mathiasandClaude Opus 5.5
b0d58d5adf
PreToolUse hook on mcp__.*: asks for repo_delete, file_delete, branch_delete, pr_merge, tbd_ship, repo_update, repo_mirror_push, workflow_run_trigger, tag_create, release_create on any server name, and for file_write_branch unless it targets a plain non-main/master branch with non-empty content under exactly one exact-case branch/content key. Fails closed; never emits allow. Refs mathias/infra#508. Reviewed in three rounds by pr-gatekeeper with execution against gitea-mcp and Gitea 1.27.1. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Co-authored-by: Mathias <mthbqv@gmail.com>
87 lines
4.3 KiB
Bash
Executable File
87 lines
4.3 KiB
Bash
Executable File
#!/usr/bin/env bash
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# PreToolUse hook (mcp__.* matcher) — makes a human approve irreversible MCP
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# verbs instead of letting the model run them unattended.
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#
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# Why (infra#508, infra ADR-0027): gitea-mcp authenticates by PAT pass-through,
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# so an agent holds the full scope of the caller's PAT. repo_delete's own guard
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# (confirm == repo) is filled in by the model on both sides: it stops typos,
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# not an injected or mistaken agent. The infra-mcp README claimed client-side
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# deny lists covered this; on 2026-10-01 none existed in any settings file.
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#
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# Policy: match gitea-mcp's verb spellings on ANY server name. The same tool is
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# reachable as mcp__plugin_skills_gitea__*, mcp__claude_ai_gitea__* or under
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# whatever name a host registers it with, so keying on the server name would
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# let a rename bypass the gate. It does NOT recognise other servers' spellings
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# of the same operation (e.g. a third-party `delete_repo`).
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#
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# file_write_branch is gated when it would land on main/master (on infra that
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# is a Flux deploy within ~60s — the same effect as pr_merge) or empty a file
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# (the same effect as file_delete). Writes to other branches pass.
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#
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# Emits "ask" or nothing; never "allow" (a hook allow would override the
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# user's own deny/ask rules). Always exits 0: Claude Code ignores a hook's
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# JSON on a non-zero exit. Anything it cannot parse — including a host
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# without jq — fails closed to "ask". The ask JSON is a literal, so the
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# fail-closed path needs no tool that might be missing.
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#
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# Scope limit, recorded in infra ADR-0027: this gates MCP calls only. A
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# session with the PAT in its environment can still reach the Gitea REST API
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# through a shell.
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set -u
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DESTRUCTIVE='repo_delete|file_delete|branch_delete|pr_merge|tbd_ship|repo_update|repo_mirror_push|workflow_run_trigger|tag_create|release_create'
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ask() {
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# $1 must contain only [A-Za-z0-9 _.:()'/-] — it is spliced into JSON verbatim.
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printf '{"hookSpecificOutput":{"hookEventName":"PreToolUse","permissionDecision":"ask","permissionDecisionReason":"%s"}}\n' "$1"
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exit 0
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}
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INPUT="$(cat)"
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command -v jq >/dev/null 2>&1 \
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|| ask "mcp-destructive-guard: jq is not installed, so this MCP call cannot be checked. Approve only if you expected it."
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if ! TOOL_NAME="$(printf '%s' "$INPUT" | jq -er '.tool_name | strings | select(length > 0)' 2>/dev/null)"; then
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ask "mcp-destructive-guard: could not read a tool name from this call, so it cannot be shown safe. Approve only if you expected it."
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fi
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case "$TOOL_NAME" in
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mcp__*__*) ;;
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*) exit 0 ;;
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esac
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verb="${TOOL_NAME##*__}"
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safe_verb="$(printf '%s' "$verb" | tr -cd 'A-Za-z0-9_-')"
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if printf '%s' "$verb" | grep -Eqix "($DESTRUCTIVE)"; then
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ask "Irreversible MCP verb '$safe_verb' (infra ADR-0027): needs your approval. The model filled in every argument, including any confirm field."
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fi
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if printf '%s' "$verb" | grep -Eqix 'file_write_branch'; then
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# Cleared only when there is exactly ONE key that case-folds to "branch" and
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# one to "content": gitea-mcp decodes with Go encoding/json, which matches
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# keys case-insensitively and lets the last one win, so {"branch":"feat/x",
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# "Branch":"main"} landed on main while a lowercase-only read saw feat/x.
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# The branch is cleared by SHAPE (allowlist), not by a blocklist of names:
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# "refs/heads/main" created a second ref that wedged API writes to main.
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verdict="$(printf '%s' "$INPUT" | jq -r '
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.tool_input as $i
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| if ($i | type) != "object" then "ask" else
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([$i | to_entries[] | select(.key | ascii_downcase == "branch")]) as $b
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| ([$i | to_entries[] | select(.key | ascii_downcase == "content")]) as $c
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| if ($b | length) != 1 or ($c | length) != 1 then "ask"
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elif ($b[0].key != "branch") or ($c[0].key != "content") then "ask"
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elif ($b[0].value | type) != "string" or ($c[0].value | type) != "string" then "ask"
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elif ($b[0].value | test("^[A-Za-z0-9][A-Za-z0-9._/-]*$") | not) then "ask"
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elif ($b[0].value | test("\\.\\.|^refs/|/$")) then "ask"
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elif ($b[0].value | ascii_downcase | . == "main" or . == "master") then "ask"
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elif ($c[0].value | test("^\\s*$")) then "ask"
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else "pass" end
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end' 2>/dev/null || echo ask)"
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[ "$verdict" = "pass" ] \
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|| ask "file_write_branch to main/master, an unusual branch name, or emptying a file (infra ADR-0027): needs your approval."
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fi
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exit 0
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