CVE
ExcelJS denial of service in worksheet parsing via sheetId
- Exploited
- No known exploitation
- Fix
- See vendor advisory
- Who is exposed
- Not reported
Do first: Workaround: The checked facts name no fix and no workaround. Until a fix is confirmed, limiting the application to loading workbooks from trusted sources removes the condition that triggers the stall.
- Published
- Last modified
- Whether it needs action now or is a reference record, by fixed rules, with why. Last checked is when cve.now last re-read it.What do these mean? →
- Reference
- Last checked
- FIRST's estimate of the chance attackers exploit it in the next 30 days, updated daily.What do these mean? →
- 0.63% · 50th percentile
- A 0-100 score for how much a CVE matters across the industry: exploitation, exposure, how widely the software is used, severity and whether a fix exists.What do these mean? →
- 31/100 · provisional
- How many real-world setups can hit the bug, from the vendor's advisory and CISA data. It does not know your setup.What do these mean? →
- Unclear
- Whether working attack code is public (Exploit-DB, Metasploit, Nuclei or GitHub). Public code makes attacks easier.What do these mean? →
- None on Exploit-DB, Metasploit or GitHub
Summary
ExcelJS through 4.4.0 contains a denial of service vulnerability that allows attackers to block the event loop by supplying an unbounded sheetId in xl/workbook.xml. Attackers can provide a workbook with a huge sheetId, making the sparse worksheet array walk stall worksheets, getWorksheet and writeBuffer for seconds.
Who should check
Teams running exceljs 4.4.0 or earlier that load workbooks from untrusted sources, such as services accepting uploaded .xlsx files.
cve.now Index
- Exploitation?/40Unknown. No CISA SSVC exploitation rating yet
- Exposure17/25No login or user action needed, reachable over the network without a login · Unknown: Advisories don't say whether default setups are affected
- Reach?/20Unknown. No tracked technology or package matched
- Severity8.7/10CVSS 8.7
- No patch5/5No fixed version listed; see vendor advisory
What would change the score
- Would score about 76 if added to CISA KEV
- Would score about 71 if CISA's SSVC rates exploitation active
- Would score about 36 if advisories confirm default setups are exposed
What to do
Workaround: The checked facts name no fix and no workaround. Until a fix is confirmed, limiting the application to loading workbooks from trusted sources removes the condition that triggers the stall.
Check your systems with an agent
Paste this into a coding or ops agent. It checks your systems read-only and reports a table per asset.
Check whether my systems are affected by CVE-2026-128342 (ExcelJS through 4.4.0 Denial of Service via Oversized Worksheet sheetId), using the cve.now data below.
Rules: work read-only. Do not run exploit code, proof-of-concept scripts or scanners that send attack payloads, and do not change any configuration, package, image or deployment. Report findings only.
What cve.now has for CVE-2026-128342 (entry last updated 2026-11-13):
Affected:
- exceljs exceljs: through 4.4.0
Fixed versions: none listed yet.
Conditions (all must be true for the bug to be hit):
- Affects exceljs versions through 4.4.0. (source: https://www.vulncheck.com/advisories/exceljs-through-4.4.0-denial-of-service-via-oversized-worksheet-sheetid)
- The application loads a workbook whose xl/workbook.xml contains a very large sheetId. (source: https://www.vulncheck.com/advisories/exceljs-through-4.4.0-denial-of-service-via-oversized-worksheet-sheetid)
Setup: unknown whether the bug needs a non-default feature or setting. Check the advisory.
Attacker needs: network access, no login, no user action.
CISA KEV: CVE-2026-128342 is not listed as of cve.now's last read.
This KEV status is for CVE-2026-128342 only. If you also check another CVE, look up its own KEV status; never assume it from this one.
How to check:
- Installed software: check what is actually running, not what a repo declares. Read the version from each host, container image and Kubernetes node (the binary's version output, the OS package database, image layers), and check whether the conditions above hold there. Distribution packages may carry a backported fix under an older version number; confirm against the distribution's own advisory.
Verify against the linked advisories before concluding, and cite the date you read each one:
- https://github.com/exceljs/exceljs
- https://github.com/mateocallec/exceljs-hardened/security/advisories/GHSA-9vv6-5g3h-prg5
- https://github.com/exceljs/exceljs/blob/v4.4.0/lib/doc/workbook.js#L200-L206
- https://github.com/exceljs/exceljs/blob/v4.4.0/lib/doc/workbook.js#L119-L125
- https://www.vulncheck.com/advisories/exceljs-through-4.4.0-denial-of-service-via-oversized-worksheet-sheetid
Do not conclude an asset is unaffected unless you checked it and the evidence covers both its version and the conditions above. If you could not check something, mark it unknown and say what is missing.
Report a table with one row per asset and these columns: Asset | Version or build | Status (affected, fixed, not applicable, unknown) | Compensating controls | Exposure (internet-facing, internal, none) | Evidence | Next action.
Entry: https://cve.now/CVE-2026-128342
For follow-up questions, use the cve.now MCP server at https://mcp.cve.now/mcp (no authentication), for example execute with name cve.get and arguments {"id": "CVE-2026-128342"}.Timeline
- CVE published
- First seen by cve.now
Affected versions
- exceljs exceljs
through 4.4.0
Evidence 7
Vendor advisory · 1
Establishes: Affected and fixed versions.
- Advisory
CVE record · 2
Establishes: The identifier and description.
- Record
- Record
Other links · 4
- Repository
- Repository
- Repository
- Link
Corrections
None so far.
Something wrong on this page? Report an error. Every fix is listed here.
Questions about CVE-2026-128342
Is CVE-2026-128342 being exploited?
It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.Source: cve.now feed data
What does this mean?
A crafted workbook with a very large sheetId in xl/workbook.xml can make ExcelJS block the Node.js event loop. While that happens, worksheet access and writeBuffer can stall for seconds, which is a denial of service.Source: vulncheck.com
Which versions are affected by CVE-2026-128342?
exceljs: through 4.4.0.Source: cve.now feed data
Am I affected, and how do I check?
Versions of exceljs through 4.4.0 are affected, and only when the application loads a workbook whose xl/workbook.xml contains a very large sheetId. To check a version, run npm ls exceljs in the project; the setup can be checked by looking at which workbooks the application reads.Source: vulncheck.com
How do I fix CVE-2026-128342?
No fixed version is listed for this vulnerability in the checked facts, so no upgrade target can be named from them. Check the exceljs advisory for a patched release or vendor patch identifiers before deciding on an update.Source: vulncheck.com
What if there's no patch?
The checked facts name no fix and no workaround. Until a fix is confirmed, limiting the application to loading workbooks from trusted sources removes the condition that triggers the stall.Source: vulncheck.com