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CVE-2026-35209— defu: Prototype pollution via `__proto__` key in defaults argument

Quick assessment

Affected
unjs defu
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.

defu是UnJS开源的一个递归合并默认值的轻量级工具库。 defu 6.1.5之前版本存在安全漏洞，该漏洞源于将未清理的用户输入传递给defu函数，可能导致原型污染。

CVSS 7.5 · High EPSS 0.52% · P43
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I. Basic Information for CVE-2026-35209

Vulnerability Information
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Vulnerability Title
defu: Prototype pollution via `__proto__` key in defaults argument
Source: CVE Program / CVE List V5
Vulnerability Description
defu is software that allows uers to assign default properties recursively. Prior to version 6.1.5, applications that pass unsanitized user input (e.g. parsed JSON request bodies, database records, or config files from untrusted sources) as the first argument to `defu()` are vulnerable to prototype pollution. A crafted payload containing a `__proto__` key can override intended default values in the merged resul. The internal `_defu` function used `Object.assign({}, defaults)` to copy the defaults object. `Object.assign` invokes the `__proto__` setter, which replaces the resulting object's `[[Prototype]]` with attacker-controlled values. Properties inherited from the polluted prototype then bypass the existing `__proto__` key guard in the `for...in` loop and land in the final result. Version 6.1.5 replaces `Object.assign({}, defaults)` with object spread (`{ ...defaults }`), which uses `[[DefineOwnProperty]]` and does not invoke the `__proto__` setter.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
Source: CVE Program / CVE List V5
Vulnerability Type
CWE-1321
Source: CVE Program / CVE List V5
Vulnerability Title
defu 安全漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
defu是UnJS开源的一个递归合并默认值的轻量级工具库。 defu 6.1.5之前版本存在安全漏洞，该漏洞源于将未清理的用户输入传递给defu函数，可能导致原型污染。
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
unjs defu < 6.1.5 -

II. Public POCs for CVE-2026-35209

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III. Intelligence Information for CVE-2026-35209

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Patches & Fixes for CVE-2026-35209 (1)

Vendor Advisories for CVE-2026-35209 (1)

IV. Related Vulnerabilities

Same vendor: unjs
Same weakness: CWE-1321

V. Comments for CVE-2026-35209

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