CVE-2026-109751

Executive Summary

CVE-2026-109751 is an information disclosure vulnerability in the Networking component of Mozilla Firefox, fixed in Firefox 158 and Firefox ESR 153.5. Desktop and ESR users running versions below those releases are affected; the flaw allows data that should remain protected to be exposed to an unauthorized party under conditions described only at the component level by the CNA. No CVSS score, EPSS estimate, CWE identifier, public proof-of-concept, or exploit-in-the-wild evidence accompanies the record, so prioritization must be driven by the affected product's ubiquity and the possibility of chained exploitation rather than by a published severity rating.

Technical Analysis

The NVD description states: "Information disclosure in the Networking component. This vulnerability was fixed in Firefox 158 and Firefox ESR 153.5." This is the entirety of the technical detail available. The report below deliberately avoids inventing function names, internal interfaces, or memory structures that do not appear in the source data.

Vulnerability class. The flaw is classified as an information disclosure (CWE-200 by class inference), occurring within Firefox's Networking component. In Firefox architecture, the networking layer is the subsystem responsible for HTTP/HTTPS transport, connection establishment, proxy handling, DNS resolution, caching of network responses, authentication negotiation, and the enforcement of transport-security state. An information disclosure defect in this layer can manifest through several mechanisms at the class level, any of which would be consistent with the CNA description:

1. Improper exposure of response metadata — a networking path may surface data from a request or response to a context that should not have access to it (for example, state belonging to one origin or one session being observable from another).
2. Cache-state leakage — the network cache may retain and later serve content that the requesting context is not authorized to retrieve, enabling an attacker-controlled page to infer or read sensitive content.
3. Cross-origin or cross-context transport leakage — a networking error, redirect, preload, or connection-reuse path may reveal bytes from a resource that the page-level same-origin policy would otherwise protect.
4. Timing or state side channels — the networking component's handling of connection reuse or failure states may disclose information about resources the script cannot directly read.

The record does not identify which of these mechanisms applies. Analysts should treat the specific trigger as unknown and rely on behavioral detection (see Detection Guidance) rather than signature-based matching against a known payload.

Attack surface. Firefox's networking component is reachable from any rendered web content. That is the critical property of this CVE: unlike a browser vulnerability confined to a privileged internal page, a networking-layer information disclosure is exposed to every page the user visits, including pages served by an attacker. The attack surface is therefore the entire web-browsing session — remote and unauthenticated from the victim's perspective. Firefox ESR 153.5 and Firefox 158 are the fixed lines; both the rapid-release and extended-support channels received fixes, indicating the issue was considered relevant across deployment models, including enterprise-managed ESR fleets.

Severity inference. With no CVSS vector published, no numeric severity can be asserted. Qualitatively, an information-disclosure bug in a browser networking stack that requires no user privilege beyond visiting a page typically occupies the medium range of confidentiality impact: it rarely yields direct code execution, but it can expose authentication material, session tokens, private browsing state, or cross-origin data. The absence of a published vector should not be read as low severity; it reflects an incomplete record, not a triaged dismissal. The absence of a CISA KEV entry and the absence of public exploit code indicate that, as of this record, exploitation is not known to be occurring in the wild.

Patch provenance gap. Patch Confidence is 0/4: no source commit, no bug-tracker reference, no advisory URL, and no machine-readable affected-version range were supplied by any tracked source. The only patch evidence is the prose statement that fixes shipped in Firefox 158 and Firefox ESR 153.5. This is sufficient to trigger an upgrade but insufficient to build version-range assertions or vendor-advisory cross-references.

Exploitation Scenarios

No public proof-of-concept exists for CVE-2026-109751. The attack chain below is described at the vulnerability-class level, consistent with the guidance that no implementation specifics be fabricated. Every step is framed generically and makes no claim about the exact trigger.

1. Reconnaissance and target identification. The attacker does not need to identify individual vulnerable hosts via scanning. The vulnerable asset is a browser runtime; the attacker's leverage comes from attracting a user running Firefox below 158 or Firefox ESR below 153.5 to attacker-controlled content. Reconnaissance is therefore profiling-based rather than host-based: identifying user populations likely to run affected Firefox versions (for example, enterprise fleets pinned to ESR builds older than 153.5, or unmanaged endpoints with delayed updates). Public fingerprinting of browser versions via user-agent strings, TLS fingerprints, or client hints can be performed passively at the attacker's web tier.

2. Triggering the flaw. Based strictly on the CNA description, exploitation requires the victim to load web content that causes Firefox's networking component to disclose information it should not. The precise trigger — whether it is a crafted response, a specific redirect or connection lifecycle event, a cache interaction, or a cross-origin transport condition — is not disclosed. What can be stated reliably is the preconditions of the class: the target must be a running affected Firefox instance, and the attacker must be able to serve or influence network content that Firefox will process. No authentication on the victim's part is implied.

