Vulnerability · Published Mar 20, 2026 · Assigned by GitHub_M
CVE-2026-33228flatted: Prototype Pollution via parse()
Critical 9.8
In plain English
A critical-severity prototype pollution in flatted from WebReflection. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.9% (higher than 59% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian:12 | node-flatted | no fix listed | DEBIAN-CVE-2026-33228 |
| Debian:13 | node-flatted | 3.2.7~ds-1+deb13u1 | DEBIAN-CVE-2026-33228 |
| Debian:14 | node-flatted | 3.4.2~ds-1 | DEBIAN-CVE-2026-33228 |
| Ubuntu:22.04:LTS | node-flatted | no fix listed | UBUNTU-CVE-2026-33228 |
| Ubuntu:24.04:LTS | node-flatted | no fix listed | UBUNTU-CVE-2026-33228 |
| Ubuntu:25.10 | node-flatted | no fix listed | UBUNTU-CVE-2026-33228 |
| Ubuntu:26.04:LTS | node-flatted | no fix listed | UBUNTU-CVE-2026-33228 |
| Ubuntu:26.10 | node-flatted | no fix listed | UBUNTU-CVE-2026-33228 |
| npm | flatted | 3.4.2 | GHSA-rf6f-7fwh-wjgh |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| WebReflection | flatted | < 3.4.2 |
Weakness: CWE-1321, CWE-915 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
References
- https://github.com/WebReflection/flatted/security/advisories/GHSA-rf6f-7fwh-wjgh x_refsource_CONFIRM
- https://github.com/WebReflection/flatted/commit/885ddcc33cf9657caf38c57c7be45ae1c5272802 x_refsource_MISC
- https://github.com/WebReflection/flatted/releases/tag/v3.4.2 x_refsource_MISC
- https://access.redhat.com/security/cve/CVE-2026-33228 vdb-entry, x_refsource_REDHAT
- RHBZ#2449872 issue-tracking, x_refsource_REDHAT
- https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-33228.json x_sadp-csaf-vex
- https://access.redhat.com/errata/RHSA-2026:34342 vendor-advisory, x_refsource_REDHAT
- https://access.redhat.com/errata/RHSA-2026:9742 vendor-advisory, x_refsource_REDHAT
- https://access.redhat.com/errata/RHSA-2026:13826 vendor-advisory, x_refsource_REDHAT