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

CVE-2026-33228 is fixed in node-flatted on 1 platform, first by DEBIAN-CVE-2026-33228, released March 20, 2026. It is not reported as exploited. Patch Radar priority: High.

- Web page: https://patchblox.com/security/cve/CVE-2026-33228
- Priority: High
- CVSS: 9.8 (Important)
- EPSS: 0.0086
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Mar 20, 2026
- Weakness: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')
- Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-33228

## 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.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | node-flatted Debian 13.5 (trixie) | 3.2.7~ds-1+deb13u1 | [DEBIAN-CVE-2026-33228](https://patchblox.com/security/data/fix/linux/debian-cve-2026-33228.md) | Mar 20, 2026 |
| Linux | node-flatted Debian 14 | 3.4.2~ds-1 | [DEBIAN-CVE-2026-33228](https://patchblox.com/security/data/fix/linux/debian-cve-2026-33228.md) | Mar 20, 2026 |

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Data as of 2026-11-08T13:06:02Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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