docs: a security policy — needs private vulnerability reporting enabled before merge

SECURITY.md

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+# Security policy
+
+## Reporting
+
+Please report anything you believe is a security issue through GitHub's private
+vulnerability reporting, from the **Security** tab of this repository
+("Report a vulnerability"). That keeps the report out of the public issue
+tracker until there is something to say about it.
+
+This is a small project without a security team, so please do not expect a
+same-day answer. What you can expect is that a report will be read, that the
+reply will say plainly whether it is in scope, and that if a fix lands the
+report will be credited unless you ask otherwise.
+
+If private reporting is unavailable to you for any reason, open a normal issue
+describing the *shape* of the problem without a working trigger, and say that you
+have more to share.
+
+## What is in scope
+
+`rbgo` aims at MRI-Ruby conformance and is also a Go library (`ruby.Run`) that
+other programs embed. Both of those shape what counts.
+
+In scope:
+
+- **The host process dying, or hanging without bound, because of Ruby input.**
+  A Go `panic` that reaches an embedder, a `fatal error:` that `recover()` cannot
+  contain, an unbounded allocation, or a call that never returns. `Run` returns
+  an `error`; anything that takes the process down instead is a defect, because
+  an embedder has no way to defend against it from the outside.
+- **Failing to raise where MRI raises**, when the difference is what a program
+  relies on to stay safe. A check that reads as "passed" because a limit was hit,
+  an allow-list that permits more than it says, a `rescue` that cannot fire
+  because the exception class is never raised. These fail *open*, which is worse
+  than failing loudly, and they are not visible to the caller.
+- **Reaching further than the documented capability set**, including any path
+  from Ruby to the embedding Go program's own state, or writing outside the
+  `io.Writer` the embedder passed.
+- Secrets or credentials committed to this repository, and anything in the
+  release or CI path that would let someone else's code run with this
+  repository's permissions.
+
+## What is not a vulnerability
+
+**`Run` is not a sandbox, and that is documented, not accidental.** See
+[*`Run` is not a sandbox*](README.md#run-is-not-a-sandbox) in the README, which
+carries the measurements. Ruby source passed to `Run` can spawn `/bin/sh`, read
+and write the filesystem as the host's uid, open sockets, `require` any absolute
+path, and change the host's `ENV` and working directory — exactly as MRI can,
+because conformance is the goal. Do not pass untrusted Ruby to it.
+
+So a report that "`system("...")` executes a command" is working as intended. A
+report that a *documented restriction* does not hold is not.
+
+By the same reasoning, faithfully reproducing an MRI behaviour that is dangerous
+in MRI — `Marshal.load` on attacker-controlled bytes, for instance — is
+conformance rather than a defect here. It is still worth telling us if the
+documentation does not warn about it.
+
+## Already known
+
+Please check the open issues before reporting; several classes are already filed
+with measurements, and a duplicate costs you more than it costs us:
+
+- no call-depth limit, so runaway recursion is a fatal Go stack overflow (#768,
+  and go-ruby-parser/parser#48 for the same thing in the parser)
+- `Net::HTTP` to a server in the program's own `Thread` hangs without bound (#771)
+- `Run` returns `nil` for `exit`/`exit!`/`abort` (#773)
+- `YAML.safe_load` instantiates arbitrary classes (#775)
+- `Regexp.timeout=` is never applied, and a fired limit reads as "no match" (#776)
+- `Array#*`/`String#*` reach `make()` with a rejected size (#777)
+
+## Supported versions
+
+There is no released version yet and no backport branch: fixes land on `main`.
+If you are embedding this, pin a commit and expect to move it.