# CVE-2026-78207: exceljs through 4.4.0 Prototype Pollution via deepMerge Reached From Note Serialization

> Attackers can achieve remote code execution or privilege escalation via prototype pollution in exceljs when processing malicious cell notes.

- **Severity:** Critical 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`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 24 Aug 2026
- **Updated:** 29 Aug 2026
- **Also known as:** GHSA-3cpv-r8x3-29fp, GHSA-qwr4-7h29-chpf

## What is CVE-2026-78207?

CVE-2026-78207 is a critical unsafe deserialization vulnerability in exceljs, published on 24 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

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.

Weakness: CWE-1321 (Improperly Controlled Modification of Object Prototype Attributes)

## Which versions are affected by CVE-2026-78207?


## How do I fix CVE-2026-78207?

No fix has been published for CVE-2026-78207 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-78207 being exploited?

There is no known exploitation of CVE-2026-78207 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-78207 and fix it if it is affected.

What it is: CVE-2026-78207 is a critical unsafe deserialization vulnerability in exceljs, published on 24 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-78207 in the wild and no public exploit code.

Affected product: exceljs. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-78207/ (Markdown: https://insights.sbomify.com/CVE-2026-78207/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-78207
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-78207
- OSV: https://osv.dev/vulnerability/CVE-2026-78207

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-78207/
Summaries are generated and may contain mistakes; the official description above is the record.