Prototype Pollution in Multiparty

CVE-2026-8161

multiparty@4.2.3 and lower versions are vulnerable to denial of service via uncaught exception. By sending a multipart/form-data request with a field name that collides with an inherited Object.prototype property such as __proto__, constructor, or toString, the parser invokes .push() on the inherited prototype value rather than an array, throwing a TypeError that propagates as an uncaught exception and crashes the process. Impact: any service accepting multipart uploads via multiparty is affected. Workarounds: none. Upgrade to multiparty@4.3.0 or higher.

Published · last modified .

CVSS v3 metric

CVSS v3 base score 7.5 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.

EPSS exploit prediction

EPSS: 0.005 (42.7th percentile), scored .

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 43rd percentile — 42.7% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS trend (30 days)EPSS over the last 30 days for CVE-2026-8161: held from 0.005 to 0.005.

EPSS over last 30 days · oldest: 0.005 · newest: 0.005 · change: +0.001

Affected products

Weakness classification (CWE)

Public proof-of-concept exploits

References

Frequently asked questions

What is CVE-2026-8161?
CVE-2026-8161 is a high-severity vulnerability in Multiparty, classified under CWE-248. CVSS score: 7.5/10. Published 2026-05-12.
How severe is CVE-2026-8161?
High severity. CVSS v3 base score is 7.5 out of 10.
Is CVE-2026-8161 known to be exploited?
1 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.