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2606122015_security-hardening-batch-full-repo.md

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id 2606122015
title Security hardening batch — 2026-06-12 full-repo audit
status ✅
summary Low/informational hardening from the 2026-06-12 full-repo audit: make the MDS040 gate non-bypassable by attacker-controlled config (S002), fix rename symlink write-through when follow-symlinks is enabled (S007), and add a warning when fix encounters recipes in an unfamiliar repo.
model sonnet

Security hardening batch — 2026-06-12 full-repo audit

Goal

Close findings S002 and S007 from the 2026-06-12 full-repo audit report.

S002 (low, confirmed). checkMDS040Gate (buildpass.go:50-84) returns true (gate open) when recipe-safety is absent or disabled: if !ok || !rc.Enabled { return true }. An attacker-controlled .mdsmith.yml can set recipe-safety: false, bypassing the shell-operator/interpreter checks and allowing recipe commands with shell interpreters to execute via mdsmith fix. The gate is the sole pre-execution safety check.

S007 (low, confirmed). writeFilePreservingMode (rename.go:356-362) uses os.WriteFile, which on POSIX follows symlinks. When --follow-symlinks is enabled and a workspace file is a symlink to an external file, mdsmith rename overwrites the external file. The fix command uses a safe temp-file-then-rename pattern (atomicWriteFile) that replaces the symlink itself — the rename command is missing the same pattern.

Tasks

S002 — MDS040 gate hardening

  • Red: write a test that disables recipe-safety in config, adds a recipe with command: sh -c 'echo pwned', and asserts that checkMDS040Gate still rejects it (returns false / returns an error).
  • Green (Option A): when build.recipes is non-empty, always run the shell-safety check regardless of the recipe-safety rule toggle. The rule can still be disabled for diagnostic reporting, but the gate should not be bypassable.
  • Alternatively (Option B): emit a prominent warning when mdsmith fix is run with recipes present and recipe-safety: false is set in config, telling the user they are running recipes without safety checks.
  • Run go test ./cmd/mdsmith/... ./internal/...; all pass.

S007 — rename symlink write-through

  • Red: write a test that enables follow-symlinks, creates a symlink in a temp workspace pointing to an external temp file, runs writeFilePreservingMode on the symlink path, and asserts the external file is NOT modified (or the function returns an error).
  • Green: replace os.WriteFile in writeFilePreservingMode with the temp-file-then-rename pattern from atomicWriteFile (fix.go): create a temp file in filepath.Dir(path), write to it, then call os.Rename(tmp, path). On POSIX, os.Rename replaces the symlink itself rather than following it.
  • Run go test ./cmd/mdsmith/...; all pass.

Acceptance Criteria

  • With recipe-safety: false in config, mdsmith fix either rejects shell-interpreter recipes or displays a clear user-facing warning before executing any recipe.
  • mdsmith rename with --follow-symlinks does not overwrite files outside the workspace via symlinks.
  • All existing tests pass.