Directory Traversal Affecting isomorphic-git package, versions <1.38.6

isomorphic-git is a pure JavaScript reimplementation of git that works in both Node.js and browser JavaScript environments. It can read and write to git repositories, fetch from and push to git remotes (such as GitHub), all without any native C++ module dependencies.

Affected versions of this package are vulnerable to Directory Traversal via git.checkout, which writes working-tree files through symlinked leading directory components without checking them. A malicious repository can place a directory symlink in one commit and then introduce a child file beneath that path in a subsequent commit; when the consumer checks out the later state, isomorphic-git mkdirs the parent (already a symlink, EEXIST ignored) and then writes the file, which the OS resolves through the symlink to a location outside the working directory. On Node.js with a POSIX filesystem this permits arbitrary host file writes and potential code execution, for example by injecting an attacker-controlled key into ~/.ssh/authorized_keys or overwriting .git/hooks to achieve RCE on the next native git invocation.

Note: Full arbitrary file write impact requires Node.js fs on Linux or macOS; Node.js on Windows typically blocks the symlink creation step without elevated privilege, and browser or in-memory fs providers are affected only when they implement both symlink creation and following, limiting impact to their virtual filesystem.

https://github.com/spiceoverflow/av1-symlink-hook-rce — open in a Codespace; it builds the malicious repo, serves it locally, and runs a victim. Output on Linux:

[AV-1] Directory symlink on disk: …/config → ../.ssh  (→ /home/node/.ssh)
[AV-1] *** SSH KEY INJECTION CONFIRMED ***

Trigger: minimal realistic path is clone (commit N) + a later pull (commit N+1) — ordinary sync of an already-trusted repo. It also collapses to a single clone when the consuming app reads or writes into the cloned tree, or when a symlink already exists on disk.

Note: The PoC states it should be opened in a Codespace, where it builds a malicious repo, serves it locally, and runs a victim on Linux.

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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