CVE-2026-33228: CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution') in WebReflection flatted
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.
AI Analysis
Technical Summary
The vulnerability (CVE-2026-33228) in WebReflection's flatted library affects versions prior to 3.4.2. The parse() function improperly handles string keys from parsed JSON by using them directly as array indices without numeric validation. When the key "__proto__" is used, it returns the Array.prototype object via JavaScript's prototype inheritance. This object is then assigned as a property of the output object, leaking a live reference to Array.prototype. Subsequent writes to this property result in prototype pollution, modifying the global Array prototype and potentially affecting all JavaScript code relying on it. This vulnerability is classified under CWE-1321 and CWE-915 and has a CVSS 4.0 score of 8.9 (high severity). The issue has been fixed in version 3.4.2.
Potential Impact
Exploitation of this vulnerability allows an attacker to modify the global Array prototype by injecting properties via the '__proto__' key in JSON input. This can lead to widespread side effects in applications using the flatted parser, as prototype pollution can alter behavior of all arrays globally, potentially causing security and stability issues. There are no known exploits in the wild at this time.
Mitigation Recommendations
An official fix is available in flatted version 3.4.2. Users should upgrade to version 3.4.2 or later to remediate this vulnerability. Patch status is confirmed by vendor advisories. No additional mitigations are specified beyond applying the update.
CVE-2026-33228: CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution') in WebReflection flatted
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 v4.0
Score 8.9high
Affected software
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AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
The vulnerability (CVE-2026-33228) in WebReflection's flatted library affects versions prior to 3.4.2. The parse() function improperly handles string keys from parsed JSON by using them directly as array indices without numeric validation. When the key "__proto__" is used, it returns the Array.prototype object via JavaScript's prototype inheritance. This object is then assigned as a property of the output object, leaking a live reference to Array.prototype. Subsequent writes to this property result in prototype pollution, modifying the global Array prototype and potentially affecting all JavaScript code relying on it. This vulnerability is classified under CWE-1321 and CWE-915 and has a CVSS 4.0 score of 8.9 (high severity). The issue has been fixed in version 3.4.2.
Potential Impact
Exploitation of this vulnerability allows an attacker to modify the global Array prototype by injecting properties via the '__proto__' key in JSON input. This can lead to widespread side effects in applications using the flatted parser, as prototype pollution can alter behavior of all arrays globally, potentially causing security and stability issues. There are no known exploits in the wild at this time.
Mitigation Recommendations
An official fix is available in flatted version 3.4.2. Users should upgrade to version 3.4.2 or later to remediate this vulnerability. Patch status is confirmed by vendor advisories. No additional mitigations are specified beyond applying the update.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- GitHub_M
- Date Reserved
- 2026-03-18T02:42:27.507Z
- Cvss Version
- 4.0
- State
- PUBLISHED
- Vendor Advisory Urls
- [{"url":"https://access.redhat.com/security/cve/CVE-2026-33228","vendor":"Red Hat"},{"url":"https://access.redhat.com/errata/RHSA-2026:9742","vendor":"Red Hat"},{"url":"https://access.redhat.com/errata/RHSA-2026:13826","vendor":"Red Hat"}]
Threat ID: 69bdd4681188d0bb0cbbf673
Added to database: 03/20/2026, 23:12:40 UTC
Last enriched: 07/31/2026, 13:00:28 UTC
Last updated: 07/31/2026, 21:28:52 UTC
Views: 244
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