{"cve":"CVE-2026-33228","nvd":{"published":"2026-03-20T23:16:46.510","modified":"2026-09-04T13:18:55.120","status":"Modified","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.","cvss_v4":{"vector":"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","score":8.9},"cvss_v3":{"version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H","score":9.8},"cwes":["CWE-1321","CWE-915"],"references":["https://github.com/WebReflection/flatted/commit/885ddcc33cf9657caf38c57c7be45ae1c5272802","https://github.com/WebReflection/flatted/releases/tag/v3.4.2","https://github.com/WebReflection/flatted/security/advisories/GHSA-rf6f-7fwh-wjgh","https://access.redhat.com/errata/RHSA-2026:13826","https://access.redhat.com/errata/RHSA-2026:34342"],"products":["webreflection:flatted"],"source":"nvd","synced_at":"2026-10-19T22:26:19.218427112+00:00"},"epss":{"cve":"CVE-2026-33228","score":0.00838,"percentile":0.57795,"date":"2026-10-31"},"kev":null,"osv":[{"id":"GHSA-rf6f-7fwh-wjgh","url":"https://osv.dev/vulnerability/GHSA-rf6f-7fwh-wjgh","summary":"Prototype Pollution via parse() in NodeJS flatted","modified":"2026-10-05T16:00:04.586755982Z","packages":[{"ecosystem":"npm","name":"flatted","purl":"pkg:npm/flatted","ranges":["<3.4.2"],"fixed":["3.4.2"]}]}],"attribution":["This product uses the NVD API but is not endorsed or certified by the NVD.","EPSS scores: FIRST Exploit Prediction Scoring System, https://www.first.org/epss","Known exploited vulnerabilities: CISA Known Exploited Vulnerabilities Catalog, https://www.cisa.gov/known-exploited-vulnerabilities-catalog","Package advisories: OSV (https://osv.dev), from the GitHub Advisory Database, PyPA, Go, RustSec and other ecosystem databases, under CC-BY 4.0 or CC0","CVE records: the CVE Program's CVE List (https://www.cve.org), under its terms of use (https://www.cve.org/Legal/TermsOfUse). CVE is a registered trademark of The MITRE Corporation; this product is not affiliated with the CVE Program."]}