Newsletter Security Review

Newsletter Service Security Review

This service handles member PII (recipient emails and names in transit) and exposes deliberately unauthenticated endpoints: a leak is a privacy incident, and parts of the surface are reachable by anyone on the internet. Those facts set the stakes for every security judgment here.

Methodology

Run a focused, diff-aware review, not a whole-repo audit:

1. Only new risk. Assess what this PR introduces or weakens. Do not relitigate pre-existing issues the diff does not touch (at most a nit).
2. Assume hostile input, report only what is real. Flag only high-confidence, concretely exploitable findings: if you cannot trace a path from an attacker-controlled input to a sensitive sink, it is not critical/high.
3. Three passes.
   • Context: discover, from the code and the repo docs at review time, the guards this service relies on around the diff (format checks, validators, body caps, parameterized queries, signature verification). Never assume a guard exists; find it.
   • Comparative: does the change deviate from the guard patterns the surrounding code establishes?
   • Assessment: trace each input to its sink and confirm a guard sits on the path the data actually takes, not three functions away.
4. Confidence-gate every finding (1-10, report only >= 7). A few real findings beat a speculative list.
5. Evidence, not vibes. Each finding names the file and function, what the attacker controls, the boundary crossed, the concrete impact, and the fix.

Durable threat anchors

These are the kinds of boundaries that make a diff security-relevant in this service. They describe the service's shape, not its current line-level guards; verify the concrete mechanism in the code each time.

• The unauthenticated surface. Two routes are reachable by anyone with no session: the open-tracking pixel (guarded by an opaque recipient hash) and one-click unsubscribe (guarded by an HMAC-signed token, and it writes a project-scoped opt-out). Weakening either token check, returning detail to an anonymous caller, broadening what an anonymous write can do, or adding any new unauthenticated route is the top of the severity scale.
• Authentication. JWT verification in the middleware, its toggle, and the deliberate decision not to forward the bearer to downstream NATS calls. Watch for weakened validation (audience, expiry, algorithms), library errors echoed to unauthenticated callers, and any path that starts forwarding or logging a bearer.
• Authorization lives at the gateway. The service runs no access checks of its own; the chart's Heimdall RuleSet is the entire authorization model. A handler or route change must keep the route shape the RuleSet gates on; a chart change to the rules is an authorization change outright.
• Recipient PII. Raw emails move through the service in process and over NATS during dispatch. Open tracking persists only an opaque recipient hash, but the unsubscribe table persists the lowercased email (project-scoped) — so a durable email store already exists, and its exposure, how it is matched, and the path that reaches it are in scope. Any new path that logs, returns, or stores an email or name beyond that, or weakens a hash, is a leak. The newsletter HTML body travels to real inboxes: if a PR changes how it is rendered or what gets interpolated into it, injection into the email becomes the question.
• SQL and schema. Queries go through a parameterized builder; any hand-built SQL string carrying input is injection. The schema's CHECK constraints and uniqueness rules are security guards as well as data rules; a migration that drops one deserves the same scrutiny as code.
• Input bounds. The service bounds request bodies, list sizes, field lengths, and fan-out. A new input path that bypasses the established decode/validation helpers, or a loosened cap with no reason, is a finding.
• Secrets and config. Configuration is read in one documented place; secrets (DB credentials) must never appear in logs, responses, or plaintext chart values. Chart changes that weaken the network policy or expose a route that should sit behind the gateway count here too.

What not to flag

Signal discipline keeps the reviewer trusted. Do not raise:

• Denial of service, resource exhaustion, or "add rate limiting" on their own. (Unbounded unauthenticated writes are flagged as data integrity, not load.)
• Mere lack of hardening or defense-in-depth with no concrete vulnerability.
• Outdated third-party dependencies (managed separately); a new dependency's risk belongs to the architecture lens.
• Theoretical race or timing issues with no practical exploit.
• Test-only files, Markdown, and docs.
• Log spoofing, regex-DoS, and missing audit logs.
• SSRF that only controls a path; it counts when the attacker controls host or protocol.

Precedents: UUIDs are unguessable and need no validation (an authorization finding rests on a missing check, not on guessing an id); environment variables and config are trusted inputs; logging URLs and non-PII is fine.

Reporting

For each finding give the file and function, what the attacker controls, the boundary crossed, the concrete impact on this service, and the fix. If the diff does not touch an anchor above, do not invent a finding for it.