CVE-2026-109723

Executive Summary

CVE-2026-109723 is an integer overflow vulnerability located in the Layout: Text and Fonts component of Mozilla Firefox and Firefox ESR. Successful exploitation typically leads to memory corruption, which in a browser rendering engine can result in application crashes (denial of service) or, under the right conditions, arbitrary code execution in the context of the browser process. Affected deployments are any Firefox build below version 158 and any Firefox ESR build below version 153.5; organizations running these versions should treat the issue as an unpatched, memory-safety class defect pending verification of the stated fix versions.

Technical Analysis

The vulnerability is classified as an integer overflow (CWE-190) within the Layout: Text and Fonts subsystem of Firefox. This component is responsible for parsing font resources and shaping/positioning text during page layout. It is a complex, memory-unsafe parser surface written in C++ that processes attacker-influenceable input in the form of web fonts, system font data referenced by a page, and the text metrics computed from them.

An integer overflow occurs when an arithmetic operation used to compute a size, length, offset, or allocation bound produces a result that exceeds the representable range of the integer type used. In memory-unsafe parsers this class of defect is dangerous because the derived value is frequently reused as an allocation length, buffer index, or bounds check. When the overflowed value wraps, the resulting bound can be smaller than the true amount of data the code subsequently writes or reads, producing an out-of-bounds write or read. In a text-and-fonts layout context, the consequence is written into heap structures associated with glyph, shaping, or layout buffers, which an attacker may be able to influence in both content and size.

The attack surface is the content rendering path: any page the user visits can supply the font and text inputs that drive the affected arithmetic. Per the description, the flaw is an integer overflow; no further implementation detail (specific functions, structures, or byte fields) is provided in the supplied CNA/NVD data, so a code-level reconstruction of the exact overflow site is not possible from available information. The vulnerability class, however, is well understood: an integer wrap on a length or offset calculation feeding a buffer operation.

Assuming exploitation reaches memory corruption, the practical outcomes for this class of bug are: (1) a renderer crash, giving denial of service and reliable proof that the overflow is reachable; and (2) potential arbitrary code execution, depending on whether the overflow can be positioned to corrupt a security-sensitive pointer or object and whether the attacker can place controlled data adjacent to the corrupted region. Browser exploitation commonly also targets the sandbox boundary, meaning code execution may be confined to the renderer process unless chained with a separate sandbox-escape vulnerability. No information in the provided data indicates a sandbox escape is involved.

Because no CWE mapping was supplied in the source data, CWE-190 is applied on the basis of the explicit "Integer overflow" wording in the NVD description. Because no CVSS vector or score was published by the vendor or NVD, no authoritative severity can be stated; any vector or subscore referenced in this report is an analyst estimate and must not be treated as official.

Exploitation Scenarios

No public proof-of-concept, Nuclei template, or exploit code is available for CVE-2026-109723 as of this reporting. The attack chain below is inferred strictly from the vulnerability class (integer overflow in a text/font layout parser) and the affected product. No specific function names, CSS properties, font table names, API endpoints, or file paths are asserted, because none are provided in the source data.

1. Reconnaissance / target identification. The attacker identifies victims running Firefox below 158 or Firefox ESR below 153.5. This can be done through passive fingerprinting of the browser (user-agent, TLS/HTTP characteristics, JS feature probing) or through tailored delivery to a known population of Firefox users, for example via a watering-hole site the target demographic is known to visit. No authenticated access or special network position is required; the attack surface is the victim's own browser.

2. Triggering the flaw. The attacker must cause the vulnerable layout/text-and-fonts code path to process input that drives the affected arithmetic into an overflow condition. At the vulnerability-class level, this means delivering crafted font or text content to the browser through a web page, a document rendered by the browser, or a font resource the page references. The specific numeric values required to cross the integer boundary would be determined by the attacker through offline analysis of the component; absent a public advisory detailing the exact overflow site, an attacker would need to reverse-engineer the fix diff once a build is available.

3. Execution / payload delivery. Delivering the overflow leads to memory corruption in the renderer. The attacker's objective at this stage is to convert the corrupted state into control of execution, typically by arranging controlled data (from the crafted content) at the location that the wrapped arithmetic causes the parser to write out of bounds. This is the least deterministic step of the chain: it depends on heap layout, the allocator state, and the exact nature of the out-of-bounds access.

4. Post-exploitation impact. If code execution is achieved, it occurs in the renderer process. The attacker would then need a second, independent vulnerability to escape the browser sandbox in order to reach the host operating system. Without a sandbox escape, the realistic impact is limited to renderer-level code execution and denial of service. If the attacker achieves only the crash path, the impact is availability: a tab or the whole browser terminates, which is remotely repeatable against any user who views the crafted content.

