fix(security): reject non-registry specs before package-source fetch

The scanner's published-source fetch passed the server's configured
package spec to `pip download` / `uv pip download` UNVALIDATED. For a
local-path, `file:`, URL, or VCS (`git+…`, ssh) spec, those commands
still invoke the package's setup.py / PEP 517 build backend to resolve
metadata — EVEN with `--only-binary=:all:` — executing untrusted code
from the server config on the meant-to-be-static scan path.

Add validatePackageSpec(ecosystem, spec): require a bare PEP 503 /
npm registry name (optionally version-pinned); reject paths, `file:`,
URLs, and VCS/ssh specs. resolveFromPackageFetch now gates both the
npm and python fetch paths through it, falling back to
tool_definitions_only (no fetch, no execution) on rejection. The
already-safe registry-name path is unchanged.

TDD: TestValidatePackageSpec (registry pass / non-registry reject) and
TestResolveFromPackageFetch_RejectsNonRegistrySpec (setup.py execution
marker stays absent for a local-path/git+ spec).

Related MCP-2442

Author: Dumbris

docs/features/security-scanner-plugins.md

@@ -259,6 +259,7 @@ Package-runner servers (`npx`, `uvx`) are the **primary** quarantine/scan target
 
 **The source is fetched but never executed.** A scanner must not run the untrusted code it is scanning. The fetch only ever downloads and unpacks archives:
 
+- **Registry name required.** Only a server whose configured spec is a **bare registry name** (PEP 503 for PyPI, npm package name — optionally version-pinned) is fetched. Local-path, `file:`, URL, and VCS (`git+…`, `[email protected]:…`) specs are **rejected** and fall back to tool definitions only. This is mandatory: for a non-registry spec, `pip download` / `uv pip download` still invokes the package's `setup.py` / PEP 517 build backend to resolve metadata **even with `--only-binary=:all:`** — executing untrusted code on the (static) scan path. `--only-binary=:all:` alone protects only bare registry names.
 - **npm (`npx`)** — `npm pack <pkg>@<version> --ignore-scripts` downloads the published tarball without running any lifecycle scripts (`install`/`postinstall`), then it is extracted (`source_method=npm_pack`).
 - **PyPI (`uvx`)** — `uv pip download <pkg>==<version> --no-deps --only-binary=:all:` (falling back to `pip download`) fetches **only a prebuilt wheel**, which is unpacked without building or running `setup.py` (`source_method=pip_download`). `--only-binary=:all:` is mandatory: downloading an sdist would invoke the package's PEP 517 build backend (`setup.py egg_info`) to resolve metadata, executing the untrusted code. A package that ships **no wheel** therefore fails the fetch and falls back to tool definitions only — sdists are never built or extracted.
 

internal/security/scanner/package_fetch.go

@@ -11,6 +11,7 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"regexp"
 	"strings"
 
 	"go.uber.org/zap"
@@ -104,6 +105,62 @@ func parsePackageSpec(spec string) (name, version string) {
 	return spec, ""
 }
 
