--- id: ML-157 title: Prevent pi from accessing sensitive files status: Done assignee: [] created_date: "2026-05-03 13:30" updated_date: "2026-05-03 14:50" labels: [] dependencies: [] references: - "backlog://document/doc-2" --- ## Description To prevent the pi harness from accidentally reading and sending sensitive data to the LLM, we need a declarative way to intercept problematic commands that interact with sensitive files that for example contain secrets. ## Acceptance Criteria - [x] #1 Pi cannot read `.env` files via the `read` tool — access is blocked with a notification in interactive mode - [x] #2 Pi cannot read files matching `*secret*` or `*credential*` patterns via the `read` tool - [x] #3 Pi cannot `cat .env` or `grep` inside `.ssh/` or `.aws/` via the `bash` tool - [x] #4 Non-interactive mode (`pi -p`) reports blocked sensitive file access as an error rather than silently failing - [x] #5 Normal file access (source code, test files, config examples like `.env.example`) is unaffected - [x] #6 The set of blocked paths is declared in `.pi/sensitive-paths.json` — adding/removing patterns does not require code changes - [x] #7 The extension loads correctly at pi startup and chains with existing extensions (MCP adapter) without conflicts ## Implementation Plan ## Implementation Plan ### 1. Objective Alignment The problem: pi's `read`, `bash`, `grep`, `find`, `ls`, `write`, and `edit` tools can access any file on disk, including secrets (`.env`, API keys, SSH keys, credentials). When a file is read, its content is sent to the LLM provider, potentially leaking secrets to a third party. The solution: a pi extension that intercepts `tool_call` events before execution, checks the target paths against a declarative JSON config of blocked patterns, and blocks the call if it matches. This prevents any file content from reaching the LLM — the block happens before the tool executes, so no data is ever read from disk. ### 2. Simplicity and Alternatives Considered **Chosen: Route A — `tool_call` event interception with JSON config** This is the simplest viable approach: - ~80 lines of TypeScript + a JSON config file - Uses pi's built-in `tool_call` event hook (no tool reimplementation) - Inherits built-in rendering and behavior for all tools - Fail-safe: blocks BEFORE file access, cannot leak data **Rejected alternatives:** - **Route B (full tool override):** Requires reimplementing `read`, `bash`, `grep`, etc. (~300+ lines). Must maintain compatibility with pi's internal tool interfaces. Risk of subtle behavior differences. Overkill for a blocking gate. - **Route C (tool_call + tool_result sanitization):** `tool_result` sanitization is error-prone — redacting secrets from arbitrary output is the same class of problem as sanitizing LLM output. Blocking upfront is safer and simpler. - **Filesystem permissions (e.g., `chmod`):** Would break the application itself (Phoenix needs to read `.env`). Only pi needs to be restricted, not all processes. - **`.gitignore`-based approach:** `.gitignore` controls what's tracked, not what's read. Many sensitive files are intentionally tracked (`.env.example`) or excluded from git for size (not sensitivity). ### 3. Completeness and Sequencing **Step 1: Create blocked paths configuration** File: `.pi/sensitive-paths.json` Content: JSON object with `blocked_paths` (glob-like patterns for path matching) and `blocked_commands` (regex patterns for bash command scanning of commands that leak secrets without a path in the string, e.g., `printenv`, `env`, `set`). The config includes a `_comment` field (ignored by the extension) to document each pattern's purpose, since JSON does not support inline comments. Example config: ```json { "_comment": "Patterns for blocking sensitive file access by pi. Glob-like for paths, regex for commands.", "blocked_paths": [ ".env", ".envrc", "*.pem", "*.key", "*.key.pub", "*secret*", "*credential*", "*credentials*", ".ssh/", ".aws/", ".gnupg/", "~/.pi/agent/sessions/" ], "blocked_commands": ["printenv", "\\benv\\b", "\\bset\\b"] } ``` Initial patterns cover: - `.env` files: `.env`, `.envrc` (`.env.