Follow-up to #1207, addressing items from the cargo validator review.
**Scoped to cargo + shared helpers — no behavior change for existing
npm/pypi/nuget/oci/mcpb publishers.** Safe to merge and promote.
### Changes
**SSRF hardening of the two-call README retrieval**
- Pin the step-2 README URL to an allowed host (`static.crates.io` in
prod; the base host under test) *before* fetching, so a metadata
response can't steer the validator at an internal/attacker host.
- `CheckRedirect` policy pins every redirect hop to the same allowlist
(the initial URL being pinned isn't enough if an upstream 3xx is
followed).
- Cap the README body with `io.LimitReader` (5 MiB).
> Not publisher-exploitable today (the URL comes from crates.io), so
this is defense-in-depth — but it makes the "host is pinned" guarantee
hold in code rather than rely on crates.io's behavior.
**Clearer status handling** (previously any non-5xx/non-200 collapsed to
"not found")
- **429** → reported as transient/retryable, at both the metadata and
README steps.
- **403** → disambiguated via the crate-version endpoint:
genuinely-missing stays "not found"; *exists-but-no-README* gets an
actionable "add a README with `mcp-name` and republish" message (mirrors
the NuGet validator). This continues the 5xx-vs-403 diagnostic split
@P4ST4S started in the #1207 review — thanks!
**Shared `containsMCPNameToken` helper**
Boundary-anchored ownership-token match (prevents prefix confusion, e.g.
a README declaring `io.github.acme/widget-pro` satisfying a claim for
`io.github.acme/widget`), used here **by the cargo validator only**.
Adopting it in PyPI/NuGet is a separate follow-up because it's a
behavior change for those already-live registries.
**Tests & docs**
Hermetic positive `ServerNameFormats`; combined fixture gains a
version-existence endpoint + cases for 403-missing vs 403-no-README,
429, and prefix-confusion rejection; new foreign-README-host (SSRF)
test; internal test for the helper. Docs: list `crates.io` in the
registry-requirements list + the `Package` model doc.
### Testing
`go build`, `go vet`, full `./internal/validators/...` suite (incl. live
cargo positive), `make check-schema`, `validate-examples` all pass. No
new `golangci-lint` findings.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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