crafted email payloads

bash

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

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