ZeptoClaw: Path boundary checks bypass via symlink, TOCTOU, and hardlink

Summary

Workspace boundary enforcement currently has three related bypass risks. This issue tracks fixing all three in one pull request.

Details

R1 - Dangling Symlink Component Bypass

• What happens: Path validation can miss dangling symlink components during traversal checks.
• Why it matters: A symlink that is unresolved at validation time can later resolve to an external location.
• Impact: Read and write operations may escape workspace boundaries.
• Affected area: src/security/path.rs (check_symlink_escape).

R2 - TOCTOU Between Validation and Use

• What happens: The path is validated first, then used later for filesystem operations.
• Why it matters: A concurrent filesystem change can swap path components after validation but before open/write.
• Impact: Race-based workspace escape is possible.
• Affected area: Filesystem and file-consuming tools that call validate_path_in_workspace before I/O.

R3 - Hardlink Alias Bypass

• What happens: A file inside workspace can be a hardlink to an inode outside the intended workspace trust boundary.
• Why it matters: Prefix and symlink checks can pass while data access still mutates or reads external content.
• Impact: Policy bypass for read/write operations.
• Affected area: Any tool that reads or writes via validated paths.

Risk Matrix

ID	Risk	Severity	Likelihood	Impact
R1	Dangling symlink component bypass	High	Medium	Workspace boundary escape for read/write
R2	Validate/use TOCTOU race	High	Medium	Race-based boundary escape during file I/O
R3	Hardlink alias bypass	Medium	Low-Medium	External inode read/write through in-workspace path

PoC

R1 - Dangling symlink component bypass

1. Create a symlink inside workspace pointing to a missing target.
2. Validate a path traversing that symlink.
3. Create the target directory outside workspace after validation.
4. Perform file operation and observe potential boundary escape if not fail-closed.

R2 - TOCTOU between validation and use

1. Validate a candidate in-workspace path.
2. Before open/write, replace an intermediate component with a link to external location.
3. Continue with the file operation.
4. Observe boundary escape if operation trusts only stale validation result.

R3 - Hardlink alias bypass

1. Place a hardlink inside workspace that points to an external inode.
2. Validate the in-workspace hardlink path.
3. Read or write through this path.
4. Observe external inode access through a path that appears in-scope.

Impacts

Unauthorized cross path boundary

Credit

@zpbrent

Patch

f50c17e11ae3e2d40c96730abac41974ef2ee2a8