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.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
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
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.
Upgrade, and only deserialise data from trusted sources or with an allow-list of types. More about Unsafe deserialization.
Is CVE-2026-78207 being exploited?
There is no known exploitation of CVE-2026-78207 in the wild and no public exploit code.
Sources
Also known as GHSA-3cpv-r8x3-29fp, GHSA-qwr4-7h29-chpf.