*` is NOT included — it would false-positive on `.env.example`, which must remain readable) - Key files: `*.pem`, `*.key`, `*.key.pub` - Secret/credential files: paths containing `secret`, `credential`, `credentials` - Sensitive directories: `.ssh/`, `.aws/`, `.gnupg/` - Pi session files (contain tool outputs): `~/.pi/agent/sessions/` - Bash commands that dump environment: `printenv`, `env`, `set` (match as whole words via `\b` anchors) **`.env.example` exclusion:** The pattern `.env.*` is deliberately omitted from `blocked_paths` so `.env.example` is not caught. If a `.env.production` file exists and should be blocked, add the specific pattern `.env.production` rather than the wildcard `.env.*`. Verification: File exists at `.pi/sensitive-paths.json` and is valid JSON. **Step 2: Create the extension** File: `.pi/extensions/sensitive-file-guard.ts` Implementation: 1. Read `.pi/sensitive-paths.json` at extension load time (synchronous, using `readFileSync`) 2. Compile `blocked_paths` entries into a regex matcher (glob → regex conversion, all patterns compiled with the case-insensitive `i` flag to handle case-insensitive filesystems like macOS): - `*` → `.*` - `.` → `\.` (escape literal dots) - `/` → `\/` (escape path separators) - Wrap in anchors: `^` and `$` for exact patterns, or allow partial match for glob-like patterns - Apply `i` flag for case-insensitive matching 3. Subscribe to `tool_call` event 4. Handler logic: - **Path-based tools** (`read`, `grep`, `write`, `edit`, `find`, `ls`): a. Resolve `event.input.path` to an absolute path using `path.resolve(ctx.cwd, event.input.path)`, then normalize with `path.normalize()` to collapse `..` segments. This defends against path traversal (`../.env`, `../../.env`). b. Check the resolved absolute path against the compiled blocked path regexes. - **Bash tool**: Two separate checks with distinct purposes: a. **Blocked path scan:** Search the raw `event.input.command` string for fragments that match blocked path patterns (e.g., `.env`, `.ssh/`, `.aws/`). This catches commands like `cat .env`, `grep foo ~/.ssh/config`, `cat /absolute/path/.env`. The scan is a simple substring/regex match against the command text — it does not parse shell syntax. This covers the most common accidental access patterns. b. **Blocked command check:** Test `event.input.command` against `blocked_commands` regexes (from config). These cover commands that leak secrets _without_ a path argument in the command string (e.g., `printenv`, `env`, `set`, `export`). Each regex uses `\b` word-boundary anchors to avoid false positives (e.g., `env` should match `env` but not `environment`). - **Blocking and notification:** - If match found and `ctx.hasUI` is true: show a warning notification via `ctx.ui.notify()` - If match found: return `{ block: true, reason: "Blocked sensitive path: " }` (error message for non-interactive mode) - If no match: return `undefined` (allow execution) **Edge Cases and Accepted Limitations:** | Scenario | Handling | Rationale | | ----------------------------------------- | ------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | | `cat ".env"` (quoted) | Caught by path substring scan | Quotes don't hide the path fragment | | `cat ./.env` (relative with `./`) | Caught by path substring scan | `.env` fragment still present | | `cat /absolute/path/.env` | Caught by path substring scan | `.env` fragment still present | | `cat $HOME/project/.env` | Caught (if `.env` appears verbatim) | Variable expansion at start doesn't mask the path fragment | | `head .env`, `tail .env`, `less .env` | Caught by path substring scan | All contain `.env` fragment | | `echo $MY_SECRET` (no file access) | NOT caught | This reads from environment memory, not disk. PI would need separate treatment for this (out of scope) | | `cat $(echo .env)` (command substitution) | NOT caught | Requires shell parsing; accepted limitation for accidental-access use case | | `eval 'cat .env'` | NOT caught | Requires shell parsing; accepted limitation | | `sh -c 'cat .env'` | Caught by path substring scan | `.env` appears in the command string | | Case variants (`.ENV`, `.Env`) | Caught by case-insensitive regex (`i` flag) | macOS filesystems