CVE-2026-109776
Executive Summary
CVE-2026-109776 is a race condition vulnerability in the XPCOM (Cross-Platform Component Object Model) component of Mozilla Firefox. This flaw affects users running Firefox versions prior to 158 and Firefox ESR prior to 153.5, potentially allowing a remote attacker to trigger memory corruption or unexpected behavior through concurrent access to shared XPCOM resources. While no CVSS score or public exploit is currently available, the nature of race conditions in browser component architecture presents a significant risk for arbitrary code execution or sandbox escape if successfully exploited.
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Technical Analysis
The vulnerability resides within XPCOM, Mozilla's cross-platform component model that enables communication between the browser's core engine and various components. XPCOM manages object lifecycles, reference counting, and inter-component messaging. A race condition in this context typically arises when multiple threads access shared XPCOM objects or their reference counts without proper synchronization, leading to use-after-free, double-free, or type confusion conditions.
The attack surface for this vulnerability is the browser's rendering engine and its interaction with web content. An attacker could craft a malicious web page that triggers concurrent operations within XPCOM, such as rapid event handling, DOM manipulation, or async JavaScript execution, causing a time-of-check-time-of-use (TOCTOU) scenario. This could corrupt memory structures, potentially leading to arbitrary code execution within the browser's sandbox.
The specific mechanism likely involves unsynchronized access to an XPCOM component's internal state, possibly during object destruction or reference count manipulation. Without detailed patch analysis or a CVE description beyond the race condition identification, the exact code path remains unknown. However, race conditions in XPCOM are historically associated with improper locking around nsISupports reference counting or component registration/deregistration.
Exploitation Scenarios
Reconnaissance/target identification: An attacker identifies targets running vulnerable Firefox versions (pre-158 or ESR pre-153.5) via user-agent strings, browser fingerprinting, or known vulnerable deployments. This can be automated through malicious advertisements or compromised websites.
Triggering the flaw class: The attacker serves a crafted web page containing JavaScript that induces high-frequency, concurrent operations within XPCOM. For example, rapid creation and destruction of DOM elements, aggressive use of Web Workers, or simultaneous async calls that stress component reference counting. The goal is to win the race window and cause memory corruption.
Payload/execution method: No public PoC is available; the attack chain is inferred from the vulnerability class. At a high level, the attacker leverages the race condition to achieve memory corruption, then uses standard exploitation primitives (e.g., heap grooming, JIT spraying) to gain code execution within the browser process. If the sandbox is bypassed, this could lead to full system compromise.
Post-exploitation impact: Successful exploitation could allow the attacker to execute arbitrary code with the privileges of the Firefox process, potentially escaping the sandbox to compromise the underlying operating system, steal credentials, install malware, or pivot into the enterprise network.
Impact Assessment
The impact of CVE-2026-109776 is potentially severe, though currently unquantified by CVSS (N/A). A race condition in XPCOM, if exploited, could lead to:
- Confidentiality: High — memory corruption could leak sensitive data from the browser process.
- Integrity: High — arbitrary code execution allows modification of system state.
- Availability: High — crashes or resource exhaustion could render the browser unusable.
- Blast radius: All users of vulnerable Firefox and Firefox ESR versions, including enterprises relying on ESR for long-term support. Given Firefox's widespread deployment, the blast radius is global.
The absence of a CVSS score suggests that the vulnerability may not have been fully assessed by NVD, but the race condition class often carries a CVSS base score of 7.0–8.8 when remote exploitation is possible. The lack of a public exploit or CISA KEV listing indicates that active exploitation has not been observed, but the risk remains significant until patched.
Detection Guidance
Sigma Rule
title: Potential Firefox XPCOM Race Condition Exploitation
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects suspicious process behavior indicative of Firefox XPCOM race condition exploitation attempts.
author: Threat Intelligence Team
date: 2026-01-01
logsource:
category: process_creation
product: windows
detection:
selection:
Image|endswith: '\firefox.exe'
CommandLine|contains:
- '-contentproc'
- '-greomni'
condition: selection
falsepositives:
- Normal Firefox multiprocess operation
level: low
YARA Rule
rule Exploit_CVE_2026_109776_XPCOM_Race
{
meta:
description = "Detects potential exploit code targeting CVE-2026-109776 in Firefox XPCOM"
author = "Threat Intelligence Team"
date = "2026-01-01"
reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-109776"
severity = "high"
strings:
$s1 = "XPCOM" ascii wide nocase
$s2 = "nsISupports" ascii wide nocase
$s3 = "race" ascii wide nocase
$s4 = "firefox" ascii wide nocase
condition:
uint16(0) == 0x5A4D and
filesize < 10MB and
any of ($s*) and
2 of ($s*)
}
Suricata IDS Rule
alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"Potential Firefox XPCOM Race Condition Exploit Attempt (CVE-2026-109776)"; flow:to_server,established; content:"User-Agent|3a|"; http_header; content:"Firefox/"; http_header; pcre:"/Firefox\/(1[0-5][0-9]|15[0-7])/"; reference:cve,2026-109776; reference:url,https://nvd.nist.gov/vuln/detail/CVE-2026-109776; classtype:attempted-user; sid:10000001; rev:1;)
Elastic Detection
process where process.name == "firefox.exe" and process.command_line : ("*contentproc*" or "*greomni*")
Splunk SPL Query
index=main sourcetype=WinEventLog:Security OR sourcetype=XmlWinEventLog:Microsoft-Windows-Sysmon/Operational
(EventCode=1 OR EventCode=4688) Image="*\\firefox.exe"
| search CommandLine="*contentproc*" OR CommandLine="*greomni*"
| stats count by ComputerName, User, CommandLine
| where count > 5
Wazuh OSSEC Rule
<group name="firefox,exploit,">
<rule id="100001" level="10">
<decoded_as>json</decoded_as>
<field name="win.eventdata.image">firefox.exe</field>
<field name="win.eventdata.commandLine">contentproc</field>
<description>Possible Firefox XPCOM race condition exploitation attempt (CVE-2026-109776)</description>
<mitre>
<id>T1190</id>
</mitre>
</rule>
</group>
Mitigation & Patch Guidance
Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). Note: The NVD description states the vulnerability was fixed in Firefox 158 and Firefox ESR 153.5, but no patch source or commit reference is available in the provided data.
Immediate mitigations:
- Disable JavaScript or use script-blocking extensions (e.g., NoScript) to reduce attack surface for web-based triggers.
- Enable Firefox's Enhanced Tracking Protection and Strict Mode to limit malicious content execution.
- Consider using a different browser for high-risk browsing until patches are applied.
Patch/upgrade steps:
- Upgrade to Firefox 158 or later, or Firefox ESR 153.5 or later, as per the NVD description.
- Monitor Mozilla's official security advisories for the exact patch release and affected version details.
- For enterprise deployments, push updates via managed software distribution tools.
Long-term hardening:
- Implement browser isolation or sandboxing technologies to contain potential exploits.
- Maintain a rapid patch management cycle for all browsers, especially ESR versions used in enterprises.
- Monitor for anomalous Firefox process behavior (e.g., unexpected child processes, memory corruption indicators) using EDR solutions.
References
- NVD — CVE-2026-109776: https://nvd.nist.gov/vuln/detail/CVE-2026-109776
- Mozilla Security Advisories: https://www.mozilla.org/en-US/security/advisories/
- CWE-362: Race Condition: https://cwe.mitre.org/data/definitions/362.html
- MITRE ATT&CK T1190: https://attack.mitre.org/techniques/T1190/
- Firefox ESR Release Notes: https://www.mozilla.org/en-US/firefox/enterprise/
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