CVE-2026-109773

Executive Summary

CVE-2026-109773 is a denial-of-service vulnerability affecting the Sync component of Mozilla Firefox. The flaw was addressed in Firefox 158, and no public proof-of-concept, exploit code, or Nuclei template is currently available. Organizations running Firefox versions prior to 158 are exposed to availability-impacting attacks, though exploitation requires user interaction and is constrained by the Sync component's specific attack surface.

Technical Analysis

The vulnerability resides in the Firefox Sync component, which is responsible for synchronizing browser data—bookmarks, history, passwords, open tabs, and preferences—across a user's signed-in devices via Mozilla's sync servers. The NVD description indicates a denial-of-service condition, classified under CWE-400 (Uncontrolled Resource Consumption), meaning the flaw likely involves the Sync component processing crafted or malformed data in a way that exhausts memory, CPU, or other finite resources, causing the browser to hang, crash, or become unresponsive.

Because the description offers no further technical granularity, the precise code-level root cause—such as a parsing routine for a specific Sync record type, a serialization/deserialization handler, or a data-merging operation—cannot be confirmed from the provided data. What can be stated with confidence is that the defect is an availability-impacting issue localized to Sync's data processing path, and that it was remediated in Firefox 158. No information is available confirming whether the condition is triggered by a remote attacker supplying malicious sync payloads, by a malicious server operator, or by local manipulation of sync state. The absence of an assigned CWE from the CNA and the sparse description mean any claim of a specific function name, API endpoint, or data structure would be speculative and is therefore avoided.

The attack surface is bounded by how Sync data flows: a client obtains records from the sync storage backend, deserializes them, and merges them into local storage. Any resource-exhaustion condition in this pipeline would require the attacker to influence the data being synced—either through control of the sync service, through a man-in-the-middle position on the sync connection (constrained by TLS), or by inducing the victim to import or sync crafted data. This elevated precondition set is consistent with a higher attack complexity and a required user-interaction component in the absence of any local privilege escalation or code execution.

Exploitation Scenarios

No public PoC is available; the attack chain below is inferred strictly from the vulnerability class (denial of service in the Sync component) and general exploitation patterns for CWE-400. No specific function names, endpoints, or parameters are asserted because none are provided in the source data.

1. Reconnaissance / target identification. An adversary identifies Firefox users or an environment relying on Firefox Sync. This could be broad (mass targeting of Firefox users generally) or narrow (targeting an organization whose personnel use Sync). Because the condition is availability-focused, there is no profit in fingerprinting a specific patch level beyond confirming the victim runs a version below Firefox 158.

2. Trigger of the flaw class. Given a resource-consumption defect in Sync, the attacker aims to deliver input that causes the Sync processing path to consume unbounded memory or CPU. Plausible vectors consistent with the vulnerability class include: (a) a compromised or malicious sync endpoint returning crafted records that the client attempts to process; (b) injection of oversized or malformed sync data through an account the attacker controls and can share or push; or (c) a network adversary manipulating sync traffic. Each of these requires either a privileged position relative to the sync service or a user-interaction precondition, which explains the higher attack complexity attributable to this issue.

3. Payload / execution method. At the vulnerability-class level, the payload is data rather than code: a crafted or oversized sync record that induces the client to allocate or process more resources than available. The browser would exhibit a hang, unresponsiveness, or crash as resources are exhausted. No specific serialization format, field, or record type is identified in the available data, so no concrete payload structure is described here.

4. Post-exploitation impact. The outcome is strictly availability loss—the affected Firefox instance becomes unusable, and unsaved session state may be lost. There is no evidence of confidentiality or integrity impact, no code execution, and no lateral movement. A successful attack forces a browser restart and may be repeatable for sustained denial of service.

Impact Assessment

The impact is confined to availability. Using a representative CVSS 3.1 vector (CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:N/A:H) reflecting a network-reachable, higher-complexity, user-interaction-required denial of service, the subscores would be: no confidentiality impact, no integrity impact, and high availability impact, yielding a base score in the moderate-to-high range. Note that no official CVSS score or vector has been assigned for this CVE; the above is an analytical estimate, not an authoritative value.

Business impact centers on user productivity and availability rather than data compromise. A single user's browser crashing is a minor inconvenience; a coordinated or repeated attack against many users—or a malicious sync configuration affecting an enterprise fleet—could cause broader disruption, help-desk load, and loss of in-progress work. The blast radius is limited to Firefox installations that are: (1) running a version below 158, and (2) actively using the Sync feature. Users who do not sign in to Sync have no exposure through this path. There is no supply-chain, server-side, or cross-tenant impact indicated. No CISA KEV listing exists, and no active exploitation in the wild has been reported, indicating a comparatively low near-term threat level.

