CVE-2026-33228flatted is a circular JSON parser.
Updated under an hour ago ()
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No exploitation reported and no EPSS estimate yet.
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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.
- CVSS
- 9.8 CRITICAL (v3.1, NVD)CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
- Weakness (CWE)
- CWE-1321, CWE-915
- Known exploited
- Not in CISA's KEV catalog
- EPSS (exploitation in the next 30 days)
- Not loaded for this CVE
- Assigned by
- Not given
- NVD analysis
- Modified
- Record last updated
- Sep 4, 2026
- First seen by InfoSec Signals
- Oct 9, 2026
Affected products
webreflection flatted webreflection:flatted | before 3.4.2 | NVD |
References
- https://github.com/WebReflection/flatted/commit/885ddcc33cf9657caf38c57c7be45ae1c5272802patch
- https://github.com/WebReflection/flatted/releases/tag/v3.4.2product
- https://github.com/WebReflection/flatted/security/advisories/GHSA-rf6f-7fwh-wjghexploit, vendor-advisory
- https://access.redhat.com/errata/RHSA-2026:13826
- https://access.redhat.com/errata/RHSA-2026:34342
- https://access.redhat.com/errata/RHSA-2026:9742
- https://access.redhat.com/security/cve/CVE-2026-33228
- https://bugzilla.redhat.com/show_bug.cgi?id=2449872
- https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-33228.json
The CVE record on cve.org NVD's entry
The CVE record is authoritative. Products marked “from the name” were identified from the CNA's free-text names, not a CPE, and are matched less certainly. This product uses the NVD API but is not endorsed or certified by the NVD.