Directory Traversal Affecting postcss package, versions <8.5.23

postcss is a PostCSS is a tool for transforming styles with JS plugins.

Affected versions of this package are vulnerable to Directory Traversal in the PreviousMap.loadFile() method of lib/previous-map.js, which nests its traversal guard inside an if (cssFile) branch that is skipped when the from option is unset, leaving only a check that the target path ends in .map. An attacker can read arbitrary .map files from the filesystem, disclosing their sources paths and sourcesContent through the generated source map, by placing an absolute or ../ path in a sourceMappingURL comment in the processed CSS. This requires the application to process attacker-influenced CSS without the from option, with map enabled and the source-map module available, and then expose result.map to the attacker, and the read is limited to JSON-parseable files ending in .map.

This vulnerability can be avoided by passing an explicit from option when processing untrusted CSS, which restores the traversal guard that is otherwise skipped.

Note: This is a bypass of the fix for the vulnerability described in CVE-2026-45623.

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

• GitHub Advisory
• GitHub Commit
• GitHub Release