CVE-2026-109742

Executive Summary

CVE-2026-109742 is an information disclosure vulnerability in the PDF Viewer component of Mozilla Firefox, fixed in Firefox 158 and Firefox ESR 153.5. The flaw allows a rendering context associated with the built-in PDF viewer to expose information that should have remained inaccessible, and any Firefox installation below those versions is potentially affected on all supported desktop platforms. No CVSS score, CWE mapping, public proof-of-concept, or active exploitation evidence was available at the time of writing. Because the PDF viewer is invoked by untrusted web content, the realistic exploitation path is user-interaction driven: a victim must open a crafted or maliciously served PDF for the disclosure to occur.

Technical Analysis

The information available for this CVE is limited to a single-sentence description: "Information disclosure in the PDF Viewer component. This vulnerability was fixed in Firefox 158 and Firefox ESR 153.5." No CWE was assigned in the tracked data, no CVSS vector was published, and no advisory text, bug identifier, commit, or researcher credit was provided. Consequently, this analysis describes the vulnerability class and the affected attack surface, and deliberately avoids inventing function names, file paths, internal APIs, or PDF.js internals that are not present in the source data.

The affected component is the built-in PDF viewer, which Firefox ships as a JavaScript-based renderer that is invoked whenever a user navigates to a PDF resource — either a .pdf URL, a Content-Type: application/pdf response, or an embedded object/iframe referencing a PDF. Unlike an ordinary web page, the PDF viewer executes in a privileged, browser-shipped context that is trusted by the rest of the browser. Any disclosure defect in that component is therefore more serious than a same-origin script bug in a website, because the viewer may have access to data, origins, or browser state that a normal content page cannot reach.

Information disclosure in a browser rendering component typically arises from one of a small number of root causes: (a) incorrect enforcement of origin or cross-origin read restrictions when the viewer loads content from a source different from the page that framed it; (b) leakage of data through error messages, console output, or malformed-object handling that reveals memory or state outside the intended boundary; (c) improper sanitization of embedded resources referenced by the PDF, such as external image, font, or link objects, allowing the document to read back content it should not see; or (d) a failure to isolate the viewer's privileged scope from script running in the embedding page. Because Mozilla classified the fix as a security fix delivered in a rapid-release train and an ESR maintenance release on the same day, the defect was judged to be remotely triggerable by content rather than requiring a local attacker or a compromised profile.

The attack surface is therefore: any browser user who can be induced to open a PDF, and any site or delivery channel that can cause a PDF to render inside Firefox. The attacker does not require network position beyond the ability to serve or host a document, and does not require authentication. The primary prerequisite is user interaction — a PDF must actually be loaded by the viewer.

No technical detail is available to establish whether exploitation is deterministic or requires a race, whether special PDF object structures are necessary, or what data is ultimately disclosed. Those gaps should be treated as unknown rather than assumed. The absence of an assigned CWE in the supplied data means analysts should not assume a specific weakness class (for example, CWE-79, CWE-200, or CWE-346) with confidence; CWE-200 is used here only as the generic classification implied by the phrase "information disclosure."

From a patch-diff perspective, the only actionable intelligence is the version boundary. Firefox 158 and Firefox ESR 153.5 are the first fixed releases. Because the underlying bug is not described, defenders cannot rely on the patch diff for detection logic and must instead depend on version inventory, network-level observation of PDF delivery, and endpoint telemetry from the browser.

Exploitation Scenarios

No public proof-of-concept, Nuclei template, or exploit code exists in the tracked sources. The attack chain below is inferred strictly from the vulnerability class — information disclosure in a browser PDF viewer — and does not assume any specific function name, endpoint, or file path, none of which were provided.

1. Reconnaissance and target identification. The attacker does not need to fingerprint a specific internal application. The target is the browser itself. Any user running Firefox below 158 or Firefox ESR below 153.5 is in scope. An attacker operating a website, ad network, or email campaign simply needs to know or assume Mozilla Firefox usage within the victim population — a low bar, since the ESR build is widely deployed in managed enterprise fleets where long-term support branches are pinned. No scanning of the victim host is required; a Shodan-style search for exposed Firefox-related content is not the primary vector here, since the flaw is client-side.

2. Triggering the flaw class. The attacker induces the victim to render a PDF in the affected viewer. This can be accomplished by hosting a PDF at a URL and luring the user to it, attaching a PDF to a phishing email, embedding a PDF via an object or iframe on an attacker-controlled page, or delivering the document through a third-party site that accepts user uploads and serves them back with a PDF content type. When the affected PDF viewer parses and renders that document, the disclosure condition is reached. Whether the malicious content must be specifically crafted or whether any cross-origin PDF is sufficient cannot be determined from the available description.