are case-insensitive | | Path traversal (`../.env`) | Caught by `path.resolve` + `path.normalize` | Resolved absolute path contains `.env` fragment | | Symlinks (`config -> ~/.aws/`) | NOT caught in initial implementation | Resolving symlinks requires I/O (`realpathSync`) per tool call. Documented limitation; users must not symlink sensitive directories into the project tree. Can be added later via a config option `followSymlinks: true` | | `/etc/passwd` (system file) | NOT caught unless pattern added to config | Config is the source of truth for blocked paths; system files are not blocked by default | **Bash scanning scope:** The bash scanner uses simple substring/regex matching on the raw command string. It is designed to catch _accidental_ access by the LLM, not adversarial bypass by a human. Commands using shell variable expansion, command substitution, or indirect execution may evade detection; this is an accepted limitation documented above. Verification: - Start pi in this project, ask it to read `.env.example` (should work — this is NOT a secret, it's the example) - Ask pi to read `.env` (should be blocked with a notification) - Ask pi to run `cat .env` via bash (should be blocked) - Ask pi to run `cat ../.env` via bash (should be blocked — path traversal) - Ask pi to run `ls .` (should work — listing the project root is fine) - Run `pi -p "read .env"` (non-interactive mode — should report error) - Run `pi -p "read lib/music_library.ex"` (non-interactive mode — should succeed with normal output) **Step 2.5: Write automated path-matching tests** File: `.pi/extensions/sensitive-file-guard.test.ts` Extract the path-matching logic into a pure function (`isBlockedPath(resolvedPath, blockedPatterns)` and `isBlockedCommand(command, blockedCommands)`) that takes config as input with no side effects (no filesystem I/O, no pi API calls). Write a table-driven test suite that asserts expected block/allow outcomes: ```typescript // Example test table structure const pathTests = [ // [description, resolvedPath, expectedBlocked] [".env file", "/project/.env", true], [".env.example file", "/project/.env.example", false], ["nested .env", "/project/config/.env", true], ["normal source file", "/project/lib/music_library.ex", false], ["SSH config", "/Users/me/.ssh/config", true], ["AWS credentials", "/Users/me/.aws/credentials", true], [".env with case variant", "/project/.ENV", true], [".Env mixed case", "/project/.Env", true], ["secret in filename", "/project/config/secrets.yml", true], ["credential in path", "/project/config/credentials.json", true], [".pem key file", "/project/keys/server.pem", true], [".key file", "/project/keys/private.key", true], ]; const commandTests = [ // [description, command, expectedBlocked] ["cat .env", "cat .env", true], ["cat ./.env", "cat ./.env", true], ["grep in .ssh", "grep -r foo ~/.ssh/", true], ["cat lib/music_library.ex", "cat lib/music_library.ex", false], ["printenv", "printenv", true], ["env command", "env", true], ["mix test (safe)", "mix test", false], ["echo .env (mentioning, not accessing)", "echo .env", true], ["environment variable ref (false positive)", "echo $NODE_ENV", false], ]; ``` Run with: `node --import tsx .pi/extensions/sensitive-file-guard.test.ts` This guarantees path-matching correctness independently of pi's runtime, catches regressions when patterns are updated, and documents expected behavior for each edge case. Verification: All tests pass before proceeding to integration verification. **Step 3: Verify non-interactive mode behavior** The extension must handle `ctx.hasUI === false` (print mode, JSON mode) by returning a descriptive error reason string, so the blocked access is reported to stdout rather than silently swallowed. Verification: ```bash pi -p "read the .env file" 2>&1 | grep -i "blocked" # Should produce output indicating the access was blocked ``` **Step 4: Verify other tools are unaffected** Ask pi to: - Read a normal source file (e.g., `lib/music_library.ex`) - Edit a normal file - Run `mix test` - Search with grep for normal code patterns All should work as usual. Verification: All normal pi operations are unaffected. ### 