Directory Traversal Affecting decompress package, versions *

decompress is a package that can be used for extracting archives.

Affected versions of this package are vulnerable to Directory Traversal and link following during archive extraction, where symlink and hardlink entries are created without checking their targets and the directory containment check relies on a string prefix comparison. An attacker can read or write files outside the target directory, and create setuid, setgid, or sticky-bit files when extraction runs as root, by supplying an archive containing out-of-target link entries, a path that shares a prefix with the output directory such as /srv/out-old against /srv/out, or entries whose modes retain special bits that mode & ~umask fails to strip. Exploitation requires extracting an untrusted or attacker-influenced archive in tar, tar.gz, tar.bz2, or zip format, and the privileged-file outcome requires extraction to run as root.

The privileged-file outcome can be avoided by running extraction as a non-root user, which prevents the permission flaw from creating setuid, setgid, or sticky-bit files, though it does not address the path traversal or link-following flaws.

A Directory Traversal attack (also known as path traversal) aims to access files and directories that are stored outside the intended folder. By manipulating files with "dot-dot-slash (../)" sequences and its variations, or by using absolute file paths, it may be possible to access arbitrary files and directories stored on file system, including application source code, configuration, and other critical system files.

Directory Traversal vulnerabilities can be generally divided into two types:

• Information Disclosure: Allows the attacker to gain information about the folder structure or read the contents of sensitive files on the system.

st is a module for serving static files on web pages, and contains a vulnerability of this type. In our example, we will serve files from the public route.

If an attacker requests the following URL from our server, it will in turn leak the sensitive private key of the root user.

curl http://localhost:8080/public/%2e%2e/%2e%2e/%2e%2e/%2e%2e/%2e%2e/root/.ssh/id_rsa

Note %2e is the URL encoded version of . (dot).

• Writing arbitrary files: Allows the attacker to create or replace existing files. This type of vulnerability is also known as Zip-Slip.

One way to achieve this is by using a malicious zip archive that holds path traversal filenames. When each filename in the zip archive gets concatenated to the target extraction folder, without validation, the final path ends up outside of the target folder. If an executable or a configuration file is overwritten with a file containing malicious code, the problem can turn into an arbitrary code execution issue quite easily.

The following is an example of a zip archive with one benign file and one malicious file. Extracting the malicious file will result in traversing out of the target folder, ending up in /root/.ssh/ overwriting the authorized_keys file:

2018-04-15 22:04:29 .....           19           19  good.txt
2018-04-15 22:04:42 .....           20           20  ../../../../../../root/.ssh/authorized_keys

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