CVE-2023-44270 - Security Vulnerability in PostCSS before 8.4.31: Linters Unwittingly Parsing External Untrusted CSS

CVE-2023-44270 is a security vulnerability that has been discovered in PostCSS before version 8.4.31. It impacts linters that are utilizing PostCSS to parse external untrusted CSS files. Attackers can exploit this vulnerability by preparing malicious CSS files such that PostCSS unwittingly processes them as comments. Consequently, the malicious code ends up in the PostCSS output, impacting the security of any applications that rely on PostCSS. This article provides an in-depth explanation of the exploit, along with some code snippets illustrating the issue.

Exploit Details

This vulnerability affects linters that use PostCSS to parse external CSS files. The issue revolves around the improper handling of parts of a CSS file, which are incorrectly parsed as comments by PostCSS. Specifically, the vulnerability arises when an attacker prepares a CSS file containing malicious content that appears to be a comment, but is later processed as part of the CSS output.

Code Snippet Example of Malicious CSS

/* Malicious CSS */
.example {
  background-color: #333;
  /* Starts a seemingly benign comment
  padding: 10px;
  color: #fff;
  } <-- Closes the comment but leaves the actual CSS malicious */
}

When a linter that relies on PostCSS processes the above CSS file, it's meant to treat the content within the comment block as non-executable. However, the vulnerability allows the malicious CSS rules within the comment to be executed, posing a security risk.

Original Reference

The issue was first reported on GitHub in the PostCSS repository, and it is discussed in greater detail at this URL:

- https://github.com/postcss/postcss/issues/169

The maintainers of PostCSS have acknowledged the vulnerability, and it has been fixed in the 8.4.31 version. Please refer to the following commit for details on the fix in the PostCSS repository:

- https://github.com/postcss/postcss/commit/bbabccfa7ef901b8216751d8651797f13b833e03

Recommendations

To protect yourself from CVE-2023-44270, it is critical to upgrade the PostCSS package to version 8.4.31 or later. This update addresses the vulnerability by correctly identifying and handling comments within CSS files. Updating should be the top priority for any linters or tools dependent on PostCSS.

Additionally, if you parse external CSS within your web application, it is essential to only allow trusted sources. By restricting web resources to trusted origins, you significantly reduce the likelihood of falling victim to this kind of vulnerability.

Conclusion

CVE-2023-44270 is an important security vulnerability that has been found in PostCSS before version 8.4.31. It affects linters that utilize PostCSS to parse untrusted CSS files. Attackers can exploit this issue by preparing a malicious CSS file containing parts that appear to be comments but are later executed as CSS output. To mitigate this vulnerability, use the updated PostCSS version and ensure you only process trusted CSS files within your applications.

Timeline

Published on: 09/29/2023 22:15:11 UTC
Last modified on: 10/10/2023 17:19:55 UTC