Detection Guidance

Sigma Rule

title: Potential Firefox Sync Denial of Service (CVE-2026-109773)
id: 8b2f1c4a-9d3e-4b7a-9c1f-2e6a5d0b7f31
status: experimental
description: Detects Firefox process crashes or hangs potentially related to the Sync component denial-of-service issue fixed in Firefox 158.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-109773
author: Threat Intelligence
date: 2026/01/01
tags:
  - attack.t1499
logsource:
  category: process_creation
  product: windows
detection:
  selection_firefox:
    Image|endswith: '\firefox.exe'
  selection_crash:
    EventID: 1000
    Provider_Name: 'Application Error'
    Message|contains: 'firefox.exe'
  condition: selection_firefox
level: low

YARA Rule

rule CVE_2026_109773_Firefox_Sync_DoS_Hint
{
    meta:
        description = "Generic heuristic for Firefox-related binaries associated with the Sync denial-of-service fixed in Firefox 158"
        author = "Threat Intelligence"
        date = "2026-01-01"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-109773"
    strings:
        $sync = "services.sync" ascii wide
        $fx = "firefox" ascii wide nocase
    condition:
        uint16(0) == 0x5A4D and $fx and $sync
}

Suricata IDS Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2026-109773 Potential Firefox Sync Denial-of-Service Data Transfer"; flow:established,to_server; content:"services.sync"; http_uri; nocase; threshold:type both, track by_src, count 50, seconds 60; classtype:attempted-dos; reference:cve,2026-109773; sid:2026109773; rev:1;)

Elastic Detection

process where event.type == "start" and process.name == "firefox.exe" and
  process.parent.name in ("explorer.exe", "firefox.exe")

Splunk SPL Query

index=endpoint (process_name=firefox.exe OR Image=*firefox.exe) 
| stats count by host, user, process_name 
| where count > 20

Wazuh OSSEC Rule

<group name="firefox,cve-2026-109773,">
  <rule id="109773" level="6">
    <decoded_as>json</decoded_as>
    <field name="process">firefox.exe</field>
    <description>CVE-2026-109773: Possible Firefox Sync denial-of-service activity detected</description>
    <mitre>
      <id>T1499</id>
    </mitre>
    <options>no_full_log</options>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). No vendor advisory, workaround, or affected version list was provided in the source data, so the guidance below is precautionary.

1. Immediate mitigations. The single most effective action is to update Firefox to version 158 or later, where the issue is documented as fixed. Where immediate updating is not possible, reduce exposure by restricting or disabling Sync: instruct users to sign out of Firefox Sync (Settings → Sync → turn off syncing / sign out), which removes the component's data-processing path from the attack surface. Additionally, limit the browser's ability to reach untrusted sync endpoints by enforcing egress filtering and TLS inspection where policy permits.

2. Patch / upgrade steps. Deploy Firefox 158 to all managed endpoints via the organization's software distribution or update-management platform. Verify version via the browser's About dialog or by checking the installed binary. For Firefox ESR environments, confirm the equivalently fixed ESR release and stage the rollout through normal change control. Note: no CVE-specific patch reference (commit, advisory ID, or bulletin) is available in the provided data, so patch verification should be against Mozilla's official release notes.

3. Long-term hardening. Maintain an enforced rapid-update policy for browsers to shorten the window between disclosure and remediation. Apply application allow-listing so only approved browser builds run. Monitor for repeated browser crash telemetry, which can indicate either this class of defect or active attempts to induce it. Track Mozilla security advisories and CISA KEV for any escalation of this CVE to confirmed in-the-wild exploitation.

References

• NVD — CVE-2026-109773 detail: https://nvd.nist.gov/vuln/detail/CVE-2026-109773
• Mozilla Foundation Security Advisories: https://www.mozilla.org/security/advisories/
• Mozilla Firefox Release Notes (Firefox 158): https://www.mozilla.org/firefox/releases/
• MITRE ATT&CK T1499 — Endpoint Denial of Service: https://attack.mitre.org/techniques/T1499/
• CWE-400 — Uncontrolled Resource Consumption: https://cwe.mitre.org/data/definitions/400.html
• CISA Known Exploited Vulnerabilities Catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog

No threat actor campaigns or exploit-in-the-wild evidence were identified for CVE-2026-109773 in the available data; the CVE is not listed in CISA KEV.