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
EPSS: 0.002 (12.7th percentile) — read the EPSS interpretation.
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.
Affected products
- Protobufjs Protobuf.js — versions >= 8.2.0, < 8.6.5
- Protobufjs_project Protobufjs
Weakness classification (CWE)
References
- security-advisories@github.com (x_refsource_CONFIRM, Mitigation, Vendor Advisory)
- security-advisories@github.com (Patch, x_refsource_MISC, Issue Tracking)
- security-advisories@github.com (Patch, x_refsource_MISC)
- security-advisories@github.com (Product, x_refsource_MISC, 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.