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Vulnerability record · CVE-2026-45623 · published 27 July 2026

CVE-2026-45623: Postcss path traversal vulnerability

Postcss · Postcss

PostCSS takes a CSS file and provides an API to analyze and modify its rules by transforming the rules into an Abstract Syntax Tree. In versions 8.5.11 and prior, the PreviousMap parses the /*# sourceMappingURL=PATH */ comment from any CSS string passed to process() and dereferences PATH against the local filesystem with no scheme, allowlist, or traversal check. An attacker who controls the CSS input can cause the host process to read any file readable by Node and leak the first ~10 bytes of its content through the resulting JSON.parse SyntaxError message. The bug also yields a precise file-existence oracle and a controllable-read primitive that may be combined with large-file targets for DoS. The behaviour is triggered with PostCSS's default options — no from, no map, no plugins required — and is therefore reachable from any pipeline that runs untrusted CSS through PostCSS (CMS themes, user-uploaded styles, browser-extension/userstyle processors, build pipelines for third-party packages, blog comment renderers, etc.). This issue has been fixed in version 8.5.12.

9.1 CVSS 3.1 Critical EPSS 0.61% · top 53.0% CWE-22 · Path traversalCWE-200 · Information exposure
9.1CVSS 3.1 base score
0.61%EPSS exploitation probability, 30 days
NoNot in CISA KEV
1Affected product versions listed by NVD
2References, 2 tagged exploit
7 Aug 2026Last modified by NVD

Description

PostCSS takes a CSS file and provides an API to analyze and modify its rules by transforming the rules into an Abstract Syntax Tree. In versions 8.5.11 and prior, the PreviousMap parses the /*# sourceMappingURL=PATH */ comment from any CSS string passed to process() and dereferences PATH against the local filesystem with no scheme, allowlist, or traversal check. An attacker who controls the CSS input can cause the host process to read any file readable by Node and leak the first ~10 bytes of its content through the resulting JSON.parse SyntaxError message. The bug also yields a precise file-existence oracle and a controllable-read primitive that may be combined with large-file targets for DoS. The behaviour is triggered with PostCSS's default options — no from, no map, no plugins required — and is therefore reachable from any pipeline that runs untrusted CSS through PostCSS (CMS themes, user-uploaded styles, browser-extension/userstyle processors, build pipelines for third-party packages, blog comment renderers, etc.). This issue has been fixed in version 8.5.12.

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H

Affected products

1 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

LinkTags
https://github.com/postcss/postcss/security/advisories/GHSA-6g55-p6wh-862q ExploitMitigationVendor Advisory
https://github.com/postcss/postcss/security/advisories/GHSA-6g55-p6wh-862q ExploitMitigationVendor Advisory

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Related vulnerabilities

Same products first, then exploited flaws of the same weakness class.

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Source: NIST National Vulnerability Database (record CVE-2026-45623), CISA KEV, FIRST EPSS (scores of 2026-09-27). This page is refreshed as NVD updates the record.