kcp: Authentication Bypass by Assumed-Immutable Data
CVE-2026-39429 is a critical-severity authentication bypass by assumed-immutable data vulnerability in kcp from Kcp-dev, with a CVSS 3.1 base score of 9.1 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, without any action from a user. Affected kcp versions: >= 0.30.0, < 0.30.3; < 0.29.3. The record does not name a fixed version, so check the vendor advisory. There is no confirmed exploitation so far. EPSS estimates a 0.44% chance of exploitation within 30 days, which is low compared with other CVEs.
Critical severity: a successful attack would have severe impact.
What to do
- Check which version of kcp 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
Version data comes from the organisation that assigned the CVE. 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
- NoneAvailability is not affected.
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N. Scored by NVD.
| Scoring system | Score | Severity | Scored by |
|---|---|---|---|
| CVSS 3.1 | 8.2 | High | CNA (github.com) |
| CVSS 3.1 | 9.1 | Critical | NVD |
Official description
kcp is a Kubernetes-like control plane for form-factors and use-cases beyond Kubernetes and container workloads. Prior to 0.30.3 and 0.29.3, the cache server is directly exposed by the root shard and has no authentication or authorization in place. This allows anyone who can access the root shard to read and write to the cache server. This vulnerability is fixed in 0.30.3 and 0.29.3.
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Authentication Bypass by Assumed-Immutable Data
The authentication scheme or implementation uses key data elements that are assumed to be immutable, but can be controlled or modified by the attacker.
How developers prevent this class of flaw
- Implement proper protection for immutable data (e.g. environment variable, hidden form fields, etc.)
More Authentication Bypass by Assumed-Immutable Data vulnerabilities (CWE-302)
Timeline
- CVE record published.
- CISA published its SSVC assessment (exploitation: none, automatable: no, technical impact: partial).
- Record last changed materially (score, affected products, status or references).
References
Advisories
- github.com/kcp-dev/kcp/security/advisories/GHSA-3j3q-wp9x-585pExploit, Mitigation
Patches and release notes
- github.com/kcp-dev/kcp/releases/tag/v0.29.3Release Notes
- github.com/kcp-dev/kcp/releases/tag/v0.30.3Release Notes
Other vulnerabilities in kcp
- 9.9 Critical severity
- 9.6 Critical severity
Recent Authentication Bypass by Assumed-Immutable Data vulnerabilities
- 7.3 High severity
- 8.1 High severity
- 7.5 High severity
- 8.1 High severity
- 9.9 Critical severity
- 8.8 High severity
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