security: fix open redirect and add small hardening (#1227)

## What this does

Fixes an open redirect plus a handful of small, mostly-invisible
hardening changes around it.

## What's in it

- **The fix**: trailing-slash redirects no longer let `//evil.com/`
redirect off-host.
- **SSRF block** on the DNS/HTTP namespace verification — refuses to
dial loopback, RFC1918, link-local, multicast, CGNAT, or
IP-literal/single-label \"domains\".
- **Slow-client guard**: `ReadTimeout` and `IdleTimeout` on the HTTP
server (no `WriteTimeout` — would risk cutting off legitimate
multi-package publishes).
- **Defence-in-depth headers** on the UI (`/`) and 404 responses.
- **5xx responses** stop leaking raw DB error text to clients (logged
server-side instead).
- **Search filter** stops treating `%` and `_` as wildcards.
- **Validators**: PyPI/NuGet identifiers URL-escaped before fetching;
MCPB no longer follows redirects.
- **Misc**: `Version` field bounded to 255 chars; logs quote user input;
`BlockedNamespaces` denylist now catches subdomain claimants.

Each change is its own commit so they can be reviewed (or reverted)
independently.

## Test plan

- [ ] CI green
- [ ] Synthetic publish in staging for each package type (npm, pypi,
nuget, oci, mcpb) — confirms validator changes don't regress
- [ ] Quick browser check of `/` after deploy to confirm UI still loads
with the new CSP
- [ ] `curl https://<host>//evil.com/` — confirm the open redirect is
closed

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

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/validators/registries/mcpb.go

@@ -54,8 +54,17 @@ func ValidateMCPB(ctx context.Context, pkg model.Package, _ string) error {
 		return fmt.Errorf("MCPB package URL must contain 'mcp': %s", pkg.Identifier)
 	}
 
-	// Verify the file exists and is publicly accessible
-	client := &http.Client{Timeout: 10 * time.Second}
+	// Verify the file exists and is publicly accessible. Refuse to follow
+	// redirects: the URL allowlist (github.com / gitlab.com) only constrains
+	// the FIRST hop, and a 30x bouncing through CDN/release-asset hosts
+	// could otherwise be steered toward attacker infrastructure or
+	// internal-only endpoints reached via DNS quirks.
+	client := &http.Client{
+		Timeout: 10 * time.Second,
+		CheckRedirect: func(_ *http.Request, _ []*http.Request) error {
+			return http.ErrUseLastResponse
+		},
+	}
 	req, err := http.NewRequestWithContext(ctx, http.MethodHead, pkg.Identifier, nil)
 	if err != nil {
 		return fmt.Errorf("failed to create request: %w", err)
@@ -69,7 +78,15 @@ func ValidateMCPB(ctx context.Context, pkg model.Package, _ string) error {
 	}
 	defer resp.Body.Close()
 
-	if resp.StatusCode != http.StatusOK {
+	// GitHub serves release assets via a 302 to a signed S3 URL; a HEAD that
+	// returns 200 directly OR a 302 with a Location header is acceptable.
+	// Any other 3xx without a usable Location is treated as inaccessible.
+	switch {
+	case resp.StatusCode == http.StatusOK:
+		// fine
+	case resp.StatusCode >= 300 && resp.StatusCode < 400 && resp.Header.Get("Location") != "":
+		// fine — first-hop allowlist + we don't actually need the body
+	default:
 		return fmt.Errorf("MCPB package '%s' is not publicly accessible (status: %d)", pkg.Identifier, resp.StatusCode)
 	}