CVE-2026-78207Disclosure

LOWCVSS 9.3 · CRITICAL

Signal is active with 2 mentions in latest observed window

Immediate actions

Patch affected systems immediately

Recommended action window: Monitor and triage in normal cycle

NVD 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.

Back to dashboard

0.5/ 10 priority

Sources & remediation

Other references

Weakness type (CWE)

CWE-1321

Priority

LOW

Exploitation

NONE

PoC

NONE

Patch

AVAILABLE

Momentum

NONE

Threat summary

Patch or workaround signal is available
2 mentions across 1 observed day

What's happening

Patch or workaround mentioned in 1 signal
Technical details provided in 2 signals
Disclosure: 2 classified signals
2 total mentions across 1 day

Deep dive

Activity timeline2 mentions / 1d

Signal classification1 categories

Disclosure

2100.0%

Referenced assets1 URL

By indicator

https://vulmon.com/vulnerabilitydetails?qid=CVE-2026-78207

Full discourse2 posts

Upwind Security MDR@UpwindMDR45d
Disclosure
🚨Critical - exceljs / exceljs-hardened Prototype Pollution via cell note deepMerge (CVE-2026-78207) exceljs and its exceljs-hardened fork share a deepMerge helper that merges cell note objects without blocking proto/constructor/prototype keys. An attacker can feed parsed JSON with a malicious proto in cell notes to poison Object.prototype during workbook processing, impacting other plain objects and potentially leading to logic/RCE chains. 👉Affected: exceljs <= 4.4.0 (no fix available), exceljs-hardened < 5.0.0 | Upgrade exceljs-hardened to 5.0.0
Post summary
The post announces CVE-2026-78207, a prototype‑pollution vulnerability in exceljs/ exceljs-hardened, providing technical details and a recommended upgrade, but no PoC, exploit code, or evidence of active exploitation.
0000086
294 followersView on X
Vulmon Vulnerability Feed@VulmonFeeds46d
Disclosure
CVE-2026-78207 Prototype Pollution in exceljs-hardened Before 5.0.0 in deepMerge Helper https://vulmon.com/vulnerabilitydetails?qid=CVE-2026-78207
Post summary
A prototype‑pollution vulnerability (CVE-2026-78207) in exceljs‑hardened prior to v5.0.0 has been disclosed, but no PoC, exploit, or patch details are provided.
00000135
4.1K followersView on X

Explore more

Back to dashboard