CVE-2026-109779

Executive Summary

CVE-2026-109779 is a race condition vulnerability in the Networking: DNS component of Mozilla Firefox, fixed in version 158. The flaw arises from improper synchronization during DNS resolution, potentially causing a browser crash or denial of service. Users running Firefox versions prior to 158 are affected, with exploitation requiring a crafted web page or network condition to trigger the race.

Technical Analysis

The vulnerability is classified as a race condition (CWE-362) within Firefox's DNS resolution subsystem. Race conditions occur when multiple threads or processes access shared resources without proper locking, leading to unpredictable behavior. In the context of DNS, this could involve concurrent resolution requests, cache updates, or socket handling that are not adequately synchronized. The NVD description provides no specific function names, memory structures, or code paths, so the exact trigger mechanism remains undisclosed. However, the impact is consistent with a denial of service: by racing DNS operations, an attacker might cause a use-after-free, double-free, or assertion failure, resulting in a browser crash. The vulnerability is remotely triggerable (AV:N) but requires user interaction (UI:R) because the victim must visit a malicious page or be subjected to a crafted network environment. Attack complexity is high (AC:H) due to the timing-sensitive nature of race conditions, and no privileges are required (PR:N). The scope is unchanged (S:U), and there is no impact on confidentiality or integrity, only availability (A:H). No public proof-of-concept or exploit code exists, and no Nuclei template is available. The CWE mapping (CWE-362) is inferred from the vulnerability class; the NVD data does not explicitly state it.

Exploitation Scenarios

1. Reconnaissance: An attacker identifies potential victims by scanning for Firefox versions below 158, using techniques such as browser fingerprinting or user-agent string analysis. Shodan dorks like product:"Firefox" or http.html:"Firefox" could help identify exposed instances, though this is a client-side vulnerability, so mass scanning is less relevant. More likely, the attacker targets specific users via phishing or malicious advertisements.
2. Triggering the race condition: The attacker crafts a web page that initiates multiple concurrent DNS requests or manipulates DNS response timing. By exploiting the lack of synchronization in Firefox's DNS component, the attacker aims to cause a race. The exact method is not detailed in the NVD description; at the vulnerability-class level, this could involve sending crafted DNS responses, using JavaScript to trigger rapid lookups, or leveraging a man-in-the-middle position to control DNS traffic.
3. Execution: The malicious page is delivered to the victim, who must interact with it (e.g., by loading the page). The race condition is triggered during DNS resolution, leading to a crash or hang. No specific payload or code pattern is known; the attack relies solely on timing and the inherent flaw.
4. Post-exploitation: The primary impact is denial of service—the browser becomes unresponsive or crashes, disrupting the user's session. There is no evidence of remote code execution or data theft. An attacker could use this to disrupt operations, force users to restart the browser, or as part of a larger campaign to cause frustration or downtime.

No public PoC is available; the attack chain is inferred from the vulnerability class. The absence of technical details means that any specific exploitation steps are speculative and should not be treated as confirmed.

Impact Assessment

The vulnerability affects Mozilla Firefox versions prior to 158. Given Firefox's significant market share (approximately 3-5% of desktop browsers), the blast radius includes individual users, enterprises, and government agencies relying on Firefox. The CVSS vector (CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:N/A:H) yields a base score of 5.3 (Medium), but the official score is not provided; this is an estimate based on the vector. The high attack complexity and required user interaction reduce the likelihood of mass exploitation. However, a successful exploit causes a denial of service, which can disrupt productivity, interrupt critical web-based workflows, and potentially lead to data loss if unsaved work is lost. In enterprise environments, repeated crashes could affect many users, though no lateral movement or privilege escalation is possible. The lack of confidentiality and integrity impact limits the severity to availability only. CISA KEV does not list this CVE, and no known threat actor campaigns are associated. EPSS is not yet scored.

Detection Guidance

Sigma Rule

title: Firefox DNS Race Condition Crash Detection
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects potential exploitation of CVE-2026-109779 via anomalous Firefox crash reports or DNS behavior
author: Threat Intelligence
date: 2026-01-01
logsource:
  product: windows
  service: application
detection:
  selection:
    EventID: 1000
    Provider_Name: 'Application Error'
    Application_Name: 'firefox.exe'
    Exception_Code: '0xc0000005'  # access violation, common in race conditions
  condition: selection
falsepositives:
  - Legitimate crashes due to other bugs
level: medium

YARA Rule

rule CVE_2026_109779_Firefox_DNS_Race
{
    meta:
        description = "Detects Firefox binaries with potential DNS race condition vulnerability"
        author = "Threat Intelligence"
        date = "2026-01-01"
        reference = "CVE-2026-109779"
    strings:
        $s1 = "nsHostResolver" ascii wide
        $s2 = "DNS" ascii wide
        $s3 = "PR_GetAddrInfoByName" ascii wide
        $s4 = "PR_GetAddrInfoByNameAsync" ascii wide
    condition:
        uint16(0) == 0x5A4D and  // MZ header
        filesize < 500KB and
        any of ($s*)
}

Note: This YARA rule is speculative; no specific strings are confirmed from the NVD description. It targets common Firefox DNS-related symbols.

Suricata IDS Rule

alert udp any any -> any 53 (msg:"CVE-2026-109779 Firefox DNS Race Condition Attempt"; content:"|00 01 00 00 00 00 00 00|"; depth:12; threshold: type both, track by_src, count 50, seconds 1; classtype:attempted-dos; sid:1000001; rev:1;)

This rule detects high-rate DNS queries that could trigger a race condition, but it is heuristic and may produce false positives.

Elastic Detection

process where process.name == "firefox.exe" and event.action == "crash"

This simple EQL rule detects Firefox crashes, which could indicate exploitation.

Splunk SPL Query

index=windows source="WinEventLog:Application" EventCode=1000 Application_Name="firefox.exe" Exception_Code="0xc0000005" | stats count by host, _time

Wazuh OSSEC Rule

<group name="firefox,">
  <rule id="100100" level="10">
    <decoded_as>json</decoded_as>
    <match>firefox.exe</match>
    <match>0xc0000005</match>
    <description>CVE-2026-109779: Firefox DNS race condition crash detected</description>
    <mitre>
      <id>T1499</id>
    </mitre>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source).

1. Immediate mitigations: Disable DNS prefetching (network.dns.disablePrefetch) in about:config; reduce DNS concurrency settings; use a DNS-over-HTTPS provider to potentially alter race conditions; ensure Firefox is updated to the latest available version even if not the specific patch. For enterprises, consider temporarily switching to an alternative browser for sensitive tasks.
2. Patch/upgrade steps: Upgrade Firefox to version 158 or later once available. Monitor Mozilla's security advisories for the official patch. If version 158 is not yet released, apply any interim updates.
3. Long-term hardening: Implement regular browser updates, use exploit mitigation tools (e.g., EMET, Windows Defender Exploit Guard), and educate users about phishing and malicious ads. Consider network-level DNS monitoring for anomalies. Engage with Mozilla's bug bounty program for early vulnerability intelligence.

References

1. NVD - CVE-2026-109779 (https://nvd.nist.gov/vuln/detail/CVE-2026-109779)
2. Mozilla Security Advisories (https://www.mozilla.org/en-US/security/advisories/)
3. CWE-362: Race Condition (https://cwe.mitre.org/data/definitions/362.html)
4. FIRST EPSS (https://www.first.org/epss/)
5. CISA Known Exploited Vulnerabilities Catalog (https://www.cisa.gov/known-exploited-vulnerabilities-catalog)

No known threat actor campaigns or exploit-in-the-wild evidence are associated with this CVE.