CVE-2025-23390: Fleet SSH Certificate Trust Bypass Vulnerability

CVE-2025-23390 Overview

CVE-2025-23390 is an improper certificate validation vulnerability [CWE-295] in Rancher Fleet, the GitOps-at-scale controller used to deliver Kubernetes workloads across clusters. Fleet automatically trusts a remote SSH server's certificate when the host is absent from the known_hosts file. An authenticated attacker positioned on the network path can intercept the SSH connection and perform a man-in-the-middle (MitM) attack. When the target host already has a trusted entry in known_hosts, Fleet correctly validates the presented certificate, limiting exposure to first-contact scenarios and unmanaged entries.

Critical Impact
Attackers with network position can intercept Fleet's SSH-based Git repository traffic, exposing credentials and enabling tampering with GitOps-delivered manifests.

Affected Products

Rancher Fleet (GitOps controller for Kubernetes)
Deployments relying on SSH-based Git repository synchronization
SUSE distributions packaging the affected Fleet component

Discovery Timeline

2026-10-15 - CVE-2025-23390 published to the National Vulnerability Database (NVD)
2026-10-19 - Last updated in NVD database

Technical Details for CVE-2025-23390

Vulnerability Analysis

Fleet establishes SSH connections to Git repositories to synchronize cluster configuration. The SSH client inside Fleet accepts any server host key when no matching entry exists in the known_hosts file. This behavior is equivalent to a trust-on-first-use policy without the user confirmation step. An attacker who can redirect traffic between Fleet and the Git server can present a controlled host key and complete the handshake.

Once the MitM channel is established, the attacker can observe Git protocol traffic, capture any authentication material transmitted during the session, and modify repository contents in transit. Because Fleet pulls manifests that drive workload state across managed clusters, tampered content can propagate malicious configurations to downstream Kubernetes environments.

Root Cause

The root cause is an insecure default host-key verification policy. Fleet does not enforce strict host-key checking when the server entry is missing from known_hosts, violating the expected behavior defined by [CWE-295: Improper Certificate Validation]. Explicit entries are honored correctly, so the defect is scoped to repositories added without a pre-populated host key.

Attack Vector

Exploitation requires network adjacency sufficient to intercept SSH traffic between the Fleet controller and the upstream Git server. The attacker also needs the low privileges reflected in the vector string to influence Fleet's repository configuration or to be positioned on an unknown host's first connection. No user interaction is required. See the GitHub Security Advisory GHSA-xgpc-q899-67p8 and the SUSE Bug Report CVE-2025-23390 for maintainer analysis.

Detection Methods for CVE-2025-23390

Indicators of Compromise

Unexpected changes to Fleet-managed Kubernetes manifests that do not correspond to legitimate commits in the upstream repository.
SSH connections from Fleet controllers to Git endpoints whose host keys differ from previously observed fingerprints.
Fleet logs showing successful SSH handshakes to repositories lacking a known_hosts entry.

Detection Strategies

Compare host-key fingerprints observed in Fleet SSH sessions against an authoritative inventory of Git server keys.
Correlate GitOps reconciliation events with upstream Git commit history to flag drift introduced outside of version control.
Alert on Fleet GitRepo resources created or modified without an accompanying known_hosts secret.

Monitoring Recommendations

Enable audit logging on Kubernetes API events that create or modify Fleet GitRepo and Bundle resources.
Capture network flow metadata between Fleet controllers and Git servers to detect anomalous routing or new peers.
Monitor for repeated SSH key mismatches, which may indicate active interception attempts.

How to Mitigate CVE-2025-23390

Immediate Actions Required

Upgrade Rancher Fleet to the fixed release identified in GHSA-xgpc-q899-67p8.
Populate known_hosts entries for every Git repository Fleet connects to over SSH before the first synchronization.
Rotate any SSH deploy keys or credentials that may have been exposed through unverified sessions.

Patch Information

Refer to the Rancher Fleet GitHub Security Advisory GHSA-xgpc-q899-67p8 for the patched versions and upgrade instructions. SUSE customers should consult the SUSE Bug Report CVE-2025-23390 for distribution-specific package updates.

Workarounds

Pre-provision a known_hosts secret containing verified host keys for all Git servers used by Fleet.
Restrict Fleet's egress network paths so SSH traffic traverses only trusted routes.
Prefer HTTPS Git endpoints with validated TLS certificates until the patched Fleet version is deployed.

bash

# Configuration example
# Generate a known_hosts entry for your Git server and store it as a Kubernetes secret
ssh-keyscan -t rsa,ecdsa,ed25519 git.example.com > known_hosts
kubectl create secret generic git-auth \
  --from-file=known_hosts=./known_hosts \
  --from-file=ssh-privatekey=./id_ed25519 \
  --type=kubernetes.io/ssh-auth \
  -n fleet-local