ML-157: implementation and configuration
This commit is contained in:
Claudio Ortolina
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---
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id: ML-157
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title: Prevent pi from accessing sensitive files
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status: Done
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assignee: []
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created_date: '2026-05-03 13:30'
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updated_date: '2026-05-03 14:50'
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labels: []
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dependencies: []
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references:
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- 'backlog://document/doc-2'
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---
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## Description
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<!-- SECTION:DESCRIPTION:BEGIN -->
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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.
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<!-- SECTION:DESCRIPTION:END -->
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## Acceptance Criteria
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<!-- AC:BEGIN -->
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- [x] #1 Pi cannot read `.env` files via the `read` tool — access is blocked with a notification in interactive mode
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- [x] #2 Pi cannot read files matching `*secret*` or `*credential*` patterns via the `read` tool
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- [x] #3 Pi cannot `cat .env` or `grep` inside `.ssh/` or `.aws/` via the `bash` tool
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- [x] #4 Non-interactive mode (`pi -p`) reports blocked sensitive file access as an error rather than silently failing
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- [x] #5 Normal file access (source code, test files, config examples like `.env.example`) is unaffected
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- [x] #6 The set of blocked paths is declared in `.pi/sensitive-paths.json` — adding/removing patterns does not require code changes
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- [x] #7 The extension loads correctly at pi startup and chains with existing extensions (MCP adapter) without conflicts
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<!-- AC:END -->
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## Implementation Plan
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<!-- SECTION:PLAN:BEGIN -->
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## Implementation Plan
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### 1. Objective Alignment
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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.
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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.
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### 2. Simplicity and Alternatives Considered
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**Chosen: Route A — `tool_call` event interception with JSON config**
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This is the simplest viable approach:
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- ~80 lines of TypeScript + a JSON config file
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- Uses pi's built-in `tool_call` event hook (no tool reimplementation)
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- Inherits built-in rendering and behavior for all tools
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- Fail-safe: blocks BEFORE file access, cannot leak data
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**Rejected alternatives:**
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- **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.
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- **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.
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- **Filesystem permissions (e.g., `chmod`):** Would break the application itself (Phoenix needs to read `.env`). Only pi needs to be restricted, not all processes.
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- **`.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).
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### 3. Completeness and Sequencing
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**Step 1: Create blocked paths configuration**
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File: `.pi/sensitive-paths.json`
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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`).
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The config includes a `_comment` field (ignored by the extension) to document each pattern's purpose, since JSON does not support inline comments.
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Example config:
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```json
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{
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"_comment": "Patterns for blocking sensitive file access by pi. Glob-like for paths, regex for commands.",
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"blocked_paths": [
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".env",
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".envrc",
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"*.pem",
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"*.key",
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"*.key.pub",
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"*secret*",
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"*credential*",
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"*credentials*",
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".ssh/",
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".aws/",
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".gnupg/",
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"~/.pi/agent/sessions/"
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],
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"blocked_commands": [
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"printenv",
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"\\benv\\b",
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"\\bset\\b"
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]
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}
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```
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Initial patterns cover:
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- `.env` files: `.env`, `.envrc` (`.env.*` is NOT included — it would false-positive on `.env.example`, which must remain readable)
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- Key files: `*.pem`, `*.key`, `*.key.pub`
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- Secret/credential files: paths containing `secret`, `credential`, `credentials`
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- Sensitive directories: `.ssh/`, `.aws/`, `.gnupg/`
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- Pi session files (contain tool outputs): `~/.pi/agent/sessions/`
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- Bash commands that dump environment: `printenv`, `env`, `set` (match as whole words via `\b` anchors)
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**`.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.*`.
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Verification: File exists at `.pi/sensitive-paths.json` and is valid JSON.
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**Step 2: Create the extension**
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File: `.pi/extensions/sensitive-file-guard.ts`
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Implementation:
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1. Read `.pi/sensitive-paths.json` at extension load time (synchronous, using `readFileSync`)
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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):
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- `*` → `.*`
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- `.` → `\.` (escape literal dots)
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- `/` → `\/` (escape path separators)
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- Wrap in anchors: `^` and `$` for exact patterns, or allow partial match for glob-like patterns
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- Apply `i` flag for case-insensitive matching
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3. Subscribe to `tool_call` event
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4. Handler logic:
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- **Path-based tools** (`read`, `grep`, `write`, `edit`, `find`, `ls`):
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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`).
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b. Check the resolved absolute path against the compiled blocked path regexes.
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- **Bash tool**: Two separate checks with distinct purposes:
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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.
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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`).
