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/nuget.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"net/url"
 	"strings"
 	"sync"
 	"time"
@@ -209,9 +210,11 @@ func validateReadme(ctx context.Context, serverName, lowerID, lowerVersion strin
 		return NoReadme, fmt.Errorf("failed to get README URL template: %w", err)
 	}
 
-	// Replace placeholders in the template
-	readmeURL := strings.ReplaceAll(readmeURLTemplate, "{lower_id}", lowerID)
-	readmeURL = strings.ReplaceAll(readmeURL, "{lower_version}", lowerVersion)
+	// Replace placeholders in the template. PathEscape both the id and version
+	// so a publisher cannot smuggle "/" / ".." through the template into a
+	// fetch against an unrelated package's README.
+	readmeURL := strings.ReplaceAll(readmeURLTemplate, "{lower_id}", url.PathEscape(lowerID))
+	readmeURL = strings.ReplaceAll(readmeURL, "{lower_version}", url.PathEscape(lowerVersion))
 	req, err := http.NewRequestWithContext(ctx, http.MethodGet, readmeURL, nil)
 	if err != nil {
 		return NoReadme, fmt.Errorf("failed to create NuGet README request: %w", err)
@@ -266,8 +269,10 @@ func validatePackageExists(ctx context.Context, lowerID, lowerVersion string, cl
 		return PackageIDNotFound, fmt.Errorf("failed to get Package Base URL: %w", err)
 	}
 
-	// Fetch the package content index to check if package ID and version exist
-	indexURL := fmt.Sprintf("%s/%s/index.json", strings.TrimRight(packageBaseURL, "/"), lowerID)
+	// Fetch the package content index to check if package ID and version exist.
+	// PathEscape so an identifier that smuggles "/" or ".." cannot redirect
+	// the metadata fetch to a different package than the one being claimed.
+	indexURL := fmt.Sprintf("%s/%s/index.json", strings.TrimRight(packageBaseURL, "/"), url.PathEscape(lowerID))
 
 	req, err := http.NewRequestWithContext(ctx, http.MethodGet, indexURL, nil)
 	if err != nil {