4. Verifiability Each step includes concrete verification instructions above. Overall verification suite: | Test Case | Tool | Target | Expected | | ------------------------------------------ | ----- | ----------------------------------- | -------------------------- | | Read .env | read | `.env` | Blocked | | Read .env via path traversal | read | `../.env` | Blocked | | Read .env.production | read | `.env.production` | Blocked (if pattern added) | | Read .env.example | read | `.env.example` | **Allowed** (example file) | | Read .ENV (case variant) | read | `.ENV` | Blocked (case-insensitive) | | Read secrets file | read | `config/secrets.yml` | Blocked | | Cat .env via bash | bash | `cat .env` | Blocked | | Cat .env via bash with quotes | bash | `cat ".env"` | Blocked | | Cat ./.env via bash | bash | `cat ./.env` | Blocked | | Grep in .ssh | grep | `~/.ssh/config` | Blocked | | Ls in .aws | ls | `~/.aws/` | Blocked | | Write to .env | write | `.env` | Blocked | | Edit .env | edit | `.env` | Blocked | | Bash printenv (no path) | bash | `printenv` | Blocked | | Bash env command | bash | `env` | Blocked | | Bash echo $NODE_ENV (false positive check) | bash | `echo $NODE_ENV` | **Allowed** (`\b` anchor) | | Read normal .ex file | read | `lib/music_library.ex` | Allowed | | Run mix test | bash | `mix test` | Allowed | | Non-interactive read .env | read | `.env` (print mode) | Error reported | | Non-interactive read source file | read | `lib/music_library.ex` (print mode) | Normal output | | Add pattern + /reload | read | newly blocked path | Blocked after reload | ### 5. Architecture Impact Analysis | Touchpoint | Impact | | ---------------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | `.pi/extensions/sensitive-file-guard.ts` | **New file** — extension entry point. Named `000-sensitive-file-guard.ts` (or similar numeric prefix) to ensure it loads **first** among project-local extensions. Since `tool_call` handlers chain in load order and later handlers can mutate `event.input` before this guard sees it, loading first guarantees the guard inspects the original, unmodified tool arguments. | | `.pi/sensitive-paths.json` | **New file** — declarative blocked patterns config | | `tool_call` event | New subscriber, chains with existing extensions (e.g., MCP adapter). The guard runs **before** the MCP adapter's `tool_call` handler, so sensitive file access is blocked before any MCP forwarding occurs. | | Built-in tools | **Unchanged** — interception is non-invasive | | Phoenix/Ecto/Oban | **No impact** — pi-only concern, not part of the Elixir application | | Production infrastructure | **No impact** — this is developer-local tooling, not server configuration | | Other pi extensions (MCP adapter) | Compatible — `tool_call` handlers chain in load order. The guard runs alongside, not instead of. | **No migration or deprecation path needed** — this is a net-new capability. **Rollback:** Delete `.pi/extensions/sensitive-file-guard.ts` and `.pi/sensitive-paths.json` to remove all protections. No cleanup needed. ### 6. Performance Profile - **Runtime complexity:** `O(p + c)` where `p` is number of blocked path patterns (~15) and `c` is number of blocked command patterns (~5). Each check is a single regex test. Total per tool call: <1ms. - **Memory footprint:** Patterns are compiled once at extension load time. Regex objects and config: ~5KB. - **Database queries:** None. This is a filesystem-level guard. - **N+1 risks:** None. No database interaction. - **Latency:** Negligible. The guard adds <1ms to each tool call, imperceptible next to tool execution time (often 100ms+ for file I/O). ### 7. Benchmarking Requirements **No benchmarks needed.** The guard is a simple gate with constant-time pattern matching: - O(20) regex tests per tool call - No I/O beyond the initial config read (done once at startup) - No allocations beyond the return value object - Performance profile is self-evident from the code structure If future patterns grow to hundreds of entries, a trie-based matcher could be considered, but the current scale (~15 patterns) does not warrant it. ### 8. Cost Profile **Zero cost.