Directory Traversal Affecting github.com/opencontainers/umoci/oci/layer package, versions <0.4.7

github.com/opencontainers/umoci/oci/layer is a reimplementation of several upstream features in upstream OCI projects. It implements features that should be maintained upstream (such as generate). To this end, this project's goal is to be completely upstreamable.

Affected versions of this package are vulnerable to Directory Traversal. By creating a malicious layer that has a symlink with the name . (or /), this package can be tricked into modifying host files. This results in the rootfs directory being replaced with an attacker-controlled symlink, because umoci deletes inodes if they change types. Subsequent image layers will then be applied on top of the target of the symlink (which could be any directory on the host filesystem the user has access to).

This vulnerability affects both umoci unpack and umoci raw unpack.

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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