Cover symlink attacks with test cases

server/filesystem/filesystem.go

@@ -332,7 +332,7 @@ func (fs *Filesystem) Delete(p string) error {
 		return errors.New("cannot delete root server directory")
 	}
 
-	if st, err := os.Stat(resolved); err != nil {
+	if st, err := os.Lstat(resolved); err != nil {
 		if !os.IsNotExist(err) {
 			fs.error(err).Warn("error while attempting to stat file before deletion")
 		}

server/filesystem/filesystem_test.go

@@ -80,6 +80,213 @@ func TestFilesystem_Path(t *testing.T) {
 	})
 }
 
+func TestFilesystem_SafePath(t *testing.T) {
+	g := Goblin(t)
+	fs, rfs := NewFs()
+	prefix := filepath.Join(rfs.root, "/server")
+
+	g.Describe("SafePath", func() {
+		g.It("returns a cleaned path to a given file", func() {
+			p, err := fs.SafePath("test.txt")
+			g.Assert(err).IsNil()
+			g.Assert(p).Equal(prefix + "/test.txt")
+
+			p, err = fs.SafePath("/test.txt")
+			g.Assert(err).IsNil()
+			g.Assert(p).Equal(prefix + "/test.txt")
+
+			p, err = fs.SafePath("./test.txt")
+			g.Assert(err).IsNil()
+			g.Assert(p).Equal(prefix + "/test.txt")
+
+			p, err = fs.SafePath("/foo/../test.txt")
+			g.Assert(err).IsNil()
+			g.Assert(p).Equal(prefix + "/test.txt")
+
+			p, err = fs.SafePath("/foo/bar")
+			g.Assert(err).IsNil()
+			g.Assert(p).Equal(prefix + "/foo/bar")
+		})
+
+		g.It("handles root directory access", func() {
+			p, err := fs.SafePath("/")
+			g.Assert(err).IsNil()
+			g.Assert(p).Equal(prefix)
+
+			p, err = fs.SafePath("")
+			g.Assert(err).IsNil()
+			g.Assert(p).Equal(prefix)
+		})
+
+		g.It("removes trailing slashes from paths", func() {
+			p, err := fs.SafePath("/foo/bar/")
+			g.Assert(err).IsNil()
+			g.Assert(p).Equal(prefix + "/foo/bar")
+		})
+
+		g.It("handles deeply nested directories that do not exist", func() {
+			p, err := fs.SafePath("/foo/bar/baz/quaz/../../ducks/testing.txt")
+			g.Assert(err).IsNil()
+			g.Assert(p).Equal(prefix + "/foo/bar/ducks/testing.txt")
+		})
+
+		g.It("blocks access to files outside the root directory", func() {
+			p, err := fs.SafePath("../test.txt")
+			g.Assert(err).IsNotNil()
+			g.Assert(errors.Is(err, ErrBadPathResolution)).IsTrue()
+			g.Assert(p).Equal("")
+
+			p, err = fs.SafePath("/../test.txt")
+			g.Assert(err).IsNotNil()
+			g.Assert(errors.Is(err, ErrBadPathResolution)).IsTrue()
+			g.Assert(p).Equal("")
+
+			p, err = fs.SafePath("./foo/../../test.txt")
+			g.Assert(err).IsNotNil()
+			g.Assert(errors.Is(err, ErrBadPathResolution)).IsTrue()
+			g.Assert(p).Equal("")
+
+			p, err = fs.SafePath("..")
+			g.Assert(err).IsNotNil()
+			g.Assert(errors.Is(err, ErrBadPathResolution)).IsTrue()
+			g.Assert(p).Equal("")
+		})
+	})
+}
+
+// We test against accessing files outside the root directory in the tests, however it
+// is still possible for someone to mess up and not properly use this safe path call. In
+// order to truly confirm this, we'll try to pass in a symlinked malicious file to all of
+// the calls and ensure they all fail with the same reason.
+func TestFilesystem_Blocks_Symlinks(t *testing.T) {
+	g := Goblin(t)
+	fs, rfs := NewFs()
+
+	if err := rfs.CreateServerFile("/../malicious.txt", "external content"); err != nil {
+		panic(err)
+	}
+
+	if err := os.Mkdir(filepath.Join(rfs.root, "/malicious_dir"), 0777); err != nil {
+		panic(err)
+	}
+
+	if err := os.Symlink(filepath.Join(rfs.root, "malicious.txt"), filepath.Join(rfs.root, "/server/symlinked.txt")); err != nil {
+		panic(err)
+	}
+
+	if err := os.Symlink(filepath.Join(rfs.root, "/malicious_dir"), filepath.Join(rfs.root, "/server/external_dir")); err != nil {
