diff --git a/pkg/inject/inject.go b/pkg/inject/inject.go
new file mode 100644
index 0000000..01f991c
--- /dev/null
+++ b/pkg/inject/inject.go
@@ -0,0 +1,38 @@
+package inject
+
+import (
+    "fmt"
+    "os"
+    "path/filepath"
+)
+
+// Injector writes node config into a mounted image (boot/root paths supplied by caller).
+type Injector struct {
+    BootPath string
+    RootPath string
+}
+
+// FileSpec describes a file to write.
+type FileSpec struct {
+    Path    string
+    Content []byte
+    Mode    os.FileMode
+    RootFS  bool // if true, write under root path; else boot path
+}
+
+func (i *Injector) Write(files []FileSpec) error {
+    for _, f := range files {
+        base := i.BootPath
+        if f.RootFS {
+            base = i.RootPath
+        }
+        target := filepath.Join(base, f.Path)
+        if err := os.MkdirAll(filepath.Dir(target), 0o755); err != nil {
+            return fmt.Errorf("mkdir %s: %w", filepath.Dir(target), err)
+        }
+        if err := os.WriteFile(target, f.Content, f.Mode); err != nil {
+            return fmt.Errorf("write %s: %w", target, err)
+        }
+    }
+    return nil
+}
diff --git a/tests/test_inject.py b/tests/test_inject.py
new file mode 100644
index 0000000..d309805
--- /dev/null
+++ b/tests/test_inject.py
@@ -0,0 +1,16 @@
+from pathlib import Path
+
+from metis.pkg.inject import Injector, FileSpec
+
+
+def test_inject_write(tmp_path):
+    boot = tmp_path / "boot"
+    root = tmp_path / "root"
+    inj = Injector(BootPath=str(boot), RootPath=str(root))
+    files = [
+        FileSpec(Path="config.txt", Content=b"bootcfg", Mode=0o644, RootFS=False),
+        FileSpec(Path="etc/hostname", Content=b"node", Mode=0o644, RootFS=True),
+    ]
+    inj.Write(files)
+    assert (boot / "config.txt").read_bytes() == b"bootcfg"
+    assert (root / "etc/hostname").read_bytes() == b"node"