+var (
+	// npmNameRe matches a bare npm registry package name: an optional
+	// "@scope/" prefix followed by the package name. It deliberately excludes
+	// any character ('/', ':', etc. beyond the single scope separator) that
+	// would appear in a path, URL, or VCS spec.
+	npmNameRe = regexp.MustCompile(`^(@[a-z0-9][a-z0-9._-]*/)?[a-z0-9][a-z0-9._-]*$`)
+	// pep503NameRe matches a PEP 503 / PEP 508 distribution name (case
+	// insensitive). No path/URL/VCS characters are permitted.
+	pep503NameRe = regexp.MustCompile(`^[A-Za-z0-9]([A-Za-z0-9._-]*[A-Za-z0-9])?$`)
+)
+
+// validatePackageSpec rejects any package spec that is not a bare registry name
+// (optionally version-pinned). This is a SECURITY gate, not a convenience check:
+// for a local-path, file:, URL, or VCS (git+/hg+/ssh) spec, `pip download` /
+// `uv pip download` STILL invoke the package's setup.py / PEP 517 build backend
+// to resolve metadata — EVEN with --only-binary=:all: — which executes untrusted
+// code from the server config on the (meant-to-be-static) scan path (MCP-2442).
+// `npm pack` of a non-registry spec is similarly unsafe. Only a bare PEP 503
+// (python) / npm registry name is guaranteed to download-without-build. On
+// rejection the caller falls back to tool_definitions_only (no fetch, no exec).
+func validatePackageSpec(ecosystem, spec string) error {
+	if spec == "" {
+		return fmt.Errorf("empty package spec")
+	}
+	// Disqualify URLs (any scheme), file:, VCS (git+/hg+/...), ssh, drive
+	// letters, and Windows paths up front. None of ':' or '\' ever appear in a
+	// bare registry name or a PEP 508 version pin, so their mere presence means
+	// the spec is not a registry name.
+	if strings.ContainsAny(spec, ":\\") {
+		return fmt.Errorf("package spec %q is not a bare registry name (contains path/URL/VCS markers)", spec)
+	}
+	// Disqualify filesystem paths (absolute, relative, home-relative) and any
+	// path traversal. A scoped npm name legitimately starts with '@', which is
+	// not matched here.
+	if strings.HasPrefix(spec, ".") || strings.HasPrefix(spec, "/") || strings.HasPrefix(spec, "~") {
+		return fmt.Errorf("package spec %q looks like a filesystem path", spec)
+	}
+	if strings.Contains(spec, "..") {
+		return fmt.Errorf("package spec %q contains a path traversal", spec)
+	}
+	name, _ := parsePackageSpec(spec)
+	switch ecosystem {
+	case "npm":
+		if !npmNameRe.MatchString(name) {
+			return fmt.Errorf("%q is not a valid npm package name", name)
+		}
+	case "python":
+		if !pep503NameRe.MatchString(name) {
+			return fmt.Errorf("%q is not a valid PEP 503 package name", name)
+		}
+	default:
+		return fmt.Errorf("unknown ecosystem %q", ecosystem)
+	}
+	return nil
+}
+
 // firstPackageArg returns the package spec from a runner command's args. It
 // honors `--from <pkg>` (uvx) and otherwise returns the first non-flag arg.
 func firstPackageArg(args []string) string {
@@ -392,8 +449,14 @@ func (r *SourceResolver) resolveFromPackageFetch(ctx context.Context, info Serve
 
 	switch cmdBase {
 	case "npx", "bunx", "pnpm":
+		if err := validatePackageSpec("npm", spec); err != nil {
+			return nil, fmt.Errorf("refusing to fetch untrusted spec for %s: %w", info.Name, err)
+		}
 		return r.fetchNpmPackage(ctx, info, spec)
 	case "uvx", "pipx":
+		if err := validatePackageSpec("python", spec); err != nil {
+			return nil, fmt.Errorf("refusing to fetch untrusted spec for %s: %w", info.Name, err)
+		}
 		return r.fetchPythonPackage(ctx, info, spec)
 	}
 	return nil, fmt.Errorf("package fetch unsupported for command %q", cmdBase)

internal/security/scanner/package_fetch_test.go

@@ -5,10 +5,13 @@ import (
 	"archive/zip"
 	"bytes"
 	"compress/gzip"
+	"context"
 	"os"
 	"path/filepath"
 	"strings"
 	"testing"
+
+	"go.uber.org/zap"
 )
 
