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CVE Record

CVE-2026-33228: flatted: Prototype Pollution via parse()

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.

CriticalCVSS 9.8Not KEV-listedUpdated
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Glexia's TakeAutomated analysiscritical

Security readout for executives and security teams

Plain-English summary

A flaw in flatted can turn specially crafted serialized data into a live reference to a shared JavaScript prototype. If application code later modifies that returned property, behavior across the process may be corrupted, potentially affecting data integrity, confidentiality, or availability. Exposure depends on whether untrusted data reaches flatted.parse() and how the result is used.

Executive priority

Prioritize rapid inventory and upgrade for internet-facing or multi-tenant services processing untrusted flatted data. Treat confirmed reachable instances as urgent. Do not assume every installation is exploitable; validate the input path and downstream writes while remediation proceeds.

Technical view

Before 3.4.2, flatted.parse() accepts attacker-controlled string values as Array index keys without ensuring they are numeric. The key "__proto__" can resolve through inheritance to Array.prototype, which is returned as parsed data. A subsequent write through that property can cause global prototype pollution. Version 3.4.2 patches the issue.

Likely exposure

Potentially exposed systems use flatted below 3.4.2, directly or transitively, and parse attacker-controlled serialized input. Risk is lower where inputs are trusted or parsed results are never subsequently written through the affected property, but dependency presence alone does not establish exploitability.

Exploitation context

The supplied data reports CVSS 3.1 score 9.8 and no KEV listing. No provided source confirms active exploitation. Successful impact requires malicious input reaching parse() and later consumer behavior that writes through the leaked prototype reference; the resulting effect depends on the application.

Researcher notes

The primitive is indirect: parsing exposes Array.prototype as a legitimate value, while later consumer writes produce pollution. Assessment should distinguish package presence, attacker-controlled parse reachability, and a usable write path. The provided evidence establishes the vulnerable logic and patched release, but not observed exploitation or a universal application-level impact.

Mitigation direction

  • Upgrade flatted to version 3.4.2 or later.
  • Update lockfiles and rebuild deployments so the corrected version is actually resolved.
  • Identify and update parent packages that introduce vulnerable transitive versions.
  • Until upgraded, prevent untrusted serialized data from reaching flatted.parse().
  • Review applicable Red Hat advisories for vendor-packaged deployments.

Validation and detection

  • Search manifests, lockfiles, SBOMs, and deployed artifacts for flatted versions below 3.4.2.
  • Confirm the production runtime resolves flatted 3.4.2 or later.
  • Trace whether external input can reach flatted.parse().
  • Review downstream code for writes to properties obtained from parsed data.
  • Use isolated regression tests to confirm rejected nonnumeric references and unchanged global prototypes.
Prepared
Confidence
high
Sources
6

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.

ATT&CK lookup starting points

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cwe · low confidence lookup

CWE-1321: Exact CWE lookup

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cwe · low confidence lookup

CWE-915: Exact CWE lookup

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cve · low confidence lookup

CVE-2026-33228 mapping review

Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.

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Vulnerability profileCVE Program record
Severity
Critical
CVSS
9.8 (3.1)
Known Exploited
No
Published

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

2CVSS vectors
5Timeline events
2ADP providers
10Source links

SSVC decision data

CISA-ADPCISA Coordinator
Timestamp
Version
2.0.3
Exploitation: pocAutomatable: yesTechnical Impact: total

CVSS vector scores

2 official scores

We collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.

ScoreVersionSeverityVectorExploitImpactSource
9.8CVSS 3.1CriticalCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H3.95.9redhat-SADP
8.9CVSS 4.0HighCVSS: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——GitHub_M

Vulnerability scoring details

Base CVSS 4.0 score

8.9High
CVSS 4.0 vector shape for CVE-2026-33228Attack VectorAttack ComplexityAttack RequirementsPrivileges RequiredUser InteractionVS ConfidentialityVS IntegrityVS AvailabilitySS ConfidentialitySS IntegritySS Availability

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

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Attack Requirements
NonePresent
Privileges Required
NoneLowHigh
User Interaction
NonePassiveActive
VS Confidentiality
HighLowNone
VS Integrity
HighLowNone
VS Availability
HighLowNone
SS Confidentiality
HighLowNone
SS Integrity
HighLowNone
SS Availability
HighLowNone

Vulnerability timeline

Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.

  1. CVE reservedCVE Program

    The CVE ID was reserved by the assigning CNA.

  2. ADP timelineredhat-SADP

    Made public.

  3. CVE publishedCVE Program

    The CVE record was published.

  4. ADP timelineredhat-SADP

    Reported to Red Hat.

  5. CVE updatedCVE Program

    The CVE record metadata indicates this as the latest update time.

ADP provider summaries

CISA-ADPCISA ADP Vulnrichment
other:ssvc
redhat-SADPflatted: Flatted: Prototype pollution vulnerability allows arbitrary code execution via crafted JSON.
other:Red Hat severity ratingcvssV3_1
  • 2026-03-21T00:01:43.424Z: Reported to Red Hat.
  • 2026-03-20T23:06:48.485Z: Made public.

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
WebReflectionflatted< 3.4.2Listed
Weakness

CWE details

CWE-1321CWE-915

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.

CWE-1321 · source CWE mapping

Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')

Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution') represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

Open CWE intelligenceSame-CWE CVEs
CWE-915 · source CWE mapping

Improperly Controlled Modification of Dynamically-Determined Object Attributes

Improperly Controlled Modification of Dynamically-Determined Object Attributes represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

Open CWE intelligenceSame-CWE CVEs