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CVE-2026-33228

CRITICAL Exploitation: PoC
9.8
CVSS 3.1
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.

Metadata

CVE ID
CVE-2026-33228
State
PUBLISHED
Assigner
GitHub_M
Reserved
2026-03-18 02:42 UTC
Published
2026-03-20 23:06 UTC
Last updated
2026-06-27 05:17 UTC
Primary CWE
CWE-1321
CWE-1321: Improperly Controlled Modification of Object Proto…
Vendor / Product
WebReflection / flatted
Sources
cve.org  ·  NVD

Severity & Metrics

9.8 CRITICAL CVSS 3.1
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
SSVC — CISA Coordinator
Exploitation
PoC
Automatable
yes
Tech. Impact
total
Affected products (1)
VendorProductPlatformVersions
WebReflection flatted — < 3.4.2
Weakness (CWE)
CWESourceDescription
CWE-1321 cna CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')
CWE-915 adp Improperly Controlled Modification of Dynamically-Determined Object Attributes
CVSS scores (2)
ScoreSeverityVersionSourceVector
9.8 CRITICAL 3.1 adp CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
8.9 HIGH 4.0 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
References (3)
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