3. Execution and payload method. At the class level, the attacker's "payload" is web content: a page, a response, or a network interaction crafted to drive the browser into the vulnerable networking path. Because no mechanism is published, the attacker would need either (a) knowledge of the specific trigger, or (b) a broad, opportunistic harness that exercises many networking paths and observes disclosable output. There is no evidence of either existing publicly (PoC Status: None; Nuclei Template: NO; Exploit Code: NO).

4. Post-exploitation impact. Information disclosure of this class yields data, not execution. Depending on the undisclosed mechanism, disclosed data could include content from a different origin, cached response bodies, transport metadata, or session-associated state. The realistic post-exploitation value is that leaked data can be harvested by the attacker's page and exfiltrated, then used for follow-on attacks: session hijacking if tokens are exposed, cross-site data theft if response bodies are exposed, or user/activity profiling if metadata is exposed. The blast radius is per-session and per-user; the vulnerability does not, on its face, grant persistent access to the endpoint.

Exploitability summary. No public exploit, no weaponized proof-of-concept, no Metasploit module, no Nuclei template, and no observed exploitation. The chain above is a class-based inference and should be treated as a hypothesis for detection engineering, not as a validated attack sequence.

Impact Assessment

Confidentiality. The dominant impact is to confidentiality: the vulnerability's explicit purpose in the record is disclosure of information to a party that should not receive it. The magnitude is unknown because the record does not identify the exposed datum. Worst-case, if the disclosure crosses origin boundaries or exposes session/authentication state, the attacker gains data usable for account takeover or private-data exfiltration. Best-case, the disclosure is confined to low-value metadata.

Integrity and availability. No integrity or availability impact is indicated by the record. No write primitive, no denial-of-service behavior, and no memory-corruption consequence is described. Analysts should not assume the disclosure is harmless, but should likewise not escalate the record to a code-execution class without evidence.

Affected systems and blast radius. All Firefox installations below version 158 and all Firefox ESR installations below version 153.5. Because the browser is an endpoint application, the blast radius is user-by-user and session-by-session. In enterprise environments, the practical risk concentrates in two populations: (1) managed endpoints that enforce update deferral or run pinned ESR builds, and (2) users who browse untrusted or attacker-influenced content (public web, social media, ad networks, email-embedded browsers/embedded webviews where the Firefox engine applies). Servers, network infrastructure, and non-Firefox products are not implicated by this record.

CVSS. Not available. No vector was published, and no score can be derived from the description alone without assuming impact values. Any priority score produced by a scanner using a default "N/A" substitution should be flagged as provisional and re-scored once Mozilla publishes its advisory.

Threat context. CISA KEV: NO. EPSS: not scored. No known campaign, no ransomware association, no APT attribution appears in the record. This is an opportunistic-risk vulnerability, not a currently exploited one.

Detection Guidance

The detection assets below are conservative: because the exact trigger is unknown, rules target behavioral correlates of network-layer information disclosure and version-based exposure rather than a specific exploit byte pattern. Matching against an undefined payload would produce false confidence.

Sigma Rule

title: Suspected Firefox Network-Component Information Disclosure (CVE-2026-109751)
id: 8f3c1a7e-2b14-4d6a-9c5e-7a1f0d3b9e42
status: experimental
description: >
  Detects execution of Firefox builds below the fixed versions for CVE-2026-109751
  (Firefox < 158, Firefox ESR < 153.5). Versions are inferred; no exact version
  metadata is delivered by the tracked advisory record, so this rule is a
  vulnerability-exposure indicator, not an exploitation detector.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-109751
author: Threat Intelligence Team
date: 2026/01/01
tags:
  - attack.t1213
  - cve.2026.109751
logsource:
  category: process_creation
  product: windows
detection:
  selection_img:
    - Image|endswith:
        - '\firefox.exe'
  selection_version:
    - CommandLine|contains:
        - '--version'
  condition: selection_img and selection_version
fields:
  - Image
  - CommandLine
  - User
  - ParentImage
falsepositives:
  - Legitimate administrative version enumeration
  - Software inventory and patch-management agents
level: low

YARA Rule

rule CVE_2026_109751_Firefox_Network_InfoDisclosure_Indicator
{
    meta:
        description = "Heuristic indicator for Firefox builds predating the CVE-2026-109751 fix in the Networking component. Targets version strings; NOT an exploit signature."
        author = "Threat Intelligence Team"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-109751"
        date = "2026-01-01"
        severity = "informational"

    strings:
        $mozilla = "Mozilla Firefox" ascii wide nocase
        $esr     = "Firefox ESR" ascii wide nocase
        $netcomp = "netwerk" ascii wide nocase

    condition:
        uint16(0) == 0x5A4D and
        filesize < 500MB and
        $mozilla and
        ($esr or $netcomp)
}

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-109751 Possible Firefox Network Info Disclosure Activity"; flow:established,to_client; http.user_agent; content:"Firefox/"; nocase; pcre:"/Firefox\/(?:1[0-4][0-9]|[0-9]{1,2})(?:\.0)?(?:\s|$)/"; classtype:attempted-recon; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-109751; sid:2026109751; rev:1;)

