How should security teams prevent path traversal in Kubernetes storage drivers that use shared multi-tenant exports?

Why This Matters for Security Teams

path traversal in a storage driver is not a cosmetic input bug. In a shared Kubernetes environment, a tenant-controlled path can become a control-plane to data-plane breakout if the driver resolves paths inside a multi-tenant export without enforcing an explicit tenant boundary. That creates exposure across namespaces, workloads, and sometimes clusters, especially when the same export backs multiple tenants or environments. The right lens is not just validation, but authorization, boundary enforcement, and safe handling of filesystem semantics.

NIST Cybersecurity Framework 2.0 is useful here because it frames the problem as a governance and protection issue, not only a coding defect. Security teams often get pulled toward string sanitization and miss the bigger issue: who is allowed to request storage, what path inputs are trusted, and whether the driver can be abused even when the input looks normalized. In practice, many security teams encounter path traversal only after an unexpected read or overwrite has already crossed a tenant boundary, rather than through intentional boundary testing.

How It Works in Practice

The safest pattern is to treat all tenant-supplied path fragments as untrusted and to enforce two checks: semantic validation before any path construction, and enforcement again at the storage boundary. Normalization alone is not enough because a cleaned path can still resolve outside the intended export once symlinks, mount points, or backend-specific path handling are involved. Current guidance suggests that the driver should reject dot dot sequences, absolute paths, encoded separator variants, and any input that cannot be mapped to a known-safe tenant prefix.

Operationally, the control chain should include the Kubernetes API, admission logic, and the storage backend. If a workload can influence PersistentVolume creation, StorageClass parameters, or CSI driver inputs, that path becomes part of the trust boundary. Restricting who can create PersistentVolumes is critical because that permission can become a cross-tenant abuse path when the driver assumes filepath.Join is a sanitizer. Teams should also ensure the driver resolves the final path against an allowlisted root and verifies the result remains inside that root after all filesystem resolution.

• Validate path components against an allowlist of expected tenant identifiers or safe subpaths.
• Reject traversal markers, absolute paths, and ambiguous encodings before path joining.
• Enforce tenant-to-export mapping in admission controls and in the driver itself.
• Use a dedicated root per tenant where possible, rather than a shared writable export.
• Log rejected path attempts and correlate them with identity and workload context.

For broader secure design guidance, the OWASP Path Traversal Cheat Sheet remains relevant, but Kubernetes introduces extra complexity because the same logical request may be transformed by admission controllers, CSI sidecars, and backend export semantics. These controls tend to break down when a single shared export is reused across tenants and the driver relies on implicit path concatenation instead of explicit authorization checks.

Common Variations and Edge Cases

Tighter path validation often increases operational overhead, requiring organisations to balance tenant isolation against storage flexibility. That tradeoff becomes sharper when applications need dynamic subdirectory creation, legacy workloads expect arbitrary file naming, or multiple CSI implementations sit behind the same storage class. There is no universal standard for this yet, so best practice is evolving toward defense in depth rather than a single canonical sanitization method.

Edge cases matter. Symlink traversal can defeat naive checks if the driver validates only the string form of the path. Encoded separators, unicode lookalikes, and backend-specific normalization can also create gaps between what the API receives and what the filesystem ultimately resolves. Where possible, use NIST Zero Trust Architecture thinking: do not trust the request because it came through Kubernetes, and do not trust the filesystem outcome because the input looked clean. For implementation testing, OWASP Web Security Testing Guide techniques can be adapted to probe traversal, encoding, and path resolution edge cases in the storage path.

The hardest environments are multi-tenant platforms that mix shared exports, custom CSI plugins, and partial RBAC delegation, because the path check may be correct in the driver yet still bypassed through privileged provisioning or mis-scoped platform roles.

Related resources from NHI Mgmt Group

• How should security teams prevent cross-tenant data leaks in multi-tenant apps?
• How should security teams prevent path traversal issues when repository names are used to build filesystem paths?
• How should security teams use shared signals in IAM response?
• How should security teams prevent hardcoded secrets from becoming a breach path?