 func TestParsePackageSpec(t *testing.T) {
@@ -465,3 +468,94 @@ func TestFindDownloadedArchive(t *testing.T) {
 		t.Errorf("wheel should be preferred: got (%q,%v)", got2, kind2)
 	}
 }
+
+// MCP-2442: a `pip download` / `uv pip download` of a NON-REGISTRY spec
+// (local path, git+, URL, file:, VCS, ssh) STILL executes the package's
+// setup.py / PEP 517 build backend even with --only-binary=:all: — arbitrary
+// code execution from untrusted server config on the (static) scan path. The
+// fetch must validate the spec is a bare PEP 503 / npm registry name and REFUSE
+// anything else, falling back to tool-definitions-only.
+func TestValidatePackageSpec(t *testing.T) {
+	cases := []struct {
+		ecosystem string
+		spec      string
+		wantOK    bool
+	}{
+		// Registry names (allowed) — must not regress.
+		{"python", "flights", true},
+		{"python", "flights==0.2.4", true},
+		{"python", "google-flights-mcp-server==0.2.4", true},
+		{"python", "Flask", true},
+		{"python", "some.dotted.name", true},
+		{"npm", "google-flights-mcp-server", true},
+		{"npm", "google-flights-mcp-server@0.2.4", true},
+		{"npm", "@modelcontextprotocol/server-everything", true},
+		{"npm", "@modelcontextprotocol/server-everything@1.0.0", true},
+		// Non-registry specs (must be REJECTED → fall back to tool-defs).
+		{"python", "./local-pkg", false},
+		{"python", "../evil", false},
+		{"python", "/abs/path/pkg", false},
+		{"python", "~/pkg", false},
+		{"python", "file:./pkg", false},
+		{"python", "git+https://github.com/user/repo.git", false},
+		{"python", "git+ssh://[email protected]/user/repo.git", false},
+		{"python", "https://example.com/pkg.tar.gz", false},
+		{"python", "[email protected]:user/repo.git", false},
+		{"python", "hg+https://example.com/repo", false},
+		{"python", `C:\Users\evil\pkg`, false},
+		{"python", "", false},
+		{"npm", "./local-pkg", false},
+		{"npm", "/abs/path", false},
+		{"npm", "git+https://github.com/user/repo.git", false},
+		{"npm", "file:../evil", false},
+		{"npm", "https://example.com/pkg.tgz", false},
+		{"npm", "", false},
+	}
+	for _, c := range cases {
+		err := validatePackageSpec(c.ecosystem, c.spec)
+		if c.wantOK && err != nil {
+			t.Errorf("validatePackageSpec(%q, %q) = %v, want OK", c.ecosystem, c.spec, err)
+		}
+		if !c.wantOK && err == nil {
+			t.Errorf("validatePackageSpec(%q, %q) = nil, want rejection", c.ecosystem, c.spec)
+		}
+	}
+}
+
+// TestResolveFromPackageFetch_RejectsNonRegistrySpec proves the resolver refuses
+// a non-registry uvx spec BEFORE invoking any download command — so the untrusted
+// package's setup.py is never executed. We point the spec at a local directory
+// containing a setup.py that drops an execution marker; the resolver must return
+// an error (caller falls back to tool_definitions_only) and the marker must be
+// absent.
+func TestResolveFromPackageFetch_RejectsNonRegistrySpec(t *testing.T) {
+	pkgDir := t.TempDir()
+	marker := filepath.Join(t.TempDir(), "EXECUTED.marker")
+	setupPy := "import pathlib\npathlib.Path(" + pyQuote(marker) + ").write_text('pwned')\n"
+	if err := os.WriteFile(filepath.Join(pkgDir, "setup.py"), []byte(setupPy), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	r := NewSourceResolver(zap.NewNop())
+	cases := []ServerInfo{
+		{Name: "evil-path", Protocol: "stdio", Command: "uvx", Args: []string{"--from", pkgDir, "evil"}},
+		{Name: "evil-git", Protocol: "stdio", Command: "uvx", Args: []string{"--from", "git+https://example.com/repo.git", "evil"}},
+		{Name: "evil-npm-path", Protocol: "stdio", Command: "npx", Args: []string{"-y", "./local-evil"}},
+	}
+	for _, info := range cases {
+		res, err := r.resolveFromPackageFetch(context.Background(), info)
+		if err == nil {
+			if res != nil && res.Cleanup != nil {
+				res.Cleanup()
+			}
+			t.Errorf("%s: expected error (fallback to tool-defs), got nil", info.Name)
+		}
+	}
+	if _, err := os.Stat(marker); err == nil {
+		t.Fatalf("setup.py was EXECUTED — marker present at %s", marker)
+	}
+}
+
+func pyQuote(s string) string {
+	return "r'" + s + "'"
+}