+		panic(err)
+	}
+
+	g.Describe("Readfile", func() {
+		g.It("cannot read a file symlinked outside the root", func() {
+			b := bytes.Buffer{}
+
+			err := fs.Readfile("symlinked.txt", &b)
+			g.Assert(err).IsNotNil()
+			g.Assert(errors.Is(err, ErrBadPathResolution)).IsTrue()
+		})
+	})
+
+	g.Describe("Writefile", func() {
+		g.It("cannot write to a file symlinked outside the root", func() {
+			r := bytes.NewReader([]byte("testing"))
+
+			err := fs.Writefile("symlinked.txt", r)
+			g.Assert(err).IsNotNil()
+			g.Assert(errors.Is(err, ErrBadPathResolution)).IsTrue()
+		})
+
+		g.It("cannot write a file to a directory symlinked outside the root", func() {
+			r := bytes.NewReader([]byte("testing"))
+
+			err := fs.Writefile("external_dir/foo.txt", r)
+			g.Assert(err).IsNotNil()
+			g.Assert(errors.Is(err, ErrBadPathResolution)).IsTrue()
+		})
+	})
+
+	g.Describe("CreateDirectory", func() {
+		g.It("cannot create a directory outside the root", func() {
+			err := fs.CreateDirectory("my_dir", "external_dir")
+			g.Assert(err).IsNotNil()
+			g.Assert(errors.Is(err, ErrBadPathResolution)).IsTrue()
+		})
+
+		g.It("cannot create a nested directory outside the root", func() {
+			err := fs.CreateDirectory("my/nested/dir", "external_dir/foo/bar")
+			g.Assert(err).IsNotNil()
+			g.Assert(errors.Is(err, ErrBadPathResolution)).IsTrue()
+		})
+
+		g.It("cannot create a nested directory outside the root", func() {
+			err := fs.CreateDirectory("my/nested/dir", "external_dir/server")
+			g.Assert(err).IsNotNil()
+			g.Assert(errors.Is(err, ErrBadPathResolution)).IsTrue()
+		})
+	})
+
+	g.Describe("Rename", func() {
+		g.It("cannot rename a file symlinked outside the directory root", func() {
+			err := fs.Rename("symlinked.txt", "foo.txt")
+			g.Assert(err).IsNotNil()
+			g.Assert(errors.Is(err, ErrBadPathResolution)).IsTrue()
+		})
+
+		g.It("cannot rename a symlinked directory outside the root", func() {
+			err := fs.Rename("external_dir", "foo")
+			g.Assert(err).IsNotNil()
+			g.Assert(errors.Is(err, ErrBadPathResolution)).IsTrue()
+		})
+
+		g.It("cannot rename a file to a location outside the directory root", func() {
+			rfs.CreateServerFile("my_file.txt", "internal content")
+
+			err := fs.Rename("my_file.txt", "external_dir/my_file.txt")
+			g.Assert(err).IsNotNil()
+			g.Assert(errors.Is(err, ErrBadPathResolution)).IsTrue()
+		})
+	})
+
+	g.Describe("Chown", func() {
+		g.It("cannot chown a file symlinked outside the directory root", func() {
+			err := fs.Chown("symlinked.txt")
+			g.Assert(err).IsNotNil()
+			g.Assert(errors.Is(err, ErrBadPathResolution)).IsTrue()
+		})
+
+		g.It("cannot chown a directory symlinked outside the directory root", func() {
+			err := fs.Chown("external_dir")
+			g.Assert(err).IsNotNil()
+			g.Assert(errors.Is(err, ErrBadPathResolution)).IsTrue()
+		})
+	})
+
+	g.Describe("Copy", func() {
+		g.It("cannot copy a file symlinked outside the directory root", func() {
+			err := fs.Copy("symlinked.txt")
+			g.Assert(err).IsNotNil()
+			g.Assert(errors.Is(err, ErrBadPathResolution)).IsTrue()
+		})
+	})
+
+	g.Describe("Delete", func() {
+		g.It("deletes the symlinked file but leaves the source", func() {
+			err := fs.Delete("symlinked.txt")
+			g.Assert(err).IsNil()
+
+			_, err = os.Stat(filepath.Join(rfs.root, "malicious.txt"))
+			g.Assert(err).IsNil()
+
+			_, err = rfs.StatServerFile("symlinked.txt")
+			g.Assert(err).IsNotNil()
+			g.Assert(errors.Is(err, os.ErrNotExist)).IsTrue()
+		})
+	})
+
+	rfs.reset()
+}
+
 func TestFilesystem_Readfile(t *testing.T) {
 	g := Goblin(t)
 	fs, rfs := NewFs()