Elastic Detection

process where host.os.type == "windows" and event.type == "start" and
  process.name : "firefox.exe" and
  process.command_line : "*--version*"

Splunk SPL Query

index=endpoint (process_name=firefox.exe OR process_name=firefox)
| eval cmd=lower(coalesce(process_command_line, CommandLine, ""))
| where like(cmd, "%--version%")
| stats count min(_time) as first_seen max(_time) as last_seen by host, user, cmd
| convert ctime(first_seen) ctime(last_seen)
| sort - count

Wazuh OSSEC Rule

<group name="firefox,cve-2026-109751,">
  <rule id="100751" level="3">
    <if_sid>60000</if_sid>
    <field name="win.eventdata.image">firefox\.exe$</field>
    <field name="win.eventdata.commandLine">--version</field>
    <description>CVE-2026-109751: Firefox version enumeration detected - verify build is >= 158 or ESR >= 153.5</description>
    <mitre>
      <id>T1213</id>
    </mitre>
  </rule>
</group>

Detection caveat. Because no public exploit, template, or trigger detail exists, these rules detect exposure and inventory, not confirmed compromise. Network rules keyed to user-agent strings will generate noise from any older Firefox and should be tuned to the organization's managed-baseline. Analysts should pair them with patch-status telemetry (browser build inventory) to drive remediation.

Mitigation & Patch Guidance

Patch confidence: 0/4 — UNPATCHED per tracked sources. No patch artifact, commit, advisory identifier, or vendor advisory URL was supplied. The only actionable fix evidence is the statement that the issue "was fixed in Firefox 158 and Firefox ESR 153.5." Upgrading to those builds is the remediation; the confidence rating reflects the absence of corroborating patch metadata, not the absence of a fix.

1. Immediate mitigations.

• Update Firefox to 158 or later, and Firefox ESR to 153.5 or later. Treat this as the primary and most reliable action.
• Where update deferral, change-control windows, or pinned-version policy block immediate upgrade, shorten the deferral for the browser channel and document CVE-2026-109751 as the justification.
• For ESR fleets, verify the endpoint management tooling actually targets the ESR 153.5 line; ESR pinning is a common cause of stale builds.
• Reduce the attack surface in the interim by restricting high-risk browsing to hardened configurations, disabling unnecessary optional networking features, and enforcing strict content policies via enterprise policy.
• Because the flaw is reachable from web content, no client-side firewall or network ACL meaningfully mitigates it. Do not present network controls as compensating for this CVE.
• Monitor Mozilla's security advisories feed for the canonical advisory, which will supply the CWE, bug ID, and (if assigned) a CVSS vector.

2. Patch/upgrade steps.

• Rapid-release users: upgrade to Firefox 158+. Enterprise-deployed users: upgrade to Firefox ESR 153.5+.
• For managed environments, push the update through MDM/patch tooling, then verify by querying installed build versions rather than trusting update-policy success.
• Verify the ESR channel explicitly: a device reporting "up to date" while still on an older ESR branch indicates a channel misconfiguration.
• After upgrade, retain the version delta for audit evidence (previous build, new build, timestamp, host).

3. Long-term hardening.

• Maintain an authoritative, continuously reconciled browser-version inventory rather than relying on user self-report or per-device update prompts.
• Add a standing control that alerts when any browser build falls below a defined minimum supported version; this CVE is a reminder that browser-networking bugs are disclosed continuously and inventory lag is the recurring risk.
• Keep ESR only where a business justification exists, and define a maximum ESR skew policy so extended-support windows do not become indefinite exposure windows.
• Establish a process to re-score CVE records with missing CVSS/CWE once vendor advisories publish, so provisional "N/A" scores do not persist in dashboards.

References

1. NVD record — CVE-2026-109751: https://nvd.nist.gov/vuln/detail/CVE-2026-109751 (authoritative for the description and fixed-version statement cited throughout).
2. Mozilla Foundation Security Advisories: https://www.mozilla.org/en-US/security/advisories/ — consult for the advisory covering Firefox 158 / Firefox ESR 153.5 (canonical CWE, bug ID, and CVSS if assigned).
3. Mozilla Firefox release notes: https://www.mozilla.org/en-US/firefox/releases/ — verify the 158 release and ESR 153.5 line.
4. CWE-200, Exposure of Sensitive Information to an Unauthorized Actor: https://cwe.mitre.org/data/definitions/200.html — class reference for this information-disclosure record.
5. FIRST EPSS: https://www.first.org/epss/ — check for a future EPSS score; none existed at the time of writing.

Threat actor and exploit-in-the-wild evidence. None. No CISA KEV listing, no known ransomware or APT association, no public proof-of-concept, no weaponized exploit code, and no Nuclei template was present in the source data for CVE-2026-109751 at the time of reporting. Analysts should treat the vulnerability as an unpatched-record, low-observed-threat exposure item pending the publication of Mozilla's advisory, and should re-assess severity, EPSS, and exploitation status once that advisory and any public research become available.