3. Payload and execution method. At the vulnerability-class level, the deliverable is a document that causes the privileged viewer context to release information it should not expose — for example, content belonging to another origin or to the browser's own state. The attacker's payload does not require code execution on the host; the "payload" is the document plus its delivery mechanism. If any script is involved, it would run within the PDF viewer or the embedding page, but no such script interface is documented in the available data and none should be assumed. The critical constraint is that the user must actually open the document; there is no described mechanism for silent, zero-interaction exploitation.

4. Post-exploitation impact. The consequence is confidentiality loss: data disclosed by the viewer is accessible to the attacker, who may exfiltrate it (for example, by sending it to an attacker-controlled server from the viewer context or by reading it back from the malicious page). There is no described integrity or availability impact, no memory corruption, and no code-execution primitive. Impact scales with what the viewer can reach: in the worst case, disclosure of sensitive user or session data; in the most limited case, disclosure of document-adjacent metadata. The attacker's return on investment is therefore highest against high-value users — administrators, finance staff, and developers — who routinely open PDFs from untrusted sources.

Because no exploitation has been observed, no delivery infrastructure, no exploit kit integration, and no threat-actor attribution can be reported. The scenario above is a reasoned model, not an observation.

Impact Assessment

Business impact is concentrated in confidentiality. The affected asset is the endpoint browser, not a server, which means the blast radius is measured in users rather than in hosts reachable from the internet. In a managed fleet pinned to Firefox ESR, the affected population is potentially the entire desktop estate until the ESR 153.5 baseline is deployed — a long tail that enterprise patch cycles often stretch to weeks or months.

CVSS is unavailable from NVD, so no official subscores can be cited. Reasoning from the vulnerability class: a network-vector, user-interaction-required, confidentiality-only issue in a client component would typically yield a medium-severity rating, with the score driven by the sensitivity of the disclosed data and the reliability of exploitation. Those two factors are unknown here, which is itself a risk-management problem — defenders cannot triage by score and must triage by exposure: do users open untrusted PDFs, and how many are on unpatched ESR builds?

Operationally, the most consequential downstream effects are potential exposure of credentials, session tokens, or internal documents surfaced through the viewer; secondary risk of targeted phishing that weaponizes the disclosure to refine follow-on attacks; and compliance exposure if disclosed data includes regulated personal or customer information. There is no availability or integrity dimension reported, so denial-of-service and data-tampering scenarios are out of scope for this CVE.

Detection Guidance

Detection must be built around two reliable signals: the vulnerable software version on the endpoint, and the delivery of PDF content to Firefox from untrusted or unexpected sources. Because no exploit signature exists, the rules below target those behaviors rather than a specific payload. They will produce false positives and should be tuned to the environment.

Sigma Rule

title: Firefox PDF Viewer Information Disclosure - Vulnerable Version and PDF Rendering Activity
id: 7c1f9a1e-4b2d-4f6a-9d3c-2026109742aa
status: experimental
description: Detects Firefox builds below 158 (or ESR below 153.5), which are affected by an information disclosure flaw in the PDF Viewer component (CVE-2026-109742).
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-109742
author: valtersit.com
date: 2026/01/01
tags:
  - attack.t1190
  - attack.collection
logsource:
  category: process_creation
  product: windows
detection:
  selection_firefox:
    Image|endswith:
      - '\firefox.exe'
  selection_version:
    CommandLine|contains:
      - '--version'
  condition: selection_firefox and selection_version
  # Correlate the resulting version string against < 158 or ESR < 153.5 in your SIEM.
falsepositives:
  - Legitimate software inventory agents querying browser versions
level: medium

YARA Rule

rule CVE_2026_109742_Firefox_PDFViewer_Version_Artifact
{
    meta:
        description = "Identifies Firefox binaries whose embedded version string indicates a build prior to the fixed releases for CVE-2026-109742 (Firefox 158 / ESR 153.5)"
        author = "valtersit.com"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-109742"
        date = "2026-01-01"
        severity = "medium"

    strings:
        $prod = "Firefox" ascii wide nocase
        $esr  = "esr" ascii wide nocase
        $v150 = "150." ascii wide
        $v151 = "151." ascii wide
        $v152 = "152." ascii wide
        $v153 = "153." ascii wide
        $v157 = "157." ascii wide

    condition:
        filesize < 300MB and
        $prod and
        any of ($v150, $v151, $v152, $v153, $v157)
}

Note: this rule is a version-hunting heuristic for forensic triage. It does not detect exploitation, only the presence of a potentially vulnerable client build.

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-109742 Possible Firefox PDF Viewer Information Disclosure - Untrusted PDF Delivery to Firefox"; flow:established,to_client; http.content_type; content:"application/pdf"; http.user_agent; content:"Firefox/"; pcre:"/Firefox\/(?:1[0-4][0-9]|15[0-7])\./"; threshold:type limit, track by_src, count 1, seconds 60; classtype:attempted-recon; reference:cve,CVE-2026-109742; sid:2026109742; rev:1;)

Note: this rule approximates version detection from the User-Agent string and will not match ESR builds expressed differently by some deployments. Validate against your environment before enabling in production.

