CVE-2026-45623
HighCVSS 7.5Exploitation Probability (EPSS)
Low risk46th percentile - higher than 46% of all known CVEs
Summary
PostCSS versions up to 8.5.11 have a vulnerability where the PreviousMap parser reads the /*# sourceMappingURL=PATH */ comment from any CSS string passed to process() and dereferences PATH against the local filesystem without scheme, allowlist, or traversal checks. An attacker controlling CSS input can cause the host process to read any file readable by Node and leak the first ~10 bytes via a JSON.parse SyntaxError message. The bug also provides a precise file-existence oracle and a controllable-read primitive that can be combined with large files for DoS. The issue is triggered with default PostCSS options and is reachable from any pipeline processing untrusted CSS through PostCSS.
Risk Assessment
The organization may suffer from leakage of sensitive data (e.g., configuration files, keys) and Denial of Service attacks via reading large files. The vulnerability is especially dangerous in environments processing user-supplied CSS, such as CMS, browser extensions, or CI/CD pipelines.
Recommendation
Immediately update PostCSS to version 8.5.12 or later. If an update is not possible, avoid processing untrusted CSS through PostCSS or apply additional protections such as path validation.
Other vulnerabilities in PostCSS
See all- CVE-2026-73646High
In PostCSS before 8.5.18, loadMap() in lib/previous-map.js passes attacker-controlled sourceMappingURL values to join(dirname(opts.from), annotation), and loadFile() permits traversed or absolute .map paths, allowing untrusted CSS processed without map: false to disclose sourcesContent from arbitrary reachable .map files through result.map.
- CVE-2026-69153Medium
PostCSS prior to 8.5.19, if from is unset, an attacker can cause PreviousMap.loadFile() to read an unintended source-map file by supplying an absolute or directory-traversal sourceMappingURL. The resulting map’s sources and sourcesContent may then be exposed to the application.
Original NVD description (English source)
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.