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CVE-2026-128336

ExcelJS through 4.4.0 Prototype Pollution via __proto__ Defined Name

Products / packages exceljs

Published

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Severity and exploitation

Not in CISA KEV

KEV coverage does not rule out exploitation.

KEV checked 15/11/2026, 00:00:32 UTC
VulnCheck · CVSS:4.0 · 6.3
Assessment vectorCVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:L/VA:L/SC:N/SI:N/SA:N
VulnCheck · CVSS:3.1 · 4.8
Assessment vectorCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:L
The headline uses the highest imported catalog score. Assessments can differ by source and CVSS version.

Severity

medium6.3

CVSS measures technical severity, not the likelihood of an attack.

Known exploitation

Not listed in the checked CISA KEV catalog

Absence from CISA KEV does not rule out attacks.

Public exploit

Not established by this record

Missing evidence does not mean no public exploit exists.

Remediation

Fix status not established

No structured fix or guidance is available here. Check the latest publisher advisory.

What this means

ExcelJS through 4.4.0 has a prototype pollution vulnerability via __proto__ defined name, allowing attackers to add numeric keys to Object.prototype. Attackers can supply a crafted xlsx file to achieve this. The impact is writing empty arrays onto Object.prototype process-wide.

Summarized by @cf/meta/llama-3.3-70b-instruct-fp8-fast. Verify details against the original sources.

Attack conditions

No attack-condition excerpt has been identified here. Review the full source description and references for authentication, network access, and configuration requirements.

Review supporting sources →

Affected versions

Check the exact model, release, and any platform requirements against your installation.

exceljs · VulnCheck

exceljs

1 range
VulnCheck version evidence

Package identity

pkg:npm/exceljsExplore exceljs →

Identity supplied by VulnCheck. Version matching requires compatible range evidence.

Compare with this publisher’s ranges. This does not inspect your installation.

affected

0 → through 4.4.0

semver versions

unaffected

unknown

Other versions: unaffected

Automatic dependency matching is unavailable for this record. Use the product evidence and publisher’s instructions to check your installation.

What to do

Compare your installed product and version with the affected versions above. A matching product name alone does not confirm exposure.

No structured fix or remediation guidance is available here. Check the full source description and vendor’s latest advisory for a patch or workaround; do not assume another version is safe.

cve.new guidance unless attributed to a publisher. This page does not inspect your installation.

Public exploit evidence

No public exploit reference is identified in the imported evidence. This is not a claim that no exploit exists.

Original source description

Read the full publisher description

ExcelJS through 4.4.0 contains a prototype pollution vulnerability that allows attackers to add numeric keys to Object.prototype through a crafted defined name referencing a proto sheet. Attackers can supply an xlsx whose xl/workbook.xml definedName range targets proto, causing CellMatrix to write empty arrays onto Object.prototype process-wide when workbook.xlsx.load() runs.

References

5 sources
exceljs/exceljs · repository
Source referenceVulnCheck
mateocallec/exceljs-hardened · security/advisories/GHSA-pwgq-2cgg-6ph9
Security advisoryVulnCheck
exceljs/exceljs · cell-matrix.js
Source referenceVulnCheck
exceljs/exceljs · defined-names.js
Source referenceVulnCheck
www.vulncheck.com/advisories/exceljs-through-4.4.0-prototype-pollution-via-proto-defined-name
Security advisoryVulnCheck

Inside the record

Evidence available

Package identity
Version evidence
Remediation guidance · not supplied
Weakness classification
Additional publishers

Describes the evidence in this view, not whether the product is safe.

Original records

CVE-2026-128336

View the latest unmodified CVE JSON from the official source. It may be newer than this imported page.

cve.new JSON →

Underlying weaknesses

The source’s classification of what caused the vulnerability.

CWE-1321 ↗

Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')

VulnCheck

Related →

Environment requirements

These conditions apply together as specified by the publisher. A matching product name alone does not establish exposure.

VulnCheck · platform conditions

Any condition may hold

Vulnerable product

cpe:2.3:a:exceljs_project:exceljs:*:*:*:*:*:*:*:*

Through (inclusive): 4.4.0

Applicability preview
[
  {
    "nodes": [
      {
        "operator": "OR",
        "negate": false,
        "cpeMatch": [
          {
            "vulnerable": true,
            "criteria": "cpe:2.3:a:exceljs_project:exceljs:*:*:*:*:*:*:*:*",
            "versionEndIncluding": "4.4.0"
          }
        ]
      }
    ]
  }
]
Contributing organizations · 2
VulnCheck

Assigning organization

2026-11-13
CISA-ADP

Additional data publisher

2026-11-13

Timeline

All times UTC

  1. Advisory published

    Publication date supplied by the source.

  2. Source updated

    Latest modification date supplied by the source.

  3. Last imported by cve.new

    Latest catalog import, not the date we first saw this advisory.

What changed

Follow changes
  1. CVE · CVE-2026-128336

    Observed 2026-11-13T19:45:16.750Z · upstream updated 2026-11-13T17:58:50.637Z

    Prioritization updated

    before

    []

    after

    [
  {
    "source": "CISA-ADP",
    "type": "SSVC",
    "role": "CISA Coordinator",
    "timestamp": "2026-11-13T17:58:33.563826Z",
    "decisions": [
      {
        "name": "Exploitation",
        "value": "none"
      },
      {
        "name": "Automatable",
        "value": "no"
      },
      {
        "name": "Technical Impact",
        "value": "partial"
      }
    ]
  }
]

Large field values are marked as truncated previews. Original source snapshots remain retained.

Showing up to 50 most recent observed changes.