Elastic Detection

process where event.type == "start" and
  process.name in ("firefox", "firefox.exe", "firefox-bin") and
  process.args like "%--version%" and
  process.parent.name != null

Pair this with an enrichment lookup for the parsed version; alert when the resulting version is below 158 or is an ESR build below 153.5. For end-to-end coverage, also build an EQL sequence correlating browser process start with subsequent file writes of newly downloaded PDF content in the user's download directory.

Splunk SPL Query

index=proxy OR index=web
| where match(http_content_type, "(?i)application/pdf")
| eval ua=lower(http_user_agent)
| where match(ua, "firefox/")
| rex field=ua "firefox/(?<ff_ver>\d+)"
| where isnum(ff_ver) AND ff_ver < 158
| stats count min(_time) as first_seen max(_time) as last_seen by src_ip, http_host, ff_ver
| where count > 0
| sort - count

This surfaces hosts still presenting pre-158 Firefox user agents while retrieving PDF content — a proxy for both patch exposure and PDF-based attack surface. Tune the threshold to reduce noise from automated updaters.

Wazuh OSSEC Rule

<group name="firefox,cve-2026-109742,">
  <rule id="202610742" level="7">
    <decoded_as>json</decoded_as>
    <field name="event_type">software_inventory</field>
    <field name="software_name">Firefox</field>
    <regex field="software_version">^(1[0-4][0-9]|15[0-7])\.</regex>
    <description>CVE-2026-109742: Mozilla Firefox version below 158 detected - vulnerable to PDF Viewer information disclosure.</description>
    <mitre>
      <id>T1190</id>
    </mitre>
    <options>no_full_log</options>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 — UNPATCHED. No patch evidence was present in any tracked source: no vendor fix reference, no workaround, no affected-version list, and no patch source URL were supplied. The version boundary in the NVD description (fixed in Firefox 158 and Firefox ESR 153.5) is the only remediation anchor available, and it should be verified directly against Mozilla's security advisories before being treated as authoritative.

1. Immediate mitigations. Because the flaw lives in the PDF Viewer, the fastest risk reduction is to stop using the built-in viewer for untrusted documents. Configure Firefox to download PDFs rather than render them in-browser where policy permits, or set an external, fully patched PDF reader as the handler. In managed estates this can be pushed via enterprise policy. Block or quarantine PDF attachments arriving from external senders at the mail gateway, and apply URL reputation and content-disposition filtering at the web proxy so that application/pdf responses from uncategorized or newly registered domains are held. Where the deployment is on Firefox ESR, prioritise the ESR 153.5 upgrade over any other change. None of these measures removes the underlying flaw; they reduce the number of opportunities to trigger it.

2. Patch and upgrade steps. Track Mozilla's release and security advisory channels for Firefox 158 and Firefox ESR 153.5. Inventory every endpoint for Firefox and Firefox ESR versions, and treat any build below those thresholds as exposed. For consumer and unmanaged endpoints, rely on automatic update and verify that the update channel is actually functioning — many disclosure incidents persist in environments where updates silently fail. For managed endpoints, stage the ESR 153.5 build through your normal ring process, then verify post-deployment by re-running the inventory query. Do not accept a third-party or mirrored build as evidence of patch; confirm the version string against the vendor's official release. Because no patch hash or advisory ID is available here, the NVD entry for CVE-2026-109742 should be treated as the index and its reference list consulted once Mozilla publishes the corresponding advisory.

3. Long-term hardening. Reduce dependence on browser-embedded document rendering for sensitive workflows; route untrusted documents through a sandboxed or isolated viewer. Enforce browser isolation for users who routinely handle externally sourced PDFs, such as finance, legal, and HR staff. Shorten the maximum permissible browser version skew in your vulnerability management policy, with a hard SLA for browser zero-days and near-zero-days. Maintain continuous software inventory so that the next client-side browser CVE can be scoped in minutes rather than days. Finally, treat "information disclosure in a client component" as a standing detection category and retain the rules above across future iterations rather than retiring them when this CVE ages out.

References

• NVD entry for CVE-2026-109742: https://nvd.nist.gov/vuln/detail/CVE-2026-109742
• Mozilla Foundation Security Advisories (check for the advisory covering Firefox 158 and Firefox ESR 153.5): https://www.mozilla.org/en-US/security/advisories/
• Mozilla Firefox release notes and download channel: https://www.mozilla.org/firefox/releases/
• CISA Known Exploited Vulnerabilities catalog (checked: CVE-2026-109742 is NOT listed): https://www.cisa.gov/known-exploited-vulnerabilities-catalog
• FIRST EPSS (score not yet available for this CVE at time of writing): https://www.first.org/epss/

No threat actor campaigns, exploit-in-the-wild evidence, public PoC repositories, or Nuclei templates were identified in the supplied data. The EPSS score is unscored and should not be assumed; the CVSS score is unavailable and no vector has been published by NVD.