Exploitation likelihood must be assessed conservatively: the flaw is a memory-safety bug in a high-value target (a major browser), which historically attracts active exploitation, but there is no evidence of in-the-wild exploitation, no KEV listing, and no public PoC for this specific CVE in the supplied data.

Impact Assessment

The impact depends entirely on exploitability, which is unproven. In the worst realistic case, integer overflow in the text/font layout code leads to out-of-bounds memory access that an attacker can steer into arbitrary code execution in the renderer process, with a possible further chain to full host compromise if a sandbox escape is available. In the minimal case, the same defect yields a reliably reachable crash, i.e., client-side denial of service against any user who loads the triggering content.

Using the estimated CVSS v3.1 vector CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H (analyst estimate; no official vector was published):

• Attack Vector: Network — content is delivered remotely over the web.
• Attack Complexity: Low — for the crash path; a working code-execution exploit would in practice raise complexity.
• Privileges Required: None.
• User Interaction: Required — the victim must load or view the crafted content, which is the normal browsing action.
• Scope: Unchanged — impact is confined to the vulnerable component's security context unless chained.
• Confidentiality / Integrity / Availability: High under a successful memory-corruption exploit; Availability High is the most defensible subscore given the realistic crash outcome. There are no official subscores, so these must be treated as estimates.

Blast radius: every unpatched Firefox installation below 158 and Firefox ESR installation below 153.5. In many enterprises, Firefox ESR is the managed browser baseline, so the ESR population (below 153.5) is the most operationally significant exposure. Impact is per-user rather than server-side: a single crafted page can affect many users simultaneously if it is hosted on a frequently visited site.

Business impact drivers: loss of browser process integrity on user endpoints, potential credential/session compromise if renderer code execution is achieved, and availability disruption if the crash path is weaponized. Because the vulnerability is client-side, it does not directly expose internal network services, but it can be used as an initial access vector against endpoints that would otherwise be hard to reach (for example, users on trusted corporate networks).

Detection Guidance

Detection for this CVE is challenging because it is a client-side memory-safety defect with no published exploit signature. The rules below focus on the classes of behavior that exploitation of an integer overflow in a text/font layout component would produce: browser crashes in the renderer, anomalous font loading activity, and repeated delivery of crafted content from a small set of origins. These rules are heuristic and require environment-specific tuning; none should be treated as a precise signature for this CVE.

Sigma Rule

title: Suspicious Firefox Renderer Crash Consistent with Memory-Corruption Exploitation Attempt
id: 5b9c1e2a-7d43-4f18-9a2b-3c6e0f1d8a77
status: experimental
description: Detects repeated Firefox content-process (renderer) terminations, which may indicate an exploitation attempt against an integer overflow in the Firefox layout/text-and-fonts component. Heuristic; tune to baseline.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-109723
author: valtersit.com Threat Intelligence
date: 2026/01/01
tags:
  - attack.t1190
  - cve.2026-109723
logsource:
  category: process_creation
  product: windows
detection:
  selection_browser:
    Image|endswith:
      - '\firefox.exe'
  selection_crash_marker:
    CommandLine|contains:
      - '-crashreporter'
      - 'MOZ_CRASHREPORTER'
  condition: selection_browser and selection_crash_marker
fields:
  - Image
  - CommandLine
  - ParentImage
  - User
falsepositives:
  - Legitimate browser crashes from unstable content or extensions
  - Crash reporter invocations during normal update cycles
level: medium

YARA Rule

rule CVE_2026_109723_Firefox_Integer_Overflow_Font_Text_Heuristic
{
    meta:
        description = "Heuristic detection of crafted font/text payloads that may target an integer overflow in Firefox layout/text-and-fonts. No public PoC exists; this rule matches font-container structures with anomalous size declarations and is intended for triage only."
        author = "valtersit.com Threat Intelligence"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-109723"
        date = "2026-01-01"
        confidence = "low"

    strings:
        $woff  = "wOFF" ascii
        $woff2 = "wOF2" ascii
        $ttf   = { 00 01 00 00 00 }
        $otf   = "OTTO" ascii

    condition:
        (
            $woff at 0 or $woff2 at 0 or $ttf at 0 or $otf at 0
        )
        and filesize < 5MB
}

Suricata IDS Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2026-109723 Mozilla Firefox Layout Text and Fonts Possible Integer Overflow - Web Font Retrieval"; flow:established,to_server; http.method; content:"GET"; http.uri; pcre:"/\.(woff2?|ttf|otf)(\?|$)/i"; threshold:type both, track by_src, count 25, seconds 60; classtype:attempted-user; reference:cve,2026-109723; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-109723; metadata:service http; sid:2026109723; rev:1;)

Note: this rule flags high-volume web-font retrieval from a single source in a short window. It is a behavioral heuristic for the delivery stage only and will not conclusively identify exploitation.

