# CVE-2026-39429: kcp-dev kcp authentication bypass in cache server

Source: https://junglewise.ai/threats/cve-2026-39429-kcp-dev-kcp-authentication-bypass-in-cache-server  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-04-08. Updated: 2026-07-24  
License: CC BY 4.0 with a link back

CVE-2026-39429 · Severity: high · CVSS 8.2 · Published 2026-04-08

kcp is a control plane used to manage workloads across different environments. A security flaw allows unauthenticated users to access the system's internal cache server, which is responsible for storing critical configuration and infrastructure data. An attacker could read sensitive information about the network topology and security rules, or potentially exploit a timing window to temporarily gain higher privileges, leading to unauthorized data access or disruption of security policies.

Technologies: [github.com/kcp-dev/kcp (Go)](https://junglewise.ai/threats/technologies/github-com-kcp-dev-kcp). Vendors: [Go](https://junglewise.ai/threats/vendors/go).

## Quick answers

**Is CVE-2026-39429 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-39429 being exploited in the wild.

**How severe is CVE-2026-39429?** High severity, with a CVSS score of 8.2, published 8 April 2026.

**Which products and versions are affected?** github.com/kcp-dev/kcp (Go) SEMVER:introduced=0,fixed=0.29.3,introduced=0.30.0,fixed=0.30.3, SEMVER:introduced=0.30.0,fixed=0.30.3.

**Is there a fix for CVE-2026-39429?** Yes. github.com/kcp-dev/kcp (Go): upgrade to 0.30.3 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-39429 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-39429?** Check whether you run github.com/kcp-dev/kcp (Go) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- github.com/kcp-dev/kcp (Go): upgrade to 0.30.3 or later

  ```sh
  go get github.com/kcp-dev/kcp@v0.30.3
  ```


## Technical details

The kcp cache server is routed within the pre-mux chain of the shard code (preHandlerChainMux), which executes before any authentication or authorization handlers. This architectural flaw allows unauthenticated network-reachable attackers to perform CRUD operations on the cache server. Impact includes full read access to replicated resources such as RBAC cluster roles, API exports, and infrastructure topology. While a replication controller acts as a self-healing mechanism by deleting unauthorized write attempts, a race condition exists where injected RBAC rules could be briefly evaluated by the GlobalAuthorizer before deletion, potentially allowing temporary privilege escalation. The vulnerability is fixed in versions 0.29.3 and 0.30.3.

## Affected products

- kcp-dev kcp < 0.29.3, >= 0.30.0, < 0.30.3

## Identifiers

- CVE-2026-39429
- GHSA-3j3q-wp9x-585p
- GO-2026-5088

## Timeline

- 2026-04-02: patched: Versions 0.29.3 and 0.30.3 released
- 2026-04-07: advisory: GitHub Security Advisory GHSA-3j3q-wp9x-585p published
- 2026-04-08: disclosed: CVE-2026-39429 published

## References

- [https://github.com/kcp-dev/kcp/releases/tag/v0.29.3](https://github.com/kcp-dev/kcp/releases/tag/v0.29.3)
- [https://github.com/kcp-dev/kcp/releases/tag/v0.30.3](https://github.com/kcp-dev/kcp/releases/tag/v0.30.3)
- [https://github.com/kcp-dev/kcp/security/advisories/GHSA-3j3q-wp9x-585p](https://github.com/kcp-dev/kcp/security/advisories/GHSA-3j3q-wp9x-585p)

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