security-hooks/docs/superpowers/specs/2026-03-26-security-hooks-design.md

Security Hooks for Claude Code

A general-purpose, distributable set of Claude Code hooks that catch prompt injection, agent autonomy drift, supply chain attacks, and data exfiltration. Ships as a single binary (Elixir daemon via Burrito) with a shell shim, a custom rule DSL, and layered command analysis (regex + AST parsing).

Threat Model

1. Prompt injection via untrusted content — a malicious README, fetched webpage, or MCP response tricks the agent into running harmful commands
2. Agent autonomy drift — the agent does something "helpful" that is destructive (force push, delete files, install malware packages)
3. Supply chain / dependency attacks — the agent installs compromised packages or runs untrusted scripts
4. Data exfiltration — the agent leaks secrets, env vars, or private code to external services

Architecture

Three components:

1. Shell shim (security-hook)

A short bash script that Claude Code invokes as a hook command. It:

• Reads the JSON hook payload from stdin
• Sends it to the daemon over a Unix socket
• Prints the daemon's JSON response to stdout

The shim is deliberately simple — it does not manage daemon lifecycle. That responsibility belongs to the platform service manager (see Daemon Lifecycle below).

Fail-closed policy: If the shim cannot reach the daemon within its timeout, it exits with code 2 (blocking error) and writes a deny reason to stderr. The system never fails open.

Timeouts: Two configurable values:

• shim_timeout_ms (default: 200ms) — steady-state timeout for a warm daemon. If a warm daemon doesn't respond in 200ms, something is wrong.
• shim_cold_start_timeout_ms (default: 3000ms) — used when the shim detects a cold start (socket activation just triggered, or fallback daemon just spawned). Allows time for BEAM boot and Burrito unpacking. The shim detects cold start by checking whether the socket existed before the connection attempt.

Socket paths:

• Linux/WSL with systemd: managed by systemd socket activation at $XDG_RUNTIME_DIR/security-hooks/sock
• macOS with launchd: managed by launchd at $TMPDIR/security-hooks/sock
• Fallback (no service manager): $XDG_RUNTIME_DIR/security-hooks/sock (Linux/WSL) or $TMPDIR/security-hooks/sock (macOS). If $XDG_RUNTIME_DIR is unset (containers, SSH sessions), falls back to /tmp/security-hooks-$UID/sock.

The socket's containing directory is created with mode 0700 to prevent other local processes from connecting.

2. Elixir daemon (security-hookd)

A long-running BEAM process distributed as a Burrito binary (single executable, no Erlang/Elixir runtime required). Target platforms: macOS (aarch64, x86_64), Linux (x86_64, aarch64), WSL (x86_64).

Components:

• Socket listener — accepts connections on Unix socket, parses JSON payloads
• Rule engine — loads rules from .rules files, evaluates them against the payload using the appropriate matching strategy (regex or AST), returns the first matching result
• Bash analyzer — parses shell commands into an AST for structural matching that catches evasion via subshells, pipes, and obfuscation (see Matching Strategies and Bash Parser Strategy below)
• Rule loader — parses the custom .rules DSL. Validator modules are compiled into the Burrito binary at build time (not loaded dynamically) to prevent code injection into the security daemon. Users who add custom validators must rebuild the binary.
• File watcher — monitors rules/ and config/ directories, triggers hot-reload on change
• Config manager — loads config.toml, merges config.local.toml overrides (see Configuration)
• Logger — writes JSONL to $XDG_STATE_HOME/security-hooks/hook.log (macOS: ~/Library/Logs/security-hooks/hook.log)

3. Rule files

Two kinds:

• Pattern rules in .rules files using a custom DSL (see Rule Format below). Rules can use regex patterns for simple matching or AST functions for structural analysis.
• Validator modules in daemon/lib/security_hooks/validators/*.ex for complex logic that cannot be expressed in the DSL. These are compiled into the binary at build time — not loaded dynamically — to prevent code injection into the security daemon.