Elastic Detection

process where event.type == "start" and
  process.name : "firefox" and
  process.command_line : ("*-crashreporter*" or "*MOZ_CRASHREPORTER*")

Companion correlation query (renderer crash rate anomaly — tune thresholds to your environment):

sequence by user.name with maxspan=5m
  [ process where event.type == "start" and process.name : "firefox" ]
  [ process where event.type == "end" and process.name : "firefox" and process.exit_code != 0 ]

Splunk SPL Query

index=endpoint (process_name=firefox OR process_name=firefox.exe)
    (process="*firefox*" OR parent_process_name="*firefox*")
    (crash="*" OR exit_code!=0 OR message="*Renderer*crash*")
| stats count values(exit_code) as exit_codes values(command_line) as cmdline by host user
| where count > 5
| sort - count

This surfaces hosts exhibiting repeated Firefox termination events consistent with repeated exploitation attempts or a persistent crash-triggering page.

Wazuh OSSEC Rule

<group name="web,browser,cve_2026_109723,">
  <rule id="109723" level="5">
    <decoded_as>json</decoded_as>
    <field name="process.name">firefox|firefox.exe</field>
    <field name="event.action">process-terminated</field>
    <field name="process.exit_code" type="pcre2">^(?!0$)\d+$</field>
    <description>CVE-2026-109723: Firefox process terminated unexpectedly - possible renderer crash or exploitation attempt</description>
    <mitre>
      <id>T1190</id>
    </mitre>
  </rule>

  <rule id="109724" level="8" frequency="5" timeframe="120">
    <if_matched_sid>109723</if_matched_sid>
    <same_field>agent.name</same_field>
    <description>CVE-2026-109723: Repeated Firefox crashes on same host - investigate for memory-corruption exploitation attempt</description>
    <mitre>
      <id>T1190</id>
    </mitre>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 — UNPATCHED. No patch evidence was available from any tracked source at the time of writing, and no vendor fix or workaround was provided in the source data.

1. Immediate mitigations

• Verify the vendor advisory directly before acting on the version numbers in the description. The NVD text states the issue was fixed in Firefox 158 and Firefox ESR 153.5; confirm against Mozilla's official security advisories that a corresponding update exists and is installable in your environment.
• Until the update is confirmed and deployed, treat the browser as an exposed attack surface: restrict browsing to trusted sites where policy allows, and block or heavily restrict web font loading from untrusted origins via enterprise browser policy.
• Enable and preserve crash reporting and renderer-termination telemetry so that repeated crashes can be triaged as potential exploitation rather than ordinary instability.
• Prioritize the managed Firefox ESR population: ESR builds below 153.5 are the primary enterprise exposure if the stated fix version is accurate.
• If the threat model warrants it, temporarily offer an alternative supported browser for high-risk web activity, and ensure the alternative itself is current.

2. Patch / upgrade steps

• Upgrade Firefox to version 158 or later.
• Upgrade Firefox ESR to version 153.5 or later.
• Confirm the installed version after upgrade using the browser's about/version interface or the package manager metadata, and verify centrally via your patch management or MDM tooling.
• Where extensions are centrally managed, re-verify extension compatibility with the new build before broad rollout.

3. Long-term hardening

• Maintain automated version tracking for internet-facing client software, including browsers, and alert on builds falling below current security baselines.
• Keep browser process sandboxing enabled and do not disable renderer sandboxing for convenience; sandboxing is the primary containment control if renderer memory corruption is achieved.
• Adopt exploit-mitigation baselines (ASLR, DEP/CFG equivalents, hardened memory allocators) on endpoints running browsers.
• Monitor vendor security advisories on a regular cadence and maintain a patch SLA for browser components; client-side browser defects are a common initial access vector.
• Log and review exploit-protection and mitigation events (for example, EDR modern exploit-protection alerts) as a compensating detection layer for memory-safety defects.

References

• NVD — CVE-2026-109723: https://nvd.nist.gov/vuln/detail/CVE-2026-109723
• Mozilla Security Advisories (verify fixed-version guidance for Firefox 158 / Firefox ESR 153.5): https://www.mozilla.org/en-US/security/advisories/
• MITRE CWE-190 — Integer Overflow or Wraparound: https://cwe.mitre.org/data/definitions/190.html
• MITRE ATT&CK T1190 — Exploit Public-Facing Application: https://attack.mitre.org/techniques/T1190/
• CISA Known Exploited Vulnerabilities Catalog (currently does not list CVE-2026-109723): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

No threat actor campaigns and no exploit-in-the-wild evidence for CVE-2026-109723 were identified in the supplied data. The CVE is not listed in CISA KEV as of this reporting.