Vulnerability · Published Jul 8, 2026 · Assigned by GitHub_M

CVE-2026-59876protobufjs: Text Format string map parsing can mutate returned map object prototype

Medium 4.8

In plain English

A medium-severity prototype pollution in protobuf.js from protobufjs. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 29% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)4.8Medium

Exploit odds (30 days)0.3%top 71.3% of all CVEs

Exploited in the wildNo reportsCISA SSVC: none

Attack toolingNoneno public tooling known

Official description

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.

Affected packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
npm	protobufjs	8.6.5	GHSA-jfj6-75fj-8934

Affected products (as reported)

Vendor	Product	Versions
protobufjs	protobuf.js	>= 8.2.0, < 8.6.5

Weakness: CWE-1321 · Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N

References

https://github.com/protobufjs/protobuf.js/security/advisories/GHSA-jfj6-75fj-8934 x_refsource_CONFIRM
https://github.com/protobufjs/protobuf.js/pull/2335 x_refsource_MISC
https://github.com/protobufjs/protobuf.js/commit/9f97fe413072d3beb52c74e62d88ea8adc9444d8 x_refsource_MISC
https://github.com/protobufjs/protobuf.js/releases/tag/protobufjs-v8.6.5 x_refsource_MISC