# CVE-2026-33228: flatted: Prototype Pollution via parse()

> Prototype pollution in flatted allows attackers to corrupt global objects, potentially leading to code execution or privilege escalation.

- **Severity:** High 8.9 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:P`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 20 Mar 2026
- **Updated:** 7 Sep 2026
- **Also known as:** AZL-80309, CLEANSTART-2026-PL93650, CLEANSTART-2026-SB96942, CLSA-2026-1791278813, DEBIAN-CVE-2026-33228, GHSA-rf6f-7fwh-wjgh, MINI-5xq4-wfwf-8jqg, MINI-7559-ch5v-xjff, MINI-j24h-9hmq-vgmg, MINI-m342-2c8x-r8c2, MINI-rvqq-p3hg-hf4h, MINI-vxh7-26gx-36vp, ROOT-APP-NPM-CVE-2026-33228, UBUNTU-CVE-2026-33228

## What is CVE-2026-33228?

CVE-2026-33228 is a high-severity vulnerability in WebReflection flatted, published on 20 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

flatted is a circular JSON parser. Prior to version 3.4.2, the parse() function in flatted can use attacker-controlled string values from the parsed JSON as direct array index keys, without validating that they are numeric. Since the internal input buffer is a JavaScript Array, accessing it with the key "__proto__" returns Array.prototype via the inherited getter. This object is then treated as a legitimate parsed value and assigned as a property of the output object, effectively leaking a live reference to Array.prototype to the consumer. Any code that subsequently writes to that property will pollute the global prototype. This issue has been patched in version 3.4.2.

Weakness: CWE-915 (Improperly Controlled Modification of Dynamically-Determined Object Attributes), CWE-1321 (Improperly Controlled Modification of Object Prototype Attributes)

## Which versions are affected by CVE-2026-33228?

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


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `flatted` | npm | All versions | 3.4.2 |
| `flatted` | npm | All versions | 3.4.2 |

Products: Webreflection Flatted.

## How do I fix CVE-2026-33228?

Upgrade flatted to 3.4.2 or later.

## Is CVE-2026-33228 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-33228 exists.

## Other vulnerabilities in WebReflection flatted

- [CVE-2026-32141](https://insights.sbomify.com/CVE-2026-32141/): flatted: Unbounded recursion DoS in parse() revive phase (high 7.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-33228 and fix it if it is affected.

What it is: CVE-2026-33228 is a high-severity vulnerability in WebReflection flatted, published on 20 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-33228 exists.

Affected packages and fixed versions:
- flatted (npm): affected all versions; fixed in 3.4.2
- flatted (npm): affected all versions; fixed in 3.4.2

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-2026-33228/ (Markdown: https://insights.sbomify.com/CVE-2026-33228/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-33228
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-33228
- OSV: https://osv.dev/vulnerability/CVE-2026-33228

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-33228/
Summaries are generated and may contain mistakes; the official description above is the record.