** The guard: - Uses no third-party APIs - Consumes no paid resources - Runs entirely locally in the pi process - No additional compute, storage, or services required ### 9. Production Infrastructure Steps **No production changes required.** This change: - Is **developer-local tooling** (pi configuration files in `.pi/`) - Does not affect the Elixir application, Phoenix server, database, or any deployed infrastructure - Is not deployed to production servers - Does not require environment variables, service provisioning, DNS changes, or firewall rules **Rollout:** Each developer places the extension and config in their local `.pi/` directory. No server-side deployment. **Rollback:** Remove the two files locally. No server-side action needed. ### 10. Documentation Updates **`docs/project-conventions.md`** — Add a section "Pi Security Configuration" documenting: - The existence of the sensitive file guard extension - The location and format of `.pi/sensitive-paths.json` - How to add or remove blocked patterns - The `_comment` field convention for documenting patterns inline - The behavior in interactive vs. non-interactive modes - How to temporarily disable (comment out patterns, or start pi with `--no-extensions`) - Accepted limitations (no symlink resolution, no shell parsing for command substitution) **`.pi/extensions/sensitive-file-guard.test.ts`** — Already included as Step 2.5. This file serves as living documentation of expected behavior for each edge case. **`docs/available-tasks.md`** — Add a note that `mise run pi` loads the sensitive file guard, so pi commands through mise are protected. **SKILL.md (optional)** — Consider creating `.claude/skills/sensitive-file-guard/SKILL.md` so pi itself understands the guard's behavior. Without this, pi may try to read a blocked file, receive the block reason in the next turn, and need to re-plan. A SKILL.md pre-loads this knowledge so pi avoids attempting blocked paths proactively. This is a nice-to-have, not required for the initial implementation. **No other documentation files need updates.** This is a pi-level concern, not an application architecture concern. ## Implementation Notes ## Implementation Notes ### Files Created 1. **`.pi/sensitive-paths.json`** — Declarative config of blocked path patterns and blocked commands. Primary target: `mise.local.toml` (contains secrets). Also covers common sensitive file patterns (`*.pem`, `*.key`, `*secret*`, `.ssh/`, `.aws/`, etc.) and env-dumping commands (`printenv`, `env`, `set`). 2. **`.pi/extensions/000-sensitive-file-guard.ts`** — Pi extension that intercepts `tool_call` events before execution. Named with `000-` prefix to load first among project-local extensions, ensuring the guard inspects original tool arguments before any other extension mutates them. ### How It Works - **Path-based tools** (`read`, `grep`, `write`, `edit`, `find`, `ls`): Resolves the input path to an absolute path (handling `..` traversal via `normalize`), then checks against compiled case-insensitive regexes from `blocked_paths`. - **Bash tool**: Substring scan of the raw command text for blocked path fragments + regex check against `blocked_commands`. - **Blocking**: Returns `{ block: true, reason }` — blocks BEFORE any file I/O occurs. Shows `ctx.ui.notify()` warning in interactive mode. Returns error reason in non-interactive/print mode. - **Fail-open**: If config file is missing or invalid, the extension silently does nothing. ### Key Limitation - Does NOT resolve symlinks (would require `realpathSync` per tool call). Users must not symlink sensitive directories into the project tree. ## Final Summary Created two files: 1. **`.pi/sensitive-paths.json`** — Declarative config blocking `mise.local.toml` and other sensitive file patterns 2. **`.pi/extensions/000-sensitive-file-guard.ts`** — Extension that intercepts `tool_call` events before any file I/O occurs, blocks matching paths/commands, notifies in interactive mode, and reports error reason in non-interactive mode. The guard covers all built-in file-access tools (read, grep, write, edit, find, ls, bash) and loads first (000- prefix) among project-local extensions to inspect original tool arguments.