CVE-2026-50568: Fission Path Validation Bypass in Multi-Tenant Kubernetes Environments

Technical summary

The SanitizeFilePath function in pkg/utils/utils.go used strings.HasPrefix() to validate that file paths remained within a designated safe directory. This lexical check fails to enforce path-separator boundaries, allowing any directory whose name begins with the safe-directory string to bypass validation. For instance, '/packages-extra/evil' would pass a check intended for '/packages' since it lexically starts with that string. The vulnerable code path affects the builder's Clean handler (pkg/builder/builder.go:208) and the fetcher's Fetch and Upload handlers (pkg/fetcher/fetcher.go), both of which operate on shared volumes in multi-tenant environments. An attacker with local access to the shared volume could exploit directory pre-creation or control to induce unauthorized read or write operations outside the intended containment boundary.

Business impact

In a shared Kubernetes cluster running Fission, this vulnerability enables a tenant to breach isolation boundaries and access or modify another tenant's function code or data stored on the shared volume. This undermines the multi-tenant security model and could lead to data exfiltration, unauthorized code execution, or denial of service. Organizations relying on Fission for isolated serverless workloads should treat this as a containment breach vector, especially if they run untrusted or competing workloads on the same cluster.

Affected systems

Fission versions prior to 1.25.0 are affected. The vulnerability is specific to Fission deployments on Kubernetes where multiple tenants share storage volumes for builder and fetcher operations. It does not affect environments where Fission runs with strict volume isolation or in single-tenant configurations.

Exploitability

Exploitation requires local access (CWE-41: Improper Restriction of Rendered UI Layers or Frames) and the ability to pre-create or control a sibling directory in the fetcher or builder's shared volume. The attacker must be an authenticated tenant within the same Kubernetes cluster. The complexity is high because successful exploitation depends on specific directory naming and knowledge of the safe-directory path structure. This is not remotely exploitable and does not require elevated privileges beyond standard tenant access.

Remediation

Upgrade Fission to version 1.25.0 or later, which implements proper path-separator boundary validation. Organizations should also audit existing deployments to determine if directory isolation controls are in place and review logs for suspicious file access patterns from tenants in the period prior to patching.

Patch guidance

Update Fission to version 1.25.0 or later. Verify the patch through the official Fission release notes and GitHub repository. Test the update in a non-production environment first to ensure compatibility with your Kubernetes configuration and any custom extensions. Consider scheduling the upgrade during a maintenance window to minimize disruption to running functions.

Detection guidance

Monitor the builder and fetcher pods for unexpected directory creation or access patterns in the shared volume. Look for file operations that reference paths with similar prefixes to the intended safe directory (e.g., '/packages' and '/packages-extra'). Enable audit logging on your Kubernetes storage layer if available. Check for unusual inter-tenant file access or modifications. Review Fission's internal logs for validation errors or warnings from the SanitizeFilePath function if debug logging is enabled.

Why prioritize this

Although assigned a LOW CVSS score (3.6) due to local-only access and high complexity, this vulnerability is a multi-tenant isolation bypass in a shared Kubernetes environment. For organizations running Fission in multi-tenant clusters, prioritize patching to maintain workload isolation and prevent cross-tenant data leakage. Single-tenant deployments are at lower immediate risk but should still be updated as part of routine maintenance.

Risk score, explained

The CVSS 3.1 score of 3.6 (LOW) reflects the attack vector being local, attack complexity being high, and impact being limited to confidentiality and integrity of a single tenant's data. The score does not capture the organizational impact of multi-tenant isolation breach, which may warrant elevated priority within specific deployment contexts. Security teams should consider their own risk model when deciding urgency.

Frequently asked questions