CVE-2026-78207 json
exceljs through 4.4.0 Prototype Pollution via deepMerge Reached From Note Serialization
published 2mo ago modified 3d ago also EUVD-2026-64682 , GHSA-qwr4-7h29-chpf
Unreviewed: the maintainers' own advisory
Published by the maintainers on their GitHub repository mateocallec/exceljs-hardened. GitHub has not reviewed it into the GitHub Advisory Database, so the affected packages and versions are the maintainers' word. Read the full advisory on GitHub.
- cvss
- 9.4
- version 3.1, CNA
- cisa kev
- Not listed
- not in CISA's catalog
- epss
- 0.6%
- EPSS 0.6% chance of exploitation in 30 days, percentile 49%
- public exploits
- 0
- none known
Summary
exceljs through 4.4.0 contains a prototype pollution vulnerability in the deepMerge helper that fails to reject __proto__, constructor, or prototype keys when merging note objects. Attackers can assign parsed JSON with a malicious __proto__ property to cell notes, modifying Object.prototype and affecting all plain objects created in the process.
Patch / fix
- npm exceljs-hardened 5.0.0
Is CVE-2026-78207 in your dependencies? Fencer shows which of your repositories are affected as soon as an advisory lands, and opens the fix as a pull request.
Check with FencerCISA SSVC
- Exploitation
- none
- Automatable
- yes
- Technical impact
- total
- Assessed
- 2mo ago
Exploit prediction
0.60% chance of exploitation in the next 30 days, higher than 49% of all scored CVEs (FIRST EPSS, 2026-11-11).
CVSS scores
| assigned by | version | score | vector |
|---|---|---|---|
| CNA | 4.0 | 9.3 | CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:L/SC:N/SI:N/SA:N |
| CNA | 3.1 | 9.4 | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:L |
| CNA | 4.0 | 9.3 | CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:L/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X |
Affected packages
| ecosystem | package | affected | fixed in |
|---|---|---|---|
| npm | exceljs-hardened | <=4.4.0 |
5.0.0 |
Affected products
- exceljs exceljs
Classification
- Weakness
-
- CWE-1321 Improperly Controlled Modification of Object Prototype Attributes
References
-
github.com
/mateocallec/exceljs-hardened/security/advisories/GHSA-qwr4-7h29-chpf
third-party-advisory, vendor-advisory, advisory
-
github.com
/exceljs/exceljs
product
-
github.com
/exceljs/exceljs/blob/v4.4.0/lib/utils/under-dash.js#L155-L181
technical-description
-
vulncheck.com
/advisories/exceljs-through-prototype-pollution-via-deepmerge-reached-from-note-serialization
third-party-advisory
Sources
- CVE List CVE-2026-78207
- NVD CVE-2026-78207
- Maintainer advisory GHSA-qwr4-7h29-chpf
Fencer Intel keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.
The maintainers of mateocallec/exceljs-hardened published their own advisory on GitHub. Reviewed sources take precedence over it.
CPEs and fixed versions use Anchore's correction of NVD's data: Added CPE configurations because not yet analyzed by NVD.