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

Quick assessment

Affected
protobufjs protobuf.js
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

protobufjs protobuf.js是protobufjs的JavaScript序列化库。 protobufjs protobuf.js 8.2.0版本至8.6.5之前版本存在输入验证错误漏洞，该漏洞源于protobufjs Text Format扩展使用普通属性分配解析字符串键映射条目，允许键为__proto__的映射条目更改返回映射对象的原型，而非创建自身的映射条目。

CVSS 4.8 · Medium EPSS 0.35% · P26

Possible ATT&CK Techniques 1 AI

T1190 · Exploit Public-Facing Application

Affected Version Matrix 1

VendorProduct Version RangeStatus
protobufjs protobuf.js >= 8.2.0, < 8.6.5 affected
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I. Basic Information for CVE-2026-59876

Vulnerability Information
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Vulnerability Title
protobufjs: Text Format string map parsing can mutate returned map object prototype
Source: CVE Program / CVE List V5
Vulnerability 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.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N
Source: CVE Program / CVE List V5
Vulnerability Type
CWE-1321
Source: CVE Program / CVE List V5
Vulnerability Title
protobufjs protobuf.js 输入验证错误漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
protobufjs protobuf.js是protobufjs的JavaScript序列化库。 protobufjs protobuf.js 8.2.0版本至8.6.5之前版本存在输入验证错误漏洞，该漏洞源于protobufjs Text Format扩展使用普通属性分配解析字符串键映射条目，允许键为__proto__的映射条目更改返回映射对象的原型，而非创建自身的映射条目。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
protobufjs protobuf.js >= 8.2.0, < 8.6.5 -

II. Public POCs for CVE-2026-59876

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III. Intelligence Information for CVE-2026-59876

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Patches & Fixes for CVE-2026-59876 (2)

Vendor Advisories for CVE-2026-59876 (1)

Vendor Pages for CVE-2026-59876 (1)

IV. Related Vulnerabilities

Same product: protobuf.js
Same vendor: protobufjs
Same weakness: CWE-1321

V. Comments for CVE-2026-59876

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