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- **Blocking and notification:**
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- If match found and `ctx.hasUI` is true: show a warning notification via `ctx.ui.notify()`
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- If match found: return `{ block: true, reason: "Blocked sensitive path: <path or command>" }` (error message for non-interactive mode)
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- If no match: return `undefined` (allow execution)
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**Edge Cases and Accepted Limitations:**
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| Scenario | Handling | Rationale |
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|----------|----------|-----------|
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| `cat ".env"` (quoted) | Caught by path substring scan | Quotes don't hide the path fragment |
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| `cat ./.env` (relative with `./`) | Caught by path substring scan | `.env` fragment still present |
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| `cat /absolute/path/.env` | Caught by path substring scan | `.env` fragment still present |
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| `cat $HOME/project/.env` | Caught (if `.env` appears verbatim) | Variable expansion at start doesn't mask the path fragment |
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| `head .env`, `tail .env`, `less .env` | Caught by path substring scan | All contain `.env` fragment |
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| `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) |
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| `cat $(echo .env)` (command substitution) | NOT caught | Requires shell parsing; accepted limitation for accidental-access use case |
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| `eval 'cat .env'` | NOT caught | Requires shell parsing; accepted limitation |
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| `sh -c 'cat .env'` | Caught by path substring scan | `.env` appears in the command string |
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| Case variants (`.ENV`, `.Env`) | Caught by case-insensitive regex (`i` flag) | macOS filesystems are case-insensitive |
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| Path traversal (`../.env`) | Caught by `path.resolve` + `path.normalize` | Resolved absolute path contains `.env` fragment |
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| 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` |
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| `/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 |
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**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.
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Verification:
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- Start pi in this project, ask it to read `.env.example` (should work — this is NOT a secret, it's the example)
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- Ask pi to read `.env` (should be blocked with a notification)
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- Ask pi to run `cat .env` via bash (should be blocked)
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- Ask pi to run `cat ../.env` via bash (should be blocked — path traversal)
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- Ask pi to run `ls .` (should work — listing the project root is fine)
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- Run `pi -p "read .env"` (non-interactive mode — should report error)
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- Run `pi -p "read lib/music_library.ex"` (non-interactive mode — should succeed with normal output)
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**Step 2.5: Write automated path-matching tests**
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File: `.pi/extensions/sensitive-file-guard.test.ts`
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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:
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```typescript
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// Example test table structure
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const pathTests = [
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// [description, resolvedPath, expectedBlocked]
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[".env file", "/project/.env", true],
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[".env.example file", "/project/.env.example", false],
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["nested .env", "/project/config/.env", true],
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["normal source file", "/project/lib/music_library.ex", false],
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["SSH config", "/Users/me/.ssh/config", true],
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["AWS credentials", "/Users/me/.aws/credentials", true],
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[".env with case variant", "/project/.ENV", true],
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[".Env mixed case", "/project/.Env", true],
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["secret in filename", "/project/config/secrets.yml", true],
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["credential in path", "/project/config/credentials.json", true],
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[".pem key file", "/project/keys/server.pem", true],
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[".key file", "/project/keys/private.key", true],
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];
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const commandTests = [
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// [description, command, expectedBlocked]
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["cat .env", "cat .env", true],
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["cat ./.env", "cat ./.env", true],
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["grep in .ssh", "grep -r foo ~/.ssh/", true],
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["cat lib/music_library.ex", "cat lib/music_library.ex", false],
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["printenv", "printenv", true],
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["env command", "env", true],
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["mix test (safe)", "mix test", false],
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["echo .env (mentioning, not accessing)", "echo .env", true],
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["environment variable ref (false positive)", "echo $NODE_ENV", false],
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];
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```
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Run with: `node --import tsx .pi/extensions/sensitive-file-guard.test.ts`
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This guarantees path-matching correctness independently of pi's runtime, catches regressions when patterns are updated, and documents expected behavior for each edge case.
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Verification: All tests pass before proceeding to integration verification.
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**Step 3: Verify non-interactive mode behavior**
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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.
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Verification:
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```bash
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pi -p "read the .env file" 2>&1 | grep -i "blocked"
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# Should produce output indicating the access was blocked
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```
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**Step 4: Verify other tools are unaffected**
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Ask pi to:
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- Read a normal source file (e.g., `lib/music_library.ex`)
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- Edit a normal file
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- Run `mix test`
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- Search with grep for normal code patterns
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All should work as usual.
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Verification: All normal pi operations are unaffected.
