CVE-2026-109773: Firefox and Thunderbird Sync DOS Vulnerability

CVE-2026-109773 Overview

CVE-2026-109773 is a denial-of-service vulnerability affecting the Sync component in Mozilla Firefox and Thunderbird. Mozilla fixed the issue in Firefox 158 and Thunderbird 158. The flaw is categorized under [CWE-400] Uncontrolled Resource Consumption and is reachable over the network with user interaction. A successful attack renders the browser or mail client unresponsive by exhausting resources through the Sync subsystem.

Critical Impact
Remote attackers can trigger availability loss in Firefox and Thunderbird clients by abusing the Sync component, requiring only that a user interacts with attacker-controlled content.

Affected Products

Mozilla Firefox versions prior to 158
Mozilla Thunderbird versions prior to 158
Mozilla Sync component (as shipped with the above products)

Discovery Timeline

2026-10-13 - CVE-2026-109773 published to the National Vulnerability Database (NVD)
2026-10-13 - Last updated in NVD database

Technical Details for CVE-2026-109773

Vulnerability Analysis

The vulnerability resides in the Sync component used by Firefox and Thunderbird to synchronize bookmarks, history, passwords, and other profile data across devices. The component fails to constrain resource consumption during specific Sync operations. An attacker can cause the client to allocate or process data in a way that exhausts memory, CPU, or other finite resources. The result is a denial-of-service condition that disrupts normal use of the application.

The weakness aligns with [CWE-400] Uncontrolled Resource Consumption. Mozilla addressed the behavior in Firefox 158 and Thunderbird 158 through code changes referenced in Mozilla Security Advisories MFSA 2026-106 and MFSA 2026-109.

Root Cause

The underlying defect is missing or insufficient limits on resources handled by the Sync subsystem. Without bounds on input size, iteration counts, or allocation requests, a crafted interaction forces the client to consume resources until the process becomes unresponsive. The Mozilla Bug Report #2066094 tracks the fix.

Attack Vector

Exploitation requires user interaction, such as visiting a malicious page or opening attacker-controlled content that engages the Sync component. The attack originates over the network and does not require authentication or elevated privileges. The impact is limited to availability; confidentiality and integrity remain intact. No public exploit or proof-of-concept is listed for this CVE at publication time.

See the security advisories for technical details on the Sync code path and the applied fix.

Detection Methods for CVE-2026-109773

Indicators of Compromise

Firefox or Thunderbird processes consuming abnormally high CPU or memory while interacting with external content
Repeated client crashes or hangs correlated with Sync activity in application logs
Unexpected restarts of browser or mail client processes on multiple endpoints shortly after visiting the same URL or opening the same message

Detection Strategies

Inventory Firefox and Thunderbird installations and flag any instance below version 158
Collect endpoint process telemetry for firefox.exe and thunderbird.exe (or platform equivalents) and alert on sustained resource spikes
Correlate browser crash reports with network connections to recently visited domains to identify suspicious triggers

Monitoring Recommendations

Forward Firefox and Thunderbird crash dumps and application logs to a centralized logging platform for trend analysis
Monitor for repeated unresponsiveness events on endpoints running vulnerable versions
Track outbound connections to Mozilla Sync endpoints and investigate anomalous volume from a single client

How to Mitigate CVE-2026-109773

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
Validate that automatic updates are enabled and functioning in enterprise deployments

Patch Information

Mozilla released fixes in Firefox 158 and Thunderbird 158. Patch details are documented in Mozilla Security Advisory MFSA 2026-106 and Mozilla Security Advisory MFSA 2026-109. The underlying code change is tracked in Mozilla Bug Report #2066094.

Workarounds

Disable Firefox Sync for affected users until the client can be upgraded, where operationally feasible
Restrict user navigation to untrusted sites through web proxy or DNS filtering policies
Advise users to avoid opening unsolicited links or email content that could trigger Sync interactions on unpatched clients

bash

# Configuration example: verify installed Firefox version on Linux endpoints
firefox --version

# Verify installed Thunderbird version
thunderbird --version

# Example: disable Firefox Sync via enterprise policy (policies.json)
# {
#   "policies": {
#     "DisableFirefoxAccounts": true
#   }
# }