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 products
webreflection: flatted
References9
- github.com/WebReflection/flatted/security/advisories/GHSA-rf6f-… ExploitVendor Advisory
- github.com/WebReflection/flatted/commit/885ddcc33cf9657caf38c57… Patch
- github.com/WebReflection/flatted/releases/tag/v3.4.2 Product
- access.redhat.com/errata/RHSA-2026:13826
- access.redhat.com/errata/RHSA-2026:34342
- access.redhat.com/errata/RHSA-2026:9742
- access.redhat.com/security/cve/CVE-2026-33228
- bugzilla.redhat.com/show_bug.cgi?id=2449872
- security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-33228…
Metrics
- CVSS 3.1
-
9.8
CRITICAL
Score source: NVD
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H - CVSS 4.0
-
8.9
HIGH
Score source: CNA
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/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X - EPSS — exploit probability
- 1% percentile: 59 · as of 12 November 2026
- CWE
- CWE-1321CWE-915
Could this be exploited in your environment?
A patch closes the known hole. A penetration test shows what an attacker can still reach — with proof.