Directory Structure

security-hooks/
├── bin/
│   └── security-hook                    # shell shim
├── service/
│   ├── security-hookd.service           # systemd user service unit
│   ├── security-hookd.socket            # systemd socket activation unit
│   └── com.security-hooks.daemon.plist  # macOS launchd agent
├── rules/
│   ├── bash.rules                       # bash command rules
│   ├── edit.rules                       # file edit rules
│   └── mcp.rules                        # MCP tool rules
├── config/
│   ├── config.toml                      # default settings
│   └── config.local.toml                # user overrides (gitignored)
├── daemon/                              # Elixir application source
│   ├── lib/
│   │   ├── security_hooks/
│   │   │   ├── application.ex
│   │   │   ├── socket_listener.ex
│   │   │   ├── rule_engine.ex
│   │   │   ├── rule_loader.ex
│   │   │   ├── bash_analyzer.ex         # AST parsing via bash Hex package
│   │   │   ├── file_watcher.ex
│   │   │   ├── config.ex
│   │   │   ├── logger.ex
│   │   │   └── validators/
│   │   │       ├── unknown_executable.ex
│   │   │       ├── dependency_mutation.ex
│   │   │       └── secret_access.ex
│   │   └── security_hooks.ex
│   ├── mix.exs
│   └── test/
├── install.sh
└── README.md

Hook Events & Claude Code Integration

Hooks are registered via install.sh into Claude Code's settings.json:

PreToolUse hooks (can allow/deny/ask)

Bash (matcher: Bash):

security-hook pre bash

Edit/Write (matcher: Edit|Write):

security-hook pre edit

MCP (matcher: mcp__.*):

security-hook pre mcp

PostToolUse hook

Deferred to a future version. Post-tool-use linting is project-specific and requires detecting the project's toolchain, choosing the right linter, handling timeouts, and distinguishing agent-introduced errors from pre-existing ones. This deserves its own design pass.

Response format

The daemon returns JSON matching Claude Code's hook output spec.

PreToolUse allow:

{
  "hookSpecificOutput": {
    "hookEventName": "PreToolUse",
    "permissionDecision": "allow"
  }
}

PreToolUse deny (tier: block):

{
  "hookSpecificOutput": {
    "hookEventName": "PreToolUse",
    "permissionDecision": "deny",
    "permissionDecisionReason": "destructive rm detected (in subshell)",
    "additionalContext": "Use trash-cli or move to a temp directory"
  }
}

PreToolUse ask (tier: suspicious):

{
  "hookSpecificOutput": {
    "hookEventName": "PreToolUse",
    "permissionDecision": "ask",
    "permissionDecisionReason": "unknown executable: foo",
    "additionalContext": "Add foo to allowed_executables in config.toml"
  }
}

Rule Format: .rules DSL

Rules use a custom DSL designed so that regex patterns are never quoted (everything after match to end of line is the pattern, verbatim) and AST-based structural matching uses readable function syntax.

Matching strategies

The rule loader inspects each match value to determine the matching strategy:

• Regex path — if the value does not start with a known DSL function name, it is treated as a regex pattern matched against the raw input string. Fast, good for simple patterns.
• AST path — if the value starts with a DSL function (command(, pipeline_to(, reads_file(, etc.), the command is parsed into an AST using the bash Hex package, and the function is evaluated against the tree. This catches evasion via subshells, pipes, quoting tricks, and obfuscation.

The two paths are distinguished unambiguously: regex patterns will never start with identifier(.

For bash rules specifically, the AST parser walks the full command tree — including subshells $(...), pipes |, logical chains &&/||, and process substitution <(...) — to find matching command nodes regardless of nesting depth.

Syntax

# Regex matching — pattern is everything after "match " to end of line
block "fork-bomb"
  match :\(\)\s*\{.*\|.*&\s*\}\s*;
  nudge "Fork bomb detected"

# AST matching — structural analysis of parsed command
block "destructive-rm"
  match command("rm") with_flags("-r", "-rf", "-fr")
  nudge "Use trash-cli or move to a temp directory"

block "pipe-to-exfil"
  match pipeline_to("curl", "wget", "nc")
  nudge "Don't pipe output to network commands"

block "curl-data-upload"
  match command("curl") with_flags("-d", "--data", "-F", "--form")
  nudge "Don't upload data via curl — only downloads are allowed"

block "eval-obfuscation"
  match command("eval", "exec")
  nudge "Don't use eval/exec — run the command directly"

