VulnerabilityModified

CVE-2026-33228

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.

HIGH 8.9EPSS 0.81%

Does this matter?

High impact if exploited, but EPSS currently rates exploitation as unlikely (0.81%). Schedule it in the normal patch cycle and watch for a rise in EPSS or a public exploit.

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 4.0: 8.9 HIGHCVSS: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/M
EPSS: 0.81% probability · 55th percentile
CISA KEV: Not listed
Weakness: CWE-1321, CWE-915
Affected: webreflection/flatted
Source: security-advisories@github.com

References

Source: NVD record, EPSS from FIRST.org, KEV from CISA. Refreshed daily.