docs: add security assessment to SECURITY.md (#70)

## Summary

Adds a Security Assessment section to SECURITY.md documenting the six
most likely and impactful threats for this project, along with existing
mitigations for each:

1. API credential exposure (HIGH)
2. Unintended write operations (HIGH)
3. Prompt injection via BookStack content (MEDIUM)
4. Insecure transport (MEDIUM)
5. Supply chain compromise (MEDIUM)
6. SSRF via BOOKSTACK_BASE_URL (LOW)

Satisfies OSPS-SA-03.01 — the project must perform a security assessment
to understand the most likely and impactful potential security problems.

## Test plan

- [ ] SECURITY.md renders correctly on GitHub

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Signed-off-by: Paradoxbound <paradoxbound@users.noreply.github.com>
Co-authored-by: Paradoxbound <paradoxbound@users.noreply.github.com>

Author: paradoxbound

SECURITY.md

@@ -9,6 +9,65 @@ Only the latest release is actively maintained with security updates.
 | 2.5.x   | yes       |
 | < 2.5   | no        |
 
+## Security Assessment
+
+This section documents the most likely and impactful security risks for this project.
+
+### 1. API credential exposure (HIGH)
+
+**Risk:** `BOOKSTACK_TOKEN_ID` and `BOOKSTACK_TOKEN_SECRET` grant full API access to the BookStack instance. If leaked — via environment variable logging, process listings, config files committed to git, or container inspection — an attacker gains complete read/write access to BookStack.
+
+**Mitigations:**
+- Tokens are read from environment variables only; never hardcoded or logged
+- `.env` is listed in `.gitignore`
+- CI/CD uses no manually stored secrets (`GITHUB_TOKEN` is auto-provisioned per run)
+- README advises HTTPS, dedicated users with limited permissions, and regular token rotation
+
+### 2. Unintended write operations (HIGH)
+
+**Risk:** Write tools (create/update/delete for books, pages, chapters, etc.) could be invoked by a malicious prompt, a confused AI model, or misconfiguration, causing data loss or corruption in BookStack.
+
+**Mitigations:**
+- Write tools are not registered unless `BOOKSTACK_ENABLE_WRITE=true` is explicitly set
+- Write operations are disabled by default; the server exposes only read tools in the default configuration
+
+### 3. Prompt injection via BookStack content (MEDIUM)
+
+**Risk:** BookStack pages returned to the MCP client could contain adversarial instructions designed to manipulate the AI model into taking unintended actions (e.g. exfiltrating content, invoking write tools).
+
+**Mitigations:**
+- The MCP server is a passive relay; it does not interpret or act on page content itself
+- Write operations require explicit opt-in (`BOOKSTACK_ENABLE_WRITE=true`), limiting the blast radius
+- Operators should use a BookStack API user scoped to the minimum required permissions
+
+### 4. Insecure transport (MEDIUM)
+
+**Risk:** If BookStack is accessed over plain HTTP, API tokens and content are transmitted in cleartext and vulnerable to interception.
+
+**Mitigations:**
+- README advises using HTTPS for all production BookStack instances
+- No enforcement at the server level; operators are responsible for BookStack TLS configuration
+
+### 5. Supply chain compromise (MEDIUM)
+
+**Risk:** A compromised npm dependency or GitHub Action could inject malicious code into the build or runtime.
+
+**Mitigations:**
+- All GitHub Actions are pinned to exact commit SHAs
+- `npm audit` runs on every CI build and fails on high/critical vulnerabilities
+- OSV Scanner and Trivy scan dependencies and the Docker image on every build
+- Dependabot opens weekly PRs for outdated packages and the Docker base image
+- SLSA Level 2 provenance attestation on every released Docker image
+- `npm ci` used in all builds to enforce the lock file
+
+### 6. Server-Side Request Forgery via BOOKSTACK_BASE_URL (LOW)
+
+**Risk:** If an operator sets `BOOKSTACK_BASE_URL` to an internal network address, the server could be used to probe or interact with internal services.
+
+**Mitigations:**
+- `BOOKSTACK_BASE_URL` is operator-supplied at deployment time; it is not accepted from end users or MCP clients
+- Operators are responsible for ensuring the URL points to a legitimate BookStack instance
+
 ## Reporting a Vulnerability
 
 Please do **not** open a public GitHub issue for security vulnerabilities.