# Regex is fine for things that can't be obfuscated
block "agent-recursion"
  match claude\s+.*--dangerously-skip-permissions
  nudge "Don't spawn Claude without permission checks"

# Config-referenced matching
suspicious "unknown-executable"
  match_base_command_not_in allowed_executables
  nudge "Unknown command '{base_command}'. Add it to allowed_executables in config.toml"

# Elixir validator for complex logic
block "dependency-mutation"
  validator SecurityHooks.Validators.DependencyMutation
  nudge "Don't modify dependencies directly — use the package manager CLI"

AST match functions

These functions operate on the parsed AST of a bash command. They match against any command node in the tree, including those nested inside subshells, pipelines, and logical chains.

Function	Description	Example
command("name", ...)	Matches if any command node has one of the given executables	command("rm", "rmdir")
with_flags("flag", ...)	Modifier: command must also have one of these flags	command("rm") with_flags("-r", "-rf")
with_args_matching(regex)	Modifier: command args must match regex	command("chmod") with_args_matching("777")
pipeline_to("name", ...)	Matches if a pipeline ends with one of these commands	pipeline_to("curl", "nc")
pipeline_from("name", ...)	Matches if a pipeline starts with one of these commands	pipeline_from("cat", "echo") pipeline_to("curl")
reads_file("path", ...)	Matches if any command reads from a sensitive path (see semantics below)	reads_file("~/.ssh", "~/.aws")
writes_file("path", ...)	Matches if any command writes to a path (see semantics below)	writes_file("/etc/", "~/.bashrc")
sets_env("var", ...)	Matches if command sets one of these env vars (see semantics below)	sets_env("LD_PRELOAD", "PATH")

reads_file semantics

Matches when a path appears in any of these AST positions:

• Input redirections: < ~/.ssh/id_rsa
• Command arguments to known file-reading commands: cat ~/.ssh/id_rsa, head /etc/shadow
• Source/dot commands: source ~/.bashrc, . ~/.profile
• Here-string file references: command <<< $(cat ~/.ssh/id_rsa)

Paths are prefix-matched: reads_file("~/.ssh") matches ~/.ssh/id_rsa, ~/.ssh/config, etc. Tilde expansion is applied before matching.

writes_file semantics

Matches when a path appears in any of these AST positions:

• Output redirections: > /etc/hosts, >> ~/.bashrc
• Command arguments to known file-writing commands: tee /etc/hosts, cp src /etc/
• The dd of= argument: dd of=/dev/sda

Same prefix-matching and tilde expansion as reads_file.

sets_env semantics

Matches all forms of environment variable assignment in bash:

• Inline assignment: PATH=/evil:$PATH command
• Export: export PATH=/evil:$PATH
• Declare: declare -x PATH=/evil:$PATH
• Env command: env PATH=/evil:$PATH command

Functions can be chained. All conditions must match (AND logic):

block "exfil-secrets-via-curl"
  match pipeline_from("cat", "echo") pipeline_to("curl", "wget")
  nudge "Don't pipe local data to network commands"

match_any works with both regex and AST functions:

block "privilege-escalation"
  match_any
    command("sudo")
    command("su") with_flags("-")
    command("chmod") with_args_matching("777|u\+s")
    command("chown") with_args_matching("root")
  nudge "Privilege escalation is not allowed"

Grammar

file          := (comment | blank | rule)*
comment       := '#' <text to end of line>
rule          := tier SP name NL clauses
tier          := "block" | "suspicious"
name          := '"' <text> '"'
clauses       := matcher nudge
matcher       := match | match_any | match_not_in | validator
match         := INDENT "match " (ast_expr | regex_pattern) NL
match_any     := INDENT "match_any" NL (INDENT2 (ast_expr | regex_pattern) NL)+
match_not_in  := INDENT ("match_base_command_not_in" | "match_server_not_in") SP <config key> NL
validator     := INDENT "validator " <elixir module name> NL
nudge         := INDENT "nudge " '"' <text with {var} interpolation> '"' NL

ast_expr      := ast_func (SP ast_func)*
ast_func      := IDENT '(' quoted_args ')' [SP modifier]*
modifier      := IDENT '(' quoted_args ')'
quoted_args   := '"' <text> '"' (',' SP '"' <text> '"')*
regex_pattern := <any text not starting with IDENT '('> <to end of line>

INDENT = 2 spaces, INDENT2 = 4 spaces.

Note: only_when / except_when conditions are deferred to a future version.

Tiers

• block — hard deny via permissionDecision: "deny". The nudge is sent as additionalContext so the agent can self-correct.
• suspicious — soft deny via permissionDecision: "ask". Falls through to Claude Code's permission prompt so the human decides. The nudge is shown as context.

Match targets

Each hook type matches against a specific field from the Claude Code JSON payload:

Hook file	Payload field matched	Example value
bash.rules	tool_input.command	rm -rf /tmp/foo
edit.rules	tool_input.file_path	/home/user/project/src/main.rs
mcp.rules	tool_name (parsed into server + tool)	mcp__context7__query-docs

For bash rules, {base_command} is extracted as the first whitespace-delimited token of tool_input.command after stripping leading environment variable assignments. For AST-matched rules, it is extracted from the parsed command node.

Evaluation

Rules are evaluated in two passes, grouped by matching strategy. First match within either pass wins.

Pass 1: Regex rules — all regex-based rules are checked in file order (fast, microsecond-level). If any matches, the verdict is returned immediately and the AST parser is never invoked.

Pass 2: AST rules — if no regex rule matched, the command is parsed into an AST (once, cached for the request). All AST-based rules are then checked in file order against the parsed tree.

This strategy-grouped evaluation means file order is respected within each group but regex rules always run before AST rules regardless of file position. This is intentional: regex serves as a fast pre-filter so the AST parser is only invoked when needed.

Place specific rules before general catch-alls within each matching strategy.

Variable interpolation in nudges

• {command} — the full command string (Bash hooks)
• {base_command} — the first token / primary executable
• {file_path} — the target file path (Edit/Write hooks)
• {tool_name} — the tool name
• {server_name} — the MCP server name (MCP hooks)

Default Rule Sets

bash.rules

Tier: block (AST-matched where evasion is a concern)

Destructive filesystem operations:

• command("rm") with_flags("-r", "-rf", "-fr") — recursive delete
• command("mkfs") — format filesystem
• command("dd") with_args_matching("of=/dev/") — raw disk write

Git history destruction:

• command("git") with_args_matching("push\\s+.*--force(?!-with-lease)") — force push (not --force-with-lease)
• command("git") with_args_matching("reset\\s+--hard") — hard reset
• command("git") with_args_matching("clean\\s+.*-f") — force clean

Package registry attacks:

• command("npm") with_args_matching("unpublish") — npm unpublish
• command("gem") with_args_matching("yank") — gem yank
• command("cargo") with_args_matching("yank") — cargo yank

Cloud resource deletion:

• command("aws") with_args_matching("delete-|terminate-|destroy") — AWS destructive ops
• command("gcloud") with_args_matching("delete") — GCloud destructive ops
• command("az") with_args_matching("delete") — Azure destructive ops
• command("fly") with_args_matching("destroy") — Fly.io destructive ops

Privilege escalation:

block "privilege-escalation"
  match_any
    command("sudo")
    command("su") with_flags("-")
    command("chmod") with_args_matching("777|u\\+s|4[0-7]{3}")
    command("chown") with_args_matching("root")
  nudge "Privilege escalation is not allowed"

Environment variable poisoning:

• sets_env("LD_PRELOAD", "LD_LIBRARY_PATH", "PATH", "NODE_OPTIONS", "PYTHONPATH", "RUBYOPT")

Data exfiltration (AST-matched to catch piped patterns):

• command("curl") with_flags("-d", "--data", "-F", "--form", "--upload-file") — data upload
• command("wget") with_flags("--post-data", "--post-file") — data upload
• pipeline_to("curl", "wget", "nc", "ncat") — piping to network commands
• reads_file("~/.ssh", "~/.aws/credentials", "~/.config/gcloud", "~/.netrc") — sensitive file access

Agent recursion:

• command("claude") with_flags("--dangerously-skip-permissions") — unguarded agent spawn

Tier: block (regex for common patterns)

These regex rules catch the most common forms. They are not evasion-proof (e.g., a renamed miner binary bypasses the regex) but provide fast first-line detection alongside the AST rules above.

