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

> Prototype pollution in defu allows attackers to override default values via crafted input, potentially leading to code execution or security bypasses.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 6 Apr 2026
- **Updated:** 6 Apr 2026
- **Also known as:** CLEANSTART-2026-IE76846, CLEANSTART-2026-KZ86783, CLSA-2026-1780879894, CLSA-2026-1784596329, GHSA-737v-mqg7-c878, MINI-2f5m-5j4r-jgq5, MINI-86mw-5jgf-fm87, MINI-rvr3-xm99-6w8p, ROOT-APP-NPM-CVE-2026-35209

## What is CVE-2026-35209?

CVE-2026-35209 is a high-severity vulnerability in unjs defu, published on 6 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-1321 (Improperly Controlled Modification of Object Prototype Attributes)

## Which versions are affected by CVE-2026-35209?

2 open source packages ship an affected version, including defu. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `defu` | npm | All versions | 6.1.5 |
| `defu` | npm | All versions | 6.1.5 |

Products: Unjs Defu.

## How do I fix CVE-2026-35209?

Upgrade defu to 6.1.5 or later.

## Is CVE-2026-35209 being exploited?

There is no known exploitation of CVE-2026-35209 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-35209 and fix it if it is affected.

What it is: CVE-2026-35209 is a high-severity vulnerability in unjs defu, published on 6 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-35209 in the wild and no public exploit code.

Affected packages and fixed versions:
- defu (npm): affected all versions; fixed in 6.1.5
- defu (npm): affected all versions; fixed in 6.1.5

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-35209/ (Markdown: https://insights.sbomify.com/CVE-2026-35209/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-35209
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-35209
- OSV: https://osv.dev/vulnerability/CVE-2026-35209

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-35209/
Summaries are generated and may contain mistakes; the official description above is the record.