Prototype Pollution in Protobufjs Protobuf.js
CVE-2026-59876
protobufjs compiles protobuf definitions into JavaScript (JS) functions. From 8.2.0 until 8.6.5, the protobufjs Text Format extension parsed string-keyed map entries using ordinary property assignment, allowing a map entry with key __proto__ to change the prototype of the returned map object instead of creating an own map entry in protobufjs/ext/textformat. This issue is fixed in version 8.6.5.
Vulnerability class: Prototype Pollution
Published · last modified .
CVSS v3 metric
CVSS v3 base score 4.8 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N.
EPSS exploit prediction
EPSS: 0.003 (26.5th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 26th percentile — 26.5% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.003 · newest: 0.003 · change: 0.000
Affected products
- Protobufjs Protobuf.js — versions >= 8.2.0, < 8.6.5
- Protobufjs Project
Weakness classification (CWE)
References
- github.com/protobufjs/protobuf.js/security/advisories/GHSA-jfj6-75fj-8934 (Mitigation, Vendor Advisory)
- github.com/protobufjs/protobuf.js/pull/2335 (Patch, Issue Tracking)
- github.com/protobufjs/protobuf.js/commit/9f97fe413072d3beb52c74e62d88ea8adc9444… (Patch)
- github.com/protobufjs/protobuf.js/releases/tag/protobufjs-v8.6.5 (Product, Release Notes)
Frequently asked questions
- What is CVE-2026-59876?
- CVE-2026-59876 is a medium-severity vulnerability in Protobufjs Protobuf.js, classified under Improperly Controlled Modification of Object Prototype Attributes (Prototype Pollution). CVSS score: 4.8/10. Published 2026-07-08.
- How severe is CVE-2026-59876?
- Medium severity. CVSS v3 base score is 4.8 out of 10.