• Fork bombs: :\(\)\s*\{.*\|.*&\s*\}\s*;
• Crypto miners: xmrig|minerd|stratum\+tcp://

Tier: suspicious

• Unknown base command not in allowed executables list (match_base_command_not_in)
• Long base64-encoded strings: [A-Za-z0-9+/]{100,}={0,2} (obfuscation signal)

edit.rules

Edit rules match against tool_input.file_path using regex on the path string.

# Tier: block

block "edit-outside-project"
  match ^(?!CLAUDE_PROJECT_DIR)
  nudge "Edits must be within the project directory"

block "edit-shell-config"
  match /\.(bashrc|zshrc|profile|bash_profile|zprofile)$
  nudge "Don't edit shell configuration files"

block "edit-env-file"
  match /\.env(\.|$)
  nudge "Don't edit .env files — manage secrets manually"

block "edit-sensitive-dir"
  match ^(~|HOME)/\.(ssh|aws|config/gcloud|gnupg)/
  nudge "Don't edit files in sensitive directories"

# Tier: suspicious

suspicious "edit-ci-config"
  match \.(github/workflows|gitlab-ci\.yml|Jenkinsfile)
  nudge "Editing CI/CD config — verify this is intentional"

suspicious "edit-dockerfile"
  match (Dockerfile|docker-compose\.yml)$
  nudge "Editing container config — verify this is intentional"

suspicious "edit-lockfile"
  match (package-lock\.json|pnpm-lock\.yaml|yarn\.lock|mix\.lock|Cargo\.lock|poetry\.lock|Gemfile\.lock|go\.sum|composer\.lock)$
  nudge "Editing lockfile directly — use the package manager instead"

suspicious "edit-dependency-manifest"
  validator SecurityHooks.Validators.DependencyMutation
  nudge "Dependency fields changed in {file_path} — use the package manager CLI"

Note: CLAUDE_PROJECT_DIR and HOME in patterns are expanded to their actual values at rule load time.

mcp.rules

MCP rules match against tool_name (format: mcp__servername__toolname). The server name and tool name are extracted and available as {server_name} and {mcp_tool} for nudge interpolation.

# Tier: block — injection patterns in MCP tool parameters
block "mcp-parameter-injection"
  match_any
    \$\(.*\)
    `.*`
    ;\s*\w+
    \|\s*\w+
  nudge "MCP tool parameters contain shell injection patterns"

# Tier: suspicious — external resource access
suspicious "mcp-url-fetch"
  match (fetch|get|read).*url
  nudge "MCP tool '{mcp_tool}' on server '{server_name}' accesses external URLs"

# Tier: block — unknown servers (catch-all, must be last)
block "unknown-mcp-server"
  match_server_not_in mcp_allowed_servers
  nudge "Unknown MCP server '{server_name}'. Add it to config.toml:\n\n[[mcp.servers]]\nname = \"{server_name}\"\ntools = [\"*\"]"

Note: MCP rules use match_server_not_in (not match_base_command_not_in) for clarity, since the match target is the server name, not a base command.

Allowed MCP servers and their tools are configured in config.toml. The install script auto-detects MCP servers from Claude Code's existing config and pre-populates the allowlist.

