docs: add security policy

A public repository with no stated disclosure channel routes a finder into
opening a public issue, which discloses a relay flaw to everyone the moment it
is filed. SECURITY.md points at the repository private vulnerability reporting
instead, with public@mixeme.ru as fallback, and states scope so operator-side
configuration (blocked port 25, missing PTR, proxy TLS) does not arrive as a
report.

No response time is promised: a deadline that cannot be honoured by a single
maintainer is worse than none. Silence is explicitly not a request for a
continued embargo.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

CHANGELOG.md

@@ -5,6 +5,14 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); version
 
 ## [Unreleased]
 
+### Added
+
+- `SECURITY.md` — how to report a vulnerability privately (GitHub private
+  vulnerability reporting, `public@mixeme.ru` as fallback), which releases get
+  fixes, and what is in and out of scope for a relay. No response time is
+  promised. Without it a finder's default move is a public issue, which
+  discloses a relay flaw to everyone the moment it is filed.
+
 ### Changed
 
 - The panel's **Account** entry is now called **Settings** — nav link, page

SECURITY.md

@@ -0,0 +1,81 @@
+# Security policy
+
+## Supported versions
+
+SelfPost follows SemVer. Fixes are issued for the **latest minor release of the
+1.x line** only; there is no backporting to earlier minors. Upgrade before
+reporting if you are behind — the image tag is `ghcr.io/mixeme/selfpost:X.Y.Z`.
+
+| Version | Supported |
+|---|---|
+| latest 1.x | yes |
+| earlier 1.x | no — upgrade first |
+| 0.x | no (pre-release) |
+
+## Reporting a vulnerability
+
+**Do not open a public issue.** Use GitHub's private vulnerability reporting:
+the *Report a vulnerability* button under the repository's
+[Security tab](https://github.com/mixeme/selfpost/security). If you cannot use
+it, mail `public@mixeme.ru` instead.
+
+Useful in a report: the image tag, the reverse proxy in front of the panel, the
+steps to reproduce, and what an attacker gains. A relevant excerpt of
+`mail.log` or the panel's system log helps; strip recipient addresses first.
+
+**No response time is promised.** SelfPost is maintained by one person, and a
+deadline that cannot be honoured is worse than none. Reports are read and
+answered as soon as the maintainer is able; a fix ships in a patch release,
+with the timeline agreed in the thread.
+
+Disclosure is coordinated by request, not by demand: please hold public details
+until a patch is out. If you get no reply, that is not a request for a
+continued embargo — disclose at your own discretion. Reporters are credited in
+the CHANGELOG unless they ask not to be.
+
+## In scope
+
+The relay's job is to accept authenticated mail from an application and hand it
+to the internet as the operator's domain, and nothing else. Breaking that is in
+scope:
+
+- **Open relay** — mail accepted from an unauthenticated sender, or relayed for
+  a domain the sending application is not bound to
+- **SASL bypass** — sending without valid credentials, or credential recovery
+  from anything the container exposes
+- **Cross-domain access** — an application or a panel session reaching a domain
+  it was not granted
+- **Secret disclosure** — DKIM private keys, the admin password hash, session
+  tokens, or backup encryption material leaking to an unauthorised party
+- **Panel authentication and session flaws** — login bypass, session fixation,
+  CSRF on state-changing routes, privilege escalation
+- **Rate-limit bypass** — evading either the Postfix-level backstop or the
+  per-domain and per-application limits
+- **Container escape** or privilege escalation from the panel's unprivileged
+  user to root
+
+## Out of scope
+
+These are the operator's responsibility or accepted trade-offs, documented in
+[docs/security.md](docs/security.md) and the
+[operator guide](docs/guide.md):
+
+- Host configuration the operator controls: a blocked port 25, a missing or
+  wrong PTR record, DNS records not published, a self-signed or expired
+  certificate on the reverse proxy
+- Anything requiring the attacker to already have root on the host or write
+  access to the `./data` bind mount
+- Missing hardening headers or TLS options on the reverse proxy — SelfPost
+  never terminates HTTPS itself
+- Deliverability outcomes: mail rejected or filtered by a receiving provider is
+  a policy decision of that provider, not a defect
+- Denial of service through sheer volume against a single-tenant relay
+- Vulnerabilities in upstream Postfix, OpenDKIM, or the base image — report
+  those upstream; if SelfPost's configuration makes an upstream issue
+  exploitable when it otherwise would not be, that *is* in scope
+
+## Reports we cannot act on
+
+Automated scanner output with no demonstrated impact, and reports whose only
+content is a version number compared against a CVE list, are closed without
+investigation.