flatted: Improperly Controlled Modification of Object Prototype Attributes
CVE-2026-33228 is a critical-severity improperly controlled modification of object prototype attributes vulnerability in flatted from WebReflection, with a CVSS 3.1 base score of 9.8 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, without any action from a user. Affected flatted versions: < 3.4.2. The record does not name a fixed version, so check the vendor advisory. There is no confirmed exploitation so far. EPSS estimates a 0.86% chance of exploitation within 30 days, higher than 59% of scored CVEs.
Critical severity: a successful attack would have severe impact.
What to do
- Check which version of flatted you run against the affected versions below, and install the vendor's latest release.
- Read the advisory on github.com for workarounds and exact fixed builds.
- Until you can update, restrict network access to the affected service so it is not reachable from untrusted networks.
Affected products and versions
| Vendor | Product | Affected versions | Fixed in |
|---|---|---|---|
| WebReflection | flatted | < 3.4.2 | Not stated |
| Red Hat | Cluster Observability Operator 1.5.0 | Not specified in the record | Not stated |
| Red Hat | Red Hat Developer Hub 1.8 | Not specified in the record | Not stated |
| Red Hat | Red Hat Developer Hub 1.9 | Not specified in the record | Not stated |
| Red Hat | Cryostat 4 | Not specified in the record | Not stated |
| Red Hat | Logging Subsystem for Red Hat OpenShift | Not specified in the record | Not stated |
| Red Hat | Multicluster Engine for Kubernetes | Not specified in the record | Not stated |
| Red Hat | Red Hat 3scale API Management Platform 2 | Not specified in the record | Not stated |
| Red Hat | Red Hat Advanced Cluster Management for Kubernetes 2 | Not specified in the record | Not stated |
| Red Hat | Red Hat AMQ Broker 7 | Not specified in the record | Not stated |
| Red Hat | Red Hat build of Apicurio Registry 2 | Not specified in the record | Not stated |
| Red Hat | Red Hat Build of Keycloak | Not specified in the record | Not stated |
| Red Hat | Red Hat build of OptaPlanner 8 | Not specified in the record | Not stated |
| Red Hat | Red Hat Build of Podman Desktop | Not specified in the record | Not stated |
| Red Hat | Red Hat Data Grid 8 | Not specified in the record | Not stated |
| Red Hat | Red Hat Directory Server 11 | Not specified in the record | Not stated |
| Red Hat | Red Hat Directory Server 12 | Not specified in the record | Not stated |
| Red Hat | Red Hat Directory Server 13 | Not specified in the record | Not stated |
| Red Hat | Red Hat Enterprise Linux 10 | Not specified in the record | Not stated |
| Red Hat | Red Hat Enterprise Linux 8 | Not specified in the record | Not stated |
| Red Hat | Red Hat Enterprise Linux 9 | Not specified in the record | Not stated |
| Red Hat | Red Hat Fuse 7 | Not specified in the record | Not stated |
| Red Hat | Red Hat JBoss Enterprise Application Platform 7 | Not specified in the record | Not stated |
| Red Hat | Red Hat JBoss Enterprise Application Platform 8 | Not specified in the record | Not stated |
| Red Hat | Red Hat JBoss Enterprise Application Platform Expansion Pack | Not specified in the record | Not stated |
Version data comes from the organisation that assigned the CVE; 7 further products are omitted here. Confirm against the vendor advisory before deciding you are unaffected.
How the severity score breaks down
- Attack vector
- NetworkCan be exploited remotely over a network, including the internet.
- Attack complexity
- LowNo special conditions are needed; the attack works reliably.
- Privileges required
- NoneNo account or login is needed.
- User interaction
- NoneNo action by a victim is required.
- Scope
- UnchangedImpact stays within the vulnerable component.
- Confidentiality
- HighAll data handled by the component can be read.
- Integrity
- HighData or code can be modified at will.
- Availability
- HighThe component can be taken down completely.
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H. Scored by NVD.
| Scoring system | Score | Severity | Scored by |
|---|---|---|---|
| CVSS 4.0 | 8.9 | High | CNA (github.com) |
| CVSS 3.1 | 9.8 | Critical | NVD |
| CVSS 3.1 | 9.8 | Critical | CNA |
Official 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.
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Improperly Controlled Modification of Object Prototype Attributes
The product receives input from an upstream component that specifies attributes that are to be initialized or updated in an object, but it does not properly control modifications of attributes of the object prototype.
How developers prevent this class of flaw
- By freezing the object prototype first (for example, Object.freeze(Object.prototype)), modification of the prototype becomes impossible.
- By blocking modifications of attributes that resolve to object prototype, such as proto or prototype, this weakness can be mitigated.
- When handling untrusted objects, validating using a schema can be used.
More Improperly Controlled Modification of Object Prototype Attributes vulnerabilities (CWE-1321)
Timeline
- CVE record published.
- CISA published its SSVC assessment (exploitation: poc, automatable: yes, technical impact: total).
- Record last changed materially (score, affected products, status or references).
References
Patches and release notes
Other references
Other vulnerabilities in flatted
- 7.5 High severity
Recent Improperly Controlled Modification of Object Prototype Attributes vulnerabilities
- 7.5 High severity
- 7.5 High severity
- 9.9 Critical severity
- 7.5 High severity
- 7.5 High severity
- 9.8 Critical severity
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