Configuration

config.toml (defaults, checked into repo)

[meta]
version = "1.0.0"

[executables]
allowed = [
  "git", "mix", "elixir", "iex", "cargo", "rustc",
  "go", "python", "pip", "uv", "node", "npm", "pnpm", "yarn",
  "rg", "fd", "jq", "cat", "ls", "head", "tail", "wc", "sort", "uniq",
  "mkdir", "cp", "mv", "touch", "echo", "grep", "sed", "awk",
  "make", "cmake", "gcc", "clang",
  "ruby", "gem", "bundler", "rake",
  "php", "composer",
  "java", "javac", "mvn", "gradle",
  "curl", "wget",
]

[secrets]
env_vars = [
  "AWS_SECRET_ACCESS_KEY", "AWS_SESSION_TOKEN", "AWS_ACCESS_KEY_ID",
  "GITHUB_TOKEN", "GH_TOKEN",
  "DATABASE_URL",
  "OPENAI_API_KEY", "ANTHROPIC_API_KEY",
  "STRIPE_SECRET_KEY",
  "PRIVATE_KEY", "SECRET_KEY",
]

[paths]
sensitive = [
  "~/.ssh",
  "~/.aws/credentials",
  "~/.config/gcloud",
  "~/.netrc",
  "/etc/shadow",
  "/etc/passwd",
]

[[mcp.servers]]
name = "context7"
tools = ["resolve-library-id", "query-docs"]

[[mcp.servers]]
name = "sequential-thinking"
tools = ["sequentialthinking"]

[daemon]
idle_timeout_minutes = 30
log_format = "jsonl"
shim_timeout_ms = 200
shim_cold_start_timeout_ms = 3000

[rules]
disabled = []

config.local.toml (user overrides, gitignored)

Merges on top of config.toml:

• Flat lists (executables, env vars, paths): support append and exclude sub-keys
• Structured arrays (MCP servers): entries are merged by name field. A local entry with the same name as a default replaces it entirely. New entries are appended. To remove a default server, add it with tools = [].
• Scalar values: overwritten directly
• rules.disabled: entries cause matching rules to be skipped

# Example: customize allowed executables
[executables]
append = ["my-custom-tool", "deno", "bun"]
exclude = ["curl", "wget"]  # force these through AST exfil checks only

# Example: add project-specific MCP servers
[[mcp.servers]]
name = "my-internal-tool"
tools = ["*"]

# Example: disable specific rules
[rules]
disabled = ["force-push"]  # I use --force intentionally

# Example: lower the shim timeout
[daemon]
shim_timeout_ms = 100

Daemon Lifecycle

The daemon is managed by the OS service manager where available, with a portable fallback. The install.sh script detects the platform and installs the appropriate mechanism.

Linux/WSL: systemd user service + socket activation

install.sh installs two systemd user units:

~/.config/systemd/user/security-hookd.socket — systemd holds the socket open at all times. When the first connection arrives, systemd starts the daemon and hands over the file descriptor. Zero cold-start latency from the caller's perspective.

[Unit]
Description=Security Hooks socket

[Socket]
ListenStream=%t/security-hooks/sock
SocketMode=0600
DirectoryMode=0700

[Install]
WantedBy=sockets.target

~/.config/systemd/user/security-hookd.service — the daemon unit. Restart=on-failure handles crashes automatically. No PID files, no health checks, no lock files.

[Unit]
Description=Security Hooks daemon
Requires=security-hookd.socket

[Service]
Type=simple
ExecStart=%h/.local/bin/security-hookd
Restart=on-failure
RestartSec=1
Environment=SECURITY_HOOKS_CONFIG=%h/.config/security-hooks

[Install]
WantedBy=default.target

After install: systemctl --user enable --now security-hookd.socket

macOS: launchd plist

install.sh installs a launchd agent:

~/Library/LaunchAgents/com.security-hooks.daemon.plist — KeepAlive restarts on crash. The Sockets key provides socket activation analogous to systemd.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>Label</key>
    <string>com.security-hooks.daemon</string>
    <key>ProgramArguments</key>
    <array>
        <string>~/.local/bin/security-hookd</string>
    </array>
    <key>Sockets</key>
    <dict>
        <key>Listeners</key>
        <dict>
            <key>SockPathName</key>
            <string>/tmp/security-hooks/sock</string>
            <key>SockPathMode</key>
            <integer>384</integer>  <!-- 0600 -->
        </dict>
    </dict>
    <key>KeepAlive</key>
    <dict>
        <key>SuccessfulExit</key>
        <false/>
    </dict>
    <key>EnvironmentVariables</key>
    <dict>
        <key>SECURITY_HOOKS_CONFIG</key>
        <string>~/.config/security-hooks</string>
    </dict>
</dict>
</plist>

