Fix path traversal in local file fetch

embedmd/content.go

@@ -36,8 +36,7 @@ type fetcher struct{}
 
 func (fetcher) Fetch(dir, path string) ([]byte, error) {
 	if !strings.HasPrefix(path, "http://") && !strings.HasPrefix(path, "https://") {
-		path = filepath.Join(dir, filepath.FromSlash(path))
-		return os.ReadFile(path)
+		return os.ReadFile(filepath.Join(dir, filepath.FromSlash(path)))
 	}
 
 	res, err := http.Get(path)

embedmd/embedmd.go

@@ -54,7 +54,10 @@ package embedmd
 import (
 	"fmt"
 	"io"
+	"os"
+	"path/filepath"
 	"regexp"
+	"strings"
 )
 
 // Process reads markdown from the given io.Reader searching for an embedmd
@@ -88,7 +91,38 @@ type embedder struct {
 	baseDir string
 }
 
+// checkPath returns an error if path is a local path that escapes dir.
+// URLs (http/https) are not checked.
+//
+// When dir is empty (e.g. when reading from stdin with no base directory set),
+// no restriction is applied: there is no meaningful boundary to enforce.
+// Callers should not feed untrusted markdown to embedmd without a base
+// directory set via WithBaseDir.
+func checkPath(dir, path string) error {
+	if strings.HasPrefix(path, "http://") || strings.HasPrefix(path, "https://") {
+		return nil
+	}
+	if dir == "" {
+		return nil
+	}
+	absResolved, err := filepath.Abs(filepath.Join(dir, filepath.FromSlash(path)))
+	if err != nil {
+		return fmt.Errorf("could not resolve path %q: %v", path, err)
+	}
+	absBase, err := filepath.Abs(dir)
+	if err != nil {
+		return fmt.Errorf("could not resolve base directory %q: %v", dir, err)
+	}
+	if !strings.HasPrefix(absResolved+string(os.PathSeparator), absBase+string(os.PathSeparator)) {
+		return fmt.Errorf("path %q escapes base directory", path)
+	}
+	return nil
+}
+
 func (e *embedder) runCommand(w io.Writer, cmd *command) error {
+	if err := checkPath(e.baseDir, cmd.path); err != nil {
+		return fmt.Errorf("could not read %s: %v", cmd.path, err)
+	}
 	b, err := e.Fetch(e.baseDir, cmd.path)
 	if err != nil {
 		return fmt.Errorf("could not read %s: %v", cmd.path, err)

embedmd/embedmd_test.go

@@ -242,6 +242,37 @@ func TestProcess(t *testing.T) {
 				"Yay!\n",
 			err: "2: could not read https://fakeurl.com\\main.go: parse \"https://fakeurl.com\\\\main.go\": invalid character \"\\\\\" in host name",
 		},
+		{
+			name: "path traversal rejected when base dir is set",
+			dir:  "sample",
+			in: "# Header\n" +
+				"[embedmd]:# (../secret.go)\n" +
+				"Yay!\n",
+			files: map[string][]byte{"secret.go": []byte(content)},
+			err:   `2: could not read ../secret.go: path "../secret.go" escapes base directory`,
+		},
+		{
+			name: "deeply nested path traversal rejected",
+			dir:  "a/b/c",
+			in: "# Header\n" +
+				"[embedmd]:# (../../../secret.go)\n" +
+				"Yay!\n",
+			err: `2: could not read ../../../secret.go: path "../../../secret.go" escapes base directory`,
+		},
+		{
+			name: "normal relative path within base dir still works",
+			dir:  "sample",
+			in: "# This is some markdown\n" +
+				"[embedmd]:# (code.go)\n" +
+				"Yay!\n",
+			files: map[string][]byte{"sample/code.go": []byte(content)},
+			out: "# This is some markdown\n" +
+				"[embedmd]:# (code.go)\n" +
+				"```go\n" +
+				string(content) +
+				"```\n" +
+				"Yay!\n",
+		},
 		{
 			name: "ignore commands in code blocks",
 			in: "# This is some markdown\n" +
@@ -285,8 +316,8 @@ type mixedContentProvider struct {
 
 func (c mixedContentProvider) Fetch(dir, path string) ([]byte, error) {
 	if !strings.HasPrefix(path, "http://") && !strings.HasPrefix(path, "https://") {
-		path = filepath.Join(dir, filepath.FromSlash(path))
-		if f, ok := c.files[path]; ok {
+		resolved := filepath.Join(dir, filepath.FromSlash(path))
+		if f, ok := c.files[resolved]; ok {
 			return f, nil
 		}
 		return nil, os.ErrNotExist