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### 4. Verifiability
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Each step includes concrete verification instructions above. Overall verification suite:
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| Test Case | Tool | Target | Expected |
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|-----------|------|--------|----------|
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| Read .env | read | `.env` | Blocked |
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| Read .env via path traversal | read | `../.env` | Blocked |
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| Read .env.production | read | `.env.production` | Blocked (if pattern added) |
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| Read .env.example | read | `.env.example` | **Allowed** (example file) |
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| Read .ENV (case variant) | read | `.ENV` | Blocked (case-insensitive) |
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| Read secrets file | read | `config/secrets.yml` | Blocked |
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| Cat .env via bash | bash | `cat .env` | Blocked |
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| Cat .env via bash with quotes | bash | `cat ".env"` | Blocked |
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| Cat ./.env via bash | bash | `cat ./.env` | Blocked |
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| Grep in .ssh | grep | `~/.ssh/config` | Blocked |
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| Ls in .aws | ls | `~/.aws/` | Blocked |
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| Write to .env | write | `.env` | Blocked |
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| Edit .env | edit | `.env` | Blocked |
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| Bash printenv (no path) | bash | `printenv` | Blocked |
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| Bash env command | bash | `env` | Blocked |
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| Bash echo $NODE_ENV (false positive check) | bash | `echo $NODE_ENV` | **Allowed** (`\b` anchor) |
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| Read normal .ex file | read | `lib/music_library.ex` | Allowed |
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| Run mix test | bash | `mix test` | Allowed |
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| Non-interactive read .env | read | `.env` (print mode) | Error reported |
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| Non-interactive read source file | read | `lib/music_library.ex` (print mode) | Normal output |
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| Add pattern + /reload | read | newly blocked path | Blocked after reload |
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### 5. Architecture Impact Analysis
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| Touchpoint | Impact |
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|------------|--------|
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| `.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. |
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| `.pi/sensitive-paths.json` | **New file** — declarative blocked patterns config |
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| `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. |
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| Built-in tools | **Unchanged** — interception is non-invasive |
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| Phoenix/Ecto/Oban | **No impact** — pi-only concern, not part of the Elixir application |
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| Production infrastructure | **No impact** — this is developer-local tooling, not server configuration |
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| Other pi extensions (MCP adapter) | Compatible — `tool_call` handlers chain in load order. The guard runs alongside, not instead of. |
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**No migration or deprecation path needed** — this is a net-new capability.
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**Rollback:** Delete `.pi/extensions/sensitive-file-guard.ts` and `.pi/sensitive-paths.json` to remove all protections. No cleanup needed.
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### 6. Performance Profile
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- **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.
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- **Memory footprint:** Patterns are compiled once at extension load time. Regex objects and config: ~5KB.
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- **Database queries:** None. This is a filesystem-level guard.
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- **N+1 risks:** None. No database interaction.
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- **Latency:** Negligible. The guard adds <1ms to each tool call, imperceptible next to tool execution time (often 100ms+ for file I/O).
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### 7. Benchmarking Requirements
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**No benchmarks needed.** The guard is a simple gate with constant-time pattern matching:
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- O(20) regex tests per tool call
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- No I/O beyond the initial config read (done once at startup)
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- No allocations beyond the return value object
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- Performance profile is self-evident from the code structure
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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.
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### 8. Cost Profile
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**Zero cost.** The guard:
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- Uses no third-party APIs
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- Consumes no paid resources
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- Runs entirely locally in the pi process
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- No additional compute, storage, or services required
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### 9. Production Infrastructure Steps
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**No production changes required.** This change:
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- Is **developer-local tooling** (pi configuration files in `.pi/`)
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- Does not affect the Elixir application, Phoenix server, database, or any deployed infrastructure
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- Is not deployed to production servers
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- Does not require environment variables, service provisioning, DNS changes, or firewall rules
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**Rollout:** Each developer places the extension and config in their local `.pi/` directory. No server-side deployment.
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**Rollback:** Remove the two files locally. No server-side action needed.
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### 10. Documentation Updates
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**`docs/project-conventions.md`** — Add a section "Pi Security Configuration" documenting:
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- The existence of the sensitive file guard extension
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- The location and format of `.pi/sensitive-paths.json`
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- How to add or remove blocked patterns
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- The `_comment` field convention for documenting patterns inline
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- The behavior in interactive vs. non-interactive modes
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- How to temporarily disable (comment out patterns, or start pi with `--no-extensions`)
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- Accepted limitations (no symlink resolution, no shell parsing for command substitution)
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**`.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.
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**`docs/available-tasks.md`** — Add a note that `mise run pi` loads the sensitive file guard, so pi commands through mise are protected.
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**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.
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**No other documentation files need updates.** This is a pi-level concern, not an application architecture concern.
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<!-- SECTION:PLAN:END -->
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## Implementation Notes
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<!-- SECTION:NOTES:BEGIN -->
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## Implementation Notes
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### Files Created
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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`).
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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.
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### How It Works
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- **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.
|
||||
<!-- SECTION:NOTES:END -->
|
||||
|
||||
## Final Summary
|
||||
|
||||
<!-- SECTION:FINAL_SUMMARY:BEGIN -->
|
||||
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.
|
||||
<!-- SECTION:FINAL_SUMMARY:END -->
|
||||
Reference in New Issue
Block a user