After install: launchctl load ~/Library/LaunchAgents/com.security-hooks.daemon.plist

Fallback: shim-managed daemon

When neither systemd nor launchd is available (rare — containers, minimal VMs), the shim falls back to managing the daemon directly:

• Acquires a lock file ($RUNTIME_DIR/security-hooks/lock) via flock (Linux) or shlock (macOS) before starting
• The daemon writes its PID to $RUNTIME_DIR/security-hooks/pid
• The shim checks socket connectivity; if the PID file is stale, it kills the old process and restarts
• Race condition between concurrent sessions is handled by the lock file

This path is less reliable than the service manager paths and is documented as a fallback only.

Common lifecycle behavior (all platforms)

Idle shutdown: The daemon exits after 30 minutes of inactivity (configurable via daemon.idle_timeout_minutes). The service manager restarts it on the next connection via socket activation. Handles SIGTERM gracefully — flushes pending log writes.

Hot-reload: A FileSystem watcher monitors rules/ and config/ directories. On change, the rule engine reloads rules and config without restarting the daemon. The AST function registry is also refreshed. Users see updated rules on the next tool call.

Socket activation support in the daemon: The daemon checks for an inherited file descriptor (systemd: $LISTEN_FDS, launchd: launch_activate_socket). If present, it uses the inherited socket. Otherwise, it opens its own (fallback path).

Logging: All decisions are written as JSONL:

{"ts":"2026-03-27T14:02:03Z","event":"PreToolUse","tool":"Bash","input":"rm -rf $(echo /)","rule":"destructive-rm","match_type":"ast","decision":"deny","nudge":"Use trash-cli or move to a temp directory"}
{"ts":"2026-03-27T14:02:05Z","event":"PreToolUse","tool":"Bash","input":"mix test","rule":null,"match_type":null,"decision":"allow"}

The match_type field records whether the rule was matched via regex, ast, config_list, or validator — useful for understanding which matching layer caught a command and for tuning rules.

Log rotation and size limits are deferred — users can manage this with external tools (logrotate, etc.) since the log path is well-defined.

Future: streaming connectors for centralized logging (stdout, webhook, syslog).

Installation

./install.sh

The install script:

1. Downloads the Burrito binary for the current platform (macOS aarch64/x86_64, Linux x86_64/aarch64) or builds from source if Elixir is available
2. Installs the binary and shell shim to ~/.local/bin/ (or user-specified location)
3. Copies default rules and config to ~/.config/security-hooks/
4. Creates config.local.toml from a template if it does not exist
5. Auto-detects installed MCP servers from Claude Code config and pre-populates the MCP allowlist in config.local.toml
6. Detects the platform and installs the appropriate service manager integration:
   • Linux/WSL with systemd: installs user units, enables socket activation
   • macOS: installs launchd plist, loads agent
   • Fallback: prints instructions noting the shim will manage the daemon directly
7. Merges hook entries into Claude Code's ~/.claude/settings.json (preserving existing hooks)
8. Prints a summary of what was configured

Existing hooks: Claude Code supports multiple hooks per event. install.sh appends security-hooks entries without removing existing user hooks. Both run on each tool call.

Uninstall: ./install.sh --uninstall removes hook entries from settings and optionally removes the config directory and binary.

Versioning & Updates

Rule files and config carry a version field:

# rules/bash.rules
# version: 1.0.0

# config/config.toml
[meta]
version = "1.0.0"

install.sh --update compares installed version against the repo version, merges new default rules (preserving config.local.toml overrides and disabled entries), and logs what changed.

Target Platforms

• macOS (aarch64, x86_64)
• Linux (x86_64, aarch64)
• WSL (x86_64) — uses Linux binary with Linux-style paths

Supported Language Ecosystems

The default allowed executables and dependency mutation validators cover:

• Rust (cargo)
• Python (pip, uv, poetry)
• TypeScript/JavaScript (npm, pnpm, yarn)
• Go (go)
• Java (maven, gradle)
• Ruby (gem, bundler)
• PHP (composer)
• C/C++ (gcc, clang, make, cmake)
• Elixir (mix)

Validator Module Interface

Validator modules implement the SecurityHooks.Validator behaviour:

@callback validate(payload :: map(), config :: map()) ::
  :allow | {:deny, reason :: String.t()} | {:ask, reason :: String.t()}

• payload — the full hook payload (tool_name, tool_input, session_id, cwd)
• config — the merged config (defaults + local overrides)
• Returns :allow to pass, {:deny, reason} to block, or {:ask, reason} for the suspicious tier

Example:

defmodule SecurityHooks.Validators.DependencyMutation do
  @behaviour SecurityHooks.Validator

  @manifest_files ~w(package.json Cargo.toml mix.exs go.mod pyproject.toml Gemfile composer.json)

  @impl true
  def validate(%{"tool_input" => %{"file_path" => path}} = _payload, _config) do
    basename = Path.basename(path)
    if basename in @manifest_files do
      {:deny, "Editing #{basename} directly — use the package manager CLI"}
    else
      :allow
    end
  end
end

Bash Parser Strategy

The AST matching layer requires a bash parser that produces a traversable tree of command nodes. This is the highest-risk technical dependency in the system.

Requirements

The parser must handle:

• Simple commands: rm -rf /foo
• Pipelines: cat file | curl -X POST
• Logical chains: cmd1 && cmd2 || cmd3
• Command substitution: $(...) and backticks
• Process substitution: <(...) and >(...)
• Subshells: (cmd1; cmd2)
• Redirections: >, >>, <, 2>&1
• Variable assignments: FOO=bar cmd, export FOO=bar
• Quoting: single quotes, double quotes, $"...", $'...'

Parser evaluation (to be validated in implementation spike)

Option A: bash Hex package — if it exposes a stable parse API with the node types above, this is the simplest integration. Risk: the package is primarily an interpreter and its internal AST may not be a stable public API.

Option B: tree-sitter-bash via NIF — the tree-sitter bash grammar is battle-tested and widely used (ShellCheck, GitHub syntax highlighting). A Rust NIF wrapping tree-sitter + tree-sitter-bash would provide a robust, well-documented AST. Higher implementation effort but lower risk for adversarial inputs.

Option C: shlex for tokenization + custom parser — use shlex (Rust NIF) for POSIX-compliant tokenization, then build a lightweight parser on top for the structural features we need (pipelines, redirections, subshells). Middle ground: less effort than tree-sitter, more robust than depending on bash internals.

Decision: Validate Option A first during the implementation spike. If the bash package's parse API is insufficient or unstable, fall back to Option B (tree-sitter-bash). The rule engine's interface to the parser is abstracted behind BashAnalyzer, so the parser can be swapped without affecting rules or the rest of the system.

Security considerations for the parser

A parser mismatch — where the security tool parses a command differently than bash executes it — is an evasion vector. Mitigations:

• The implementation must include a comprehensive test suite of evasion attempts: quoting tricks, Unicode homoglyphs, ANSI escape sequences, null bytes, newlines in arguments, and variable expansion edge cases.
• Tests should validate that the parser's interpretation matches actual bash behavior for each case.
• Regex rules provide a defense-in-depth fallback: even if the AST parser is evaded, regex patterns on the raw string may still catch the attack.

CLI Commands

In addition to the hook entry point, the shim supports diagnostic and testing commands:

• security-hook status — check if the daemon is running, show loaded rule count, config path, socket path, and uptime
• security-hook test "<command>" — dry-run a command against the rules and print the verdict (decision, matching rule, match type) without affecting logs
• security-hook reload — trigger a manual rule/config reload
• security-hook log [--tail N] — print recent log entries

Dependencies

Elixir/Hex packages required by the daemon:

• jason — JSON encoding/decoding
• toml — TOML config parsing
• file_system — cross-platform file watcher for hot-reload
• burrito — compile to single-binary for distribution
• Bash parser — one of: bash (Hex), tree-sitter-bash (via Rust NIF), or shlex + custom parser (see Bash Parser Strategy)