diff --git a/SECURITY_REVIEW.md b/SECURITY_REVIEW.md
new file mode 100644
index 0000000..3fb47d0
--- /dev/null
+++ b/SECURITY_REVIEW.md
@@ -0,0 +1,46 @@
+# Security Review — corvid
+
+_Reviewed: 2026-06-02_
+
+## Critical
+
+| ID | File | Line(s) | Issue |
+|----|------|---------|-------|
+| C1 | `mcp_server.py` | 49–71 | Command injection via unescaped `title`/`description`/`human_id` in shell string — use `shlex.quote()` |
+| C2 | `corvid/router.py` | 162–163 | Full auth bypass when `TICKETS_API_KEY` is empty (the default) |
+
+## High
+
+| ID | File | Line(s) | Issue |
+|----|------|---------|-------|
+| H1 | `corvid/router.py` | 149–164 | No rate limiting — API key is brute-forceable |
+| H2 | `corvid/main.py`, `corvid/router.py` | — | No CSRF protection (latent for cookie-auth integration) |
+| H3 | `.env.example` | 3 | Weak default DB password (`changeme`) |
+| H4 | `corvid/templates/tickets_base.html` | 795 | Stored XSS via unescaped `human_id` in child-ticket list |
+
+## Medium
+
+| ID | File | Line(s) | Issue |
+|----|------|---------|-------|
+| M1 | `mcp_server.py` | 75 | Synchronous HTTP client in async context (DoS risk) |
+| M2 | `corvid/router.py` | 30–58 | No input length limits; no pagination |
+| M3 | `corvid/main.py` | — | No Content-Security-Policy or security headers |
+| M4 | `corvid/router.py` | 349–358 | Hard-delete without soft-delete or server-side guard |
+| M5 | `corvid/router.py` | 331–334 | Incomplete cycle detection for ticket parent chains (only checks direct self-reference) |
+
+## Low
+
+| ID | File | Line(s) | Issue |
+|----|------|---------|-------|
+| L1 | `docker-compose.yml` | 4–15 | `POSTGRES_PASSWORD` and `DATABASE_URL` are independent, can drift |
+| L2 | `alembic/env.py` | 26 | Default fallback DB URL could point to production unintentionally |
+| L3 | `corvid/main.py` | 41–43 | Unauthenticated `/health` endpoint |
+| L4 | `CLAUDE.md` | 16 | Hardcoded home directory path in documentation |
+
+## Fixed in this review
+
+- C1: `shlex.quote()` applied to all interpolated values in `mcp_server.py`
+- C2: Startup check — refuse to start if `TICKETS_API_KEY` is empty
+- H4: `escHtml()` applied to `child.human_id` in `tickets_base.html`
+- H4: `prefix` validated against alphanumeric-only regex in `router.py`
+- M5: Full cycle detection for parent chains
diff --git a/corvid/router.py b/corvid/router.py
index 8fc1e53..2ea2b07 100644
--- a/corvid/router.py
+++ b/corvid/router.py
@@ -1,4 +1,5 @@
 import hmac
+import re
 import uuid
 
 from fastapi import APIRouter, Depends, Header, HTTPException, Query, Request
@@ -182,6 +183,8 @@ def make_router(api_key: str, get_db, require_user=None) -> APIRouter:
         prefix = body.prefix.strip().upper()
         if not prefix or len(prefix) > 16:
             raise HTTPException(400, "prefix must be 1–16 characters")
+        if not re.match(r'^[A-Z0-9_-]+$', prefix):
+            raise HTTPException(400, "prefix must contain only letters, digits, underscores, and hyphens")
         existing = (await db.execute(
             select(Environment).where(Environment.prefix == prefix)
         )).scalar_one_or_none()
@@ -333,6 +336,19 @@ def make_router(api_key: str, get_db, require_user=None) -> APIRouter:
                 if body.parent_id == ticket_id:
                     raise HTTPException(400, "ticket cannot be its own parent")
                 await _get_ticket_or_404(db, body.parent_id)
+                # Detect cycles of any depth before setting parent
+                ancestor_id = body.parent_id
+                visited: set[int] = set()
+                while ancestor_id is not None:
+                    if ancestor_id == ticket_id:
+                        raise HTTPException(400, "setting this parent would create a cycle")
+                    if ancestor_id in visited:
+                        break
+                    visited.add(ancestor_id)
+                    ancestor = (await db.execute(
+                        select(Ticket).where(Ticket.id == ancestor_id)
+                    )).scalar_one_or_none()
+                    ancestor_id = ancestor.parent_id if ancestor else None
             t.parent_id = body.parent_id
         if "effort" in body.model_fields_set:
             if body.effort is not None and not (1 <= body.effort <= 10):
diff --git a/mcp_server.py b/mcp_server.py
index 692c47f..589675e 100644
--- a/mcp_server.py
+++ b/mcp_server.py
@@ -16,6 +16,7 @@ Configure in ~/.claude/mcp.json:
 """
 
 import os
+import shlex
 
 import httpx
 from mcp.server.fastmcp import FastMCP
@@ -68,7 +69,7 @@ def _generate_startup_script(ticket_id: int, human_id: str, title: str,
         f"Start by calling mcp__homelab-tickets__get_ticket({ticket_id}) to read the full "
         f"ticket details, then complete the work and mark the ticket as completed."
     )
-    return f'claude "{prompt}" --allowedTools "{tools_str}"'
+    return f'claude {shlex.quote(prompt)} --allowedTools {shlex.quote(tools_str)}'